Showing papers in "Phytopathology in 2000"

Economic and social impacts of fusarium head blight: changing farms and rural communities in the northern great plains.

Abstract: Fusarium head blight, also known as scab, can be a devastating disease on all classes of wheat and barley. Among the species of Fusarium that cause scab, F. graminearum is the predominant pathogen. Wheat and barley are highly susceptible to infection when the crop is in the flowering to soft dough stages and when weather includes frequent precipitation, high humidity, or heavy dews. Multiple symptoms occur: reduced yields, discolored and shriveled kernels (tombstones), depressed seed weights, and reduced seed quality and vigor. The mycotoxin deoxynivalenol (DON, commonly known as vomitoxin) also can be produced by F. graminearum on infected grain. Grain contaminated with DON often is unsuitable for flour, cereals, or malt and is too toxic as feed for nonruminant animals (19). Distribution of Fusarium head blight. In recent years, Fusarium head blight has reemerged worldwide as a disease of economic importance (19). In the United States and Canada, the reemergence of Fusarium head blight in the 1990s has caused epidemics of varying severity on barley and on all classes of wheat. It is documented as causing epidemics in 26 states (19) and the Canadian provinces of Quebec (A. Comeau, Agriculture and Agri-Food Canada, Ste. Foy, Quebec, Canada, personal communication), Ontario (A. W. Schaafsma, University of Guelph, Ridgetown College, Guelph, Ontario, personal communication), and Manitoba and Saskatchewan (R. Clear, Canadian Grain Commission, Winnipeg, Manitoba, personal communication). The United States Department of Agriculture (USDA) ranks Fusarium head blight as the worst plant disease to hit the nation since the stem rust epidemics of the 1950s (35). Wheat and barley losses caused by scab epidemics in the United States during the 1990s are estimated at close to $3 billion. American wheat farmers lost over 500 million bushels of wheat valued at about $2.5 billion (T. Sayler, Prairie Ag Communications, Fargo, ND, personal communication) and Midwestern barley producers lost $400 million on barley (J. Mittleider, North Dakota Barley Council, Fargo, personal communication). Economic losses to wheat producers in Canada in the 1990s are estimated in U.S. dollars at $220 million in Quebec and Ontario (A. Comeau, Agriculture and Agri-Food Canada, Ste. Foy, Quebec, personal communication) and, from 1993 to 1998, at $300 million in Manitoba (A. Tekauz, Agriculture and Agri-Food Canada, Winnipeg, Manitoba, personal communication). Why Fusarium head blight has been so severe. The disease has been favored by a combination of factors including unseasonably wet weather, particularly when plants are most susceptible. The increased practice of conservation tillage has favored inoculum of the pathogen to survive in residues of host crops such as wheat, barley, and corn. A high percentage of land has been planted to small grains, with short intervals between crops. Also, plant resistance to scab has been limited or absent in wheat and barley cultivars. These factors have resulted in the buildup and abundance of inoculum of F. graminearum. “Breadbasket of the World” under siege. The Northern Great Plains has a short growing season, so much of the land is tied to cereal production that includes hard red spring wheat, durum wheat, winter wheat, and barley. Beginning in 1993, northwestern Minnesota, eastern North Dakota, northeastern South Dakota, and southern Manitoba in Canada were particularly hard hit by Fusarium head blight and disease severities reached 20 to 80% (19). Epidemics of Fusarium head blight, however, have continued to reoccur every year from 1993 to 1998 at varying intensities, particularly in northeastern North Dakota, northwestern Minnesota, and southern Manitoba. In the last couple of summers, F. graminearum has spread further westward across North Dakota (M. McMullen, North Dakota State University, Fargo, personal communication) and from western Manitoba into Saskatchewan (R. Clear, Canadian Grain Commission, Winnipeg, Manitoba, personal communication). In the 1999 season, Fusarium head blight was a minor disease problem in Minnesota and North Dakota (although infections occurred as far west as the central portion of the state) and less of a problem in Manitoba compared with previous years. The Red River Valley, which I refer to occasionally in this presentation, is a 40to 60-mile region on each side the Red River that separates North Dakota and Minnesota and flows north into Manitoba. The flatlands of the Red River Valley are the lake bed of the prehistoric Lake Agassiz. It’s black loamy soil is among the richest farmland in the world. This region is the “Breadbasket of the World” and farmers here typically are prosperous. I live and work in northeastern North Dakota and northwestern Minnesota and have witnessed the impact of Fusarium head blight on the region. It has severely crippled the wheat and barley industries, driven producers to financial ruin and human hardship, and resulted in rural communities dwindling away. The epidemic has generated nearly $10 million in new public and private funding to support Fusarium head blight research and education in the United States (T. Sayler, Prairie Ag Communications, Fargo, ND, personal communication) and also has resulted in lobbying to change agricultural policies. In this address, I will focus on the development and impact of this crisis, with an emphasis on northeastern North Dakota and northwestern Minnesota. The summer of 1993 held great promise. Fields of wheat and barley were lush and green, and yield potentials promised to meet or exceed the bumper crops harvested in 1992. At harvest, growers were stunned and distressed when they found severe scab infecContribution No. 9910102 of the Minnesota Agricultural Experiment Station.

Nematicidal Activity of Essential Oils and Their Components Against the Root-Knot Nematode

Abstract: Nematicidal activity of essential oils extracted from 27 spices and aromatic plants were evaluated in vitro and in pot experiments. Twelve of the twenty-seven essential oils immobilized more than 80% of juveniles of the root-knot nematode Meloidogyne javanica at a concentration of 1,000 μl/liter. At this concentration, most of these oils also inhibited nematode hatching. Essential oils of Carum carvi, Foeniculum vulgare, Mentha rotundifolia, and Mentha spicata showed the highest nematicidal activity among the in vitro tested oils. These oils and those from Origanum vulgare, O. syriacum, and Coridothymus capitatus mixed in sandy soil at concentrations of 100 and 200 mg/kg reduced the root galling of cucumber seedlings in pot experiments. The main components of these essential oils were tested for their nematicidal activity. Carvacrol, t-anethole, thymol, and (+)-carvone immobilized the juveniles and inhibited hatching at >125 μl/liter in vitro. Most of these components mixed in sandy soil at conce...

Gene Genealogies and AFLP Analyses in the Fusarium oxysporum Complex Identify Monophyletic and Nonmonophyletic Formae Speciales Causing Wilt and Rot Disease

Abstract: The monophyletic origin of host-specific taxa in the plant-pathogenic Fusarium oxysporum complex was tested by constructing nuclear and mitochondrial gene genealogies and amplified fragment length polymorphism (AFLP)-based phylogenies for 89 strains representing the known genetic and pathogenic diversity in 8 formae speciales associated with wilt diseases and root and bulb rot. We included strains from clonal lineages of F. oxysporum f. spp. asparagi, dianthi, gladioli, lilii, lini, opuntiarum, spinaciae, and tulipae. Putatively nonpathogenic strains from carnation and lily were included and a reference strain from each of the three main clades identified previously in the F. oxysporum complex; sequences from related species were used as outgroups. DNA sequences from the nuclear translation elongation factor 1alpha and the mitochondrial small subunit (mtSSU) ribosomal RNA genes were combined for phylogenetic analysis. Strains in vegetative compatibility groups (VCGs) shared identical sequences and AFLP profiles, supporting the monophyly of the two single-VCG formae speciales, lilii and tulipae. Identical genotypes were also found for the three VCGs in F. oxysporum f. sp. spinaciae. In contrast, multiple evolutionary origins were apparent for F. oxysporum f. spp. asparagi, dianthi, gladioli, lini, and opuntiarum, although different VCGs within each of these formae speciales often clustered close together or shared identical EF-1alpha and mtSSU rDNA haplotypes. Kishino-Hasegawa analyses of constraints forcing the monophyly of these formae speciales supported the exclusive origin of F. oxysporum f. sp. opuntiarum but not the monophyly of F. oxysporum f. spp. asparagi, dianthi, gladioli, and lini. Most of the putatively nonpathogenic strains from carnation and lily, representing unique VCGs, were unrelated to F. oxysporum f. spp. dianthi and lilii, respectively. Putatively nonpathogenic or rot-inducing strains did not form exclusive groups within the molecular phylogeny. Parsimony analyses of AFLP fingerprint data supported the gene genealogy-based phylogram; however, AFLP-based phylogenies were considerably more homoplasious than the gene genealogies. The predictive value of the forma specialis naming system within the F. oxysporum complex is questioned.

Control of soilborne plant pathogens by incorporating fresh organic amendments followed by tarping

Abstract: A new method for the control of soilborne plant pathogens was tested for its efficacy in two field experiments during two years. Plots were amended with fresh broccoli or grass (3.4 to 4.0 kg fresh weight m(-2)) or left nonamended, and covered with an airtight plastic cover (0.135 mm thick) or left noncovered. In plots amended with broccoli or grass and covered with plastic sheeting, anaerobic and strongly reducing soil conditions developed quickly, as indicated by rapid depletion of oxygen and a decrease in redox potential values to as low as -200 mV. After 15 weeks, survival of Fusarium oxysporum f. sp. asparagi, Rhizoctonia solani, and Verticillium dahliae in inoculum samples buried 15 cm deep was strongly reduced in amended, covered plots in both experiments. The pathogens were not or hardly inactivated in amended, noncovered soil or nonamended, covered soil. The latter indicates that thermal inactivation due to increased soil temperatures under the plastic cover was not involved in pathogen inactivation. The results show the potential for this approach to control various soilborne pathogens and that it may serve as an alternative to chemical soil disinfestation for high-value crops under conditions where other alternatives, such as solarization or soil flooding, are not effective or not feasible.

Induction of terpenoid synthesis in cotton roots and control of Rhizoctonia solani by seed treatment with Trichoderma virens. [Erratum: 2003 Dec., v. 93, no. 12, p. 1606.]

Abstract: Research on the mechanisms employed by the biocontrol agent Trichoderma virens to suppress cotton (Gossypium hirsutum) seedling disease incited by Rhizoctonia solani has shown that mycoparasitism and antibiotic production are not major contributors to successful biological control. In this study, we examined the possibility that seed treatment with T. virens stimulates defense responses, as indicated by the synthesis of terpenoids in cotton roots. We also examined the role of these terpenoid compounds in disease control. Analysis of extracts of cotton roots and hypocotyls grown from T. virens-treated seed showed that terpenoid synthesis and peroxidase activity were increased in the roots of treated plants, but not in the hypocotyls of these plants or in the untreated controls. Bioassay of the terpenoids for toxicity to R. solani showed that the pathway intermediates desoxyhemigossypol (dHG) and hemigossypol (HG) were strongly inhibitory to the pathogen, while the final product gossypol (G) was toxic only at a much higher concentration. Strains of T. virens and T. koningii were much more resistant to HG than was R. solani, and they thoroughly colonized the cotton roots. A comparison of biocontrol efficacy and induction of terpenoid synthesis in cotton roots by strains of T. virens, T. koningii, T. harzianum, and protoplast fusants indicated that there was a strong correlation (+0.89) between these two phenomena. It, therefore, appears that induction of defense response, particularly terpenoid synthesis, in cotton roots by T. virens may be an important mechanism in the biological control by this fungus of R. solani-incited cotton seedling disease.

Induction of Terpenoid Synthesis in Cotton Roots and Control of Rhizoctonia solani by Seed Treatment with Trichoderma virens.

Abstract: Research on the mechanisms employed by the biocontrol agent Trichoderma virens to suppress cotton (Gossypium hirsutum) seedling disease incited by Rhizoctonia solani has shown that mycopar...

Expression of Endochitinase from Trichoderma harzianum in Transgenic Apple Increases Resistance to Apple Scab and Reduces Vigor.

Abstract: The goal of this research was to improve scab resistance of apple by transformation with genes encoding chitinolytic enzymes from the bio-control organism Trichoderma harzianum. The endochitinase gene, as cDNA and genomic clones, was transferred into apple cv. Marshall McIntosh by Agrobacterium-transformation. A total of 15 lines were identified as transgenic by NPTII enzyme-linked immunosorbent assay and polymerase chain reaction and confirmed by Southern analysis. Substantial differences in endochitinase activity were detected among different lines by enzymatic assay and western analysis. Eight lines propagated as grafted and own-rooted plants were inoculated with Venturia inaequalis. Six of these transgenic lines expressing endochitinase were more resistant than nontransformed cv. Marshall McIntosh. Disease severity compared with cv. Marshall McIntosh was reduced by 0 to 99.7% (number of lesions), 0 to 90% (percentage of leaf area infected), and 1 to 56% (conidia recovered) in the transgenic lines tested. Endochitinase also had negative effects on the growth of both inoculated and uninoculated plants. There was a significant negative correlation between the level of endochitinase production and both the amount of disease and plant growth.

Detection of Potato mop top virus and Tobacco rattle virus Using a Multiplex Real-Time Fluorescent Reverse-Transcription Polymerase Chain Reaction Assay.

Abstract: Mumford, R. A., Walsh, K., Barker, I., and Boonham, N. 2000. Detection of Potato mop top virus and Tobacco rattle virus using a multiplex realtime fluorescent reverse-transcription polymerase chain reaction assay. Phytopathology 90:448-453. Tobacco rattle virus (TRV) and Potato mop top virus (PMTV) are important diseases of potato that are difficult to diagnose reliably by visual symptoms. Effective control strategies rely on accurate dia gnosis. This paper describes the development of a multiplex assay for the detection of TRV and PMTV directly from potato tubers and leaves by polymerase chain reaction (PCR) combined with in-tube fluorescent product detection (TaqMan). This technology obviates any post-PCR manipulations and has many advantages including reducing contamination risks, eliminating the need for ethidium bromide staining, and removing the time and cost of gel running. The new assay also allows the replacement of the two separate tests (a TRV reverse-transcription-PCR and a PMTV enzyme-linked immunosorbent assay) currently used with a single-tube multiplex format. In addition to greatly simplifying the detection of these two viruses, the multiplex TaqMan assay was also shown to be more sensitive than either of the tests that it replaces, allowing 100- and 10,000-fold increases in sensitivity for TRV and PMTV detection, respectively. The test reliably detected over 40 different isolates of TRV and PMTV obtained from a wide range of different cultivars and geographical locations, including some samples in which existing tests failed to detect virus. The use of an assay of this kind in routine diagnosis helps to speed up and streamline the diagnostic laboratory; in add ition, more reliable diagnosis should help in the control of this damaging disease.

Mefenoxam Insensitivity and the Sexual Stage of Phytophthora capsici in Michigan Cucurbit Fields.

Abstract: The potential for outcrossing, occurrence of oospores, and inheritance of mefenoxam sensitivity was assessed in naturally occurring populations of Phytophthora capsici. Between 1997 and 1998, 14 farms were sampled, with 473 isolates recovered from cucurbit hosts and 30 from bell pepper. The A1 and A2 compatibility types were recovered in a roughly 1:1 ratio in 8 of 14 farms with sample sizes larger than 15. In 1997, one isolate was designated as insensitive and four as sensitive to mefenoxam. In 1998, 55% of the 498 isolates sampled were sensitive, 32% were intermediate, and 13% were fully insensitive to mefenoxam. In vitro characterization of mefenoxam sensitivity was conducted by crossing field isolates. Chi-square analysis of crosses between sensitive, intermediately sensitive, and insensitive isolates indicate that mefenoxam insensitivity segregated as an incompletely dominant trait unlinked to compatibility type (P = 0.05). Oospores were observed in diseased cucurbit fruit from four farms in 1998, and 223 oospore progeny were recovered from a single diseased cucumber. All six mefenoxam sensitivity-compatibility type combinations were present in these oospore progeny and within single fields. Based on these findings, we conclude that oospores likely play a role in the survival of P. capsici and that sexual recombination may significantly influence population structure.

Bacterial-mediated induced resistance in cucumber: beneficial effect of the endophytic bacterium Serratia plymuthica on the protection against infection by Pythium ultimum

Abstract: The potential of the endophytic bacterium Serratia plymuthica strain R1GC4 in stimulating defense reactions in cucumber (Cucumis sativus) seedlings inoculated with the soilborne pathogen Pythium ultimum was explored at the cellular level. Bacterial treatment prior to Pythium inoculation resulted in less seedling disease development as compared with that in nontreated control plants, in which typical root symptoms were visible by 3 days after inoculation with the pathogen. Histological investigations of root samples revealed striking differences in the extent of plant defense reactions between bacterized and nonbacterized plants. These observations were further confirmed at the ultrastructural level with the demonstration that restriction of fungal colonization to the outermost root tissues of bacterized seedlings correlated with the deposition of enlarged callose-enriched wall appositions at sites of potential pathogen penetration and the accumulation of an osmiophilic material in the colonized areas. Hyphae of the pathogen, surrounded by this electron-opaque material, exhibited considerable changes including cytoplasm disorganization and, in many cases, loss of the protoplasm. However, labeling with the beta-1,4-exoglucanase resulted in a regular labeling of Pythium cell walls, even at a time when these walls were entirely coated by the osmiophilic material. This material was also found to infiltrate into the invading hyphae to form either an internal coating of the cell wall or a network of polymorphic droplets in the area previously occupied by the cytoplasm. Cytochemical investigations revealed that callose, pectin, and cellulose appeared in the wall appositions. In addition, glucosides, lipids, and phenolics were detected in the electron-dense aggregates forming the core of most wall appositions. Finally, galactose residues were among the minor polysaccharidic compounds detected in the wall appositions. Evidence is provided in this study showing that treatment with S. plymuthica sensitizes susceptible cucumber plants to react more rapidly and more efficiently to Pythium attack through the formation of physical and chemical barriers at sites of potential fungal entry.

Characterizing the mechanism of biological control of postharvest diseases on fruits with a simple method to study competition for nutrients.

Abstract: Janisiewicz, W. J., Tworkoski, T. J., and Sharer, C. 2000. Characterizing the mechanism of biological control of postharvest diseases on fruits with a simple method to study competition for nutrients. Phytopathology 90:1196-1200. Biocontrol agents may compete with pathogens for nutrients and space to delay or prevent decay of fruits after harvest. These mechanisms of biological control have been difficult to study b ecause no method has been available to determine the significance of each of the com ponents of competition. We developed a nondestructive method using tissue culture plates with cylinder inserts containing defusing membrane at one end to study competition for nutrients wit hout competition for space. Other biocontrol mechanisms in which direct contact between an antagonist and a pathogen is not required also can be studied. The method was used to determine the competition between the yeastlike biocontrol agent, Aureobasidium pullulans, and Penicillium expansum for limited nutrients in apple juice during 24 h incubation, simulating a fruit wound. The antagonist depleted amino acids and inhibited germination of P. expansum conidia. Exposing these conidia to fresh apple juice increased conidial germination to the level comparable to that exhibited by conidia which were not exposed to the antagonist. Because the culture plate method was nondestructive, follow-up experiments in an agar diffusion test were conducted. Juice in which the antagonist grew did not inhibit germination of P. expansum conidia that were seeded on the plates. This corroborates findings from the culture plate method that inhib ition of the conidia germination resulted from competition for nutrients. The new met hod can be coupled with existing techniques to improve understanding of antagonist‐pathogen interaction for biological control of postharvest diseases.

Riboflavin Induces Disease Resistance in Plants by Activating a Novel Signal Transduction Pathway

Abstract: The role of riboflavin as an elicitor of systemic resistance and an activator of a novel signaling process in plants was demonstrated. Following treatment with riboflavin, Arabidopsis thaliana developed systemic resistance to Peronospora parasitica and Pseudomonas syringae pv. Tomato, and tobacco developed systemic resistance to Tobacco mosaic virus (TMV) and Alternaria alternata. Riboflavin, at concentrations necessary for resistance induction, did not cause cell death in plants or directly affect growth of the culturable pathogens. Riboflavin induced expression of pathogenesis-related (PR) genes in the plants, suggesting its ability to trigger a signal transduction pathway that leads to systemic resistance. Both the protein kinase inhibitor K252a and mutation in the NIM1/NPR1 gene which controls transcription of defense genes, impaired responsiveness to riboflavin. In contrast, riboflavin induced resistance and PR gene expression in NahG plants, which fail to accumulate salicylic acid (SA). Thus, riboflavin-induced resistance requires protein kinase signaling mechanisms and a functional NIM1/NPR1 gene, but not accumulation of SA. Riboflavin is an elicitor of systemic resistance, and it triggers resistance signal transduction in a distinct manner.

Suppression of Phytophthora cinnamomi in Potting Mixes Amended with Uncomposted and Composted Animal Manures

Abstract: We examined the effects of fresh and composted animal manures on the development of root rot, dieback, and plant death caused by Phytophthora cinnamomi. Fresh chicken manure, or chicken manure composted for 5 weeks before incorporation into the potting mix (25%, vol/vol), significantly reduced pathogen survival and the development of symptoms on Lupinus albus seedlings. Chicken manure composted for 2 weeks was less suppressive. Cow, sheep, and horse manure, whether fresh or composted, did not consistently suppress populations of P. cinnamomi or disease symptoms at the rates used (25%, vol/vol). All composts increased organic matter content, total biological activity, and populations of actinomycetes, fluorescent pseudomonads, and fungi. Only chicken manure stimulated endospore-forming bacteria, a factor that was strongly associated with seedling survival. Fallowing the potting mix for an additional 8 weeks after the first harvest increased the survival of lupin seedlings in a second bioassay, with survival rates in chicken manure compost-amended potting mix exceeding 90%. These data suggest that the ability of composted manure to stimulate sustained biological activity, in particular the activity of endospore-forming bacteria, is the key factor in reducing disease symptoms caused by P. cinnamomi. Supporting these results, the survival of rooted cuttings of Thryptomene calycina was significantly higher in sand-peat potting mix following amendment with commercially available chicken manure (15% vol/vol). However, this protection was reduced if the potting mix was steam pasteurized before amendment, indicating that suppression was due to endogenous as well as introduced microbes. Chicken manure compost incorporated at 5% (vol/vol) or more was strongly phytotoxic to young Banksia spinulosa plants and is not suitable as an amendment for phosphorus-sensitive plants.

Survival of Ralstonia solanacearum Biovar 2, the Causative Agent of Potato Brown Rot, in Field and Microcosm Soils in Temperate Climates.

Abstract: After outbreaks of potato brown rot in three different fields in the Netherlands, the fate of the brown rot pathogen, Ralstonia solanacearum biovar 2, was monitored in soil by immunofluorescence colony staining (IFC) supported by R. solanacearum division-2 specific polymerase chain reaction. In selected areas of all fields, the R. solanacearum population densities were initially on the order 104 to 106 per g of topsoil. These population densities then declined progressively over time. In two fields, however, the pathogen persisted for periods of 10 to 12 months. The survival of a selected R. solanacearum biovar 2 isolate, strain 1609, in three soils, a loamy sand and two different silt loam soils, was further studied in soil microcosm experiments. The effects of temperature and soil moisture content were assessed. At 12 or 15 and 20°C, a gradual decline of the population densities was observed in all three soils, from the established 105 to 106 CFU g-1 of dry soil to significantly reduced levels,...

Synopsis on the taxonomy of the genus xanthomonas.

Abstract: The genus Xanthomonas exhibits a high phytopathogenic diversity in contrast to a phenotypic uniformity, which has hampered the genesis of a stable classification for a long time. In past decades, a large number of Xanthomonas strains have been characterized by a variety of phenotypic and genotypic methods in a multitude of studies. Extensive DNA hybridization studies and repetitive sequence-based polymerase chain reaction and amplified fragment length polymorphism genomic fingerprinting have clearly revealed the genomic diversity and relationships within the genus. A review of the current classification of the genus Xanthomonas based on the synopsis of these studies is given here.

Molecular Analyses of Colletotrichum Species from Almond and Other Fruits

Abstract: Isolates of Colletotrichum spp. from almond, avocado, and strawberry from Israel and isolates of the pink subpopulation from almond from the United States were characterized by various molecular methods and compared with morphological identification. Taxon-specific primer analysis grouped the avocado isolates within the species C. gloeosporioides and the U.S. almond and Israeli strawberry isolates within the species C. acutatum. However, the Israeli almond isolates, previously identified morphologically as C. gloeosporioides, reacted with C. acutatum-specific primers. Arbitrarily primed polymerase chain reaction and A+T-rich DNA analyses determined that each population from almond and strawberry was distinct and clonal. Sequence analysis of the complete internal transcribed spacer (ITS) region (ITS 1–5.8S–ITS 2) revealed a similarity of between 97.03 and 98.72% among almond isolates from Israel, C. acutatum almond isolates from the United States, and C. acutatum strawberry isolates from Israel. S...

Yield loss in chickpeas in relation to development of fusarium wilt epidemics.

Abstract: Navas-Cortes, J. A., Hau, B., and Jimenez-Diaz, R. M. 2000. Yield loss in chickpeas in relation to development of Fusarium wilt epidemics. Phytopathology 90:1269-1278. Development of 108 epidemics of Fusarium wilt of chickpea caused by Fusarium oxysporum f. sp. ciceris were studied on cvs. P-2245 and PV-61 in field microplots artificially infested with races 0 and 5 of F. oxysporum f. sp. ciceris in 1986 to 1989. Disease progression data were f itted to the Richards model using nonlinear regression. The shape parameter was influenced primarily by date of sowing and, to a lesser extent, by chickpea cultivars and races of F. oxysporum f. sp. ciceris. Fusarium wilt reduced chickpea yield by decreasing both seed yield and seed weight. These effects were related to sowing date, chickpea cultivar, and virulence of the prevalent F. oxysporum f. sp. ciceris race. Regression models were developed to relate chickpea yield to Fusarium wilt disease intensity with the following independent variables: time to initial symptoms ( tis), time to inflection point ( tip) of the disease intensity index ( DII) progress curve, final DII (DIIfinal), standardized area under DII progress curve (SAUDPC), and the Richards weighted mean absolute rate of disease progression ( rho). Irrespective of the chickpea cultivar ◊ pat hogen race combination, the absolute and relative seed yields decreased primarily by delayed sowing. The relative seed yield increased with the delay in tis and tip and decreased with increasing DIIfinal, SAUDPC, and rho. A response surface was developed in which seed yield loss decreased in a linear relationship with the delay in tis and increased exponentially with the increase of rho.

A theoretical assessment of the effects of vector-virus transmission mechanism on plant virus disease epidemics.

Abstract: Madden, L. V., Jeger, M. J., and van den Bosch, F. 2000. A theoretical assessment of the effects of vector-virus transmission mechanism on plant virus disease epidemics. Phytopathology 90:576-594. A continuous-time and deterministic model was used to characterize plant virus disease epidemics in relation to virus transmission mechanism and population dynamics of the insect vectors. The model can be wr itten as a set of linked differential equations for healthy (virus-free), latently infected, infectious, and removed (postinfectious) plant categories, and virus-free, latent, and infective insects, with parameters based on the transmission classes, vector population dynamics, immigration/emigration rates, and virus-plant interactions. The rate of change in diseased plants is a function of the density of infective insects, the number of plants visited per time, and the probability of transmitting the virus per plant visit. The rate of change in infective insects is a function of the density of infectious plants, the number of plants visited per time by an insect, and the probability of acquiring the virus per plant visit. Numerical solutions of the differential equations were used to determine transitional and steady-state levels of disease incidence ( d*); d* was also determined directly from the model parameters. Clear differences were found in disease development among the four transmission classes: nonpersistently transm itted (styletborne [NP]); semipersistently transmitted (foregut-borne [SP]); circulative, persistently transmitted (CP); and propagative, persistently transmitted (PP), with the highest disease incidence ( d) for the SP and CP classes relative to the others, especially at low insect density when there was no insect migration or when the vector status of emigrating insects was the same as that of immigrating ones. The PP and CP viruses were most affected by changes in vector longevity, rates of acquis ition, and inoculation of the virus by vectors, whereas the PP viruses were least affected by changes in insect mobility. When vector migration was explicitly considered, results depended on the fraction of infective insects in the immigration pool and the fraction of dying and emigrating vectors repl aced by immigrants. The PP and CP viruses were most sensitive to changes in these factors. Based on model parameters, the basic reproductive number (R0) number of new infected plants resulting from an infected plant introduced into a susceptible plant population was derived for some circumstances and used to determine the steady-state level of disease incidence and an approximate exponential rate of disease increase early in the epidemic. Results can be used to evaluate disease management strategies.

A Point Mutation in the FRNK Motif of the Potyvirus Helper Component-Protease Gene Alters Symptom Expression in Cucurbits and Elicits Protection Against the Severe Homologous Virus.

Abstract: Sequence comparison had previously shown three amino acid changes in conserved motifs in the 455-amino acid sequence of the helper component-protease (HC-Pro) between a severe field strain of Zucchini yellow mosaic virus (ZYMV-NAT) and a mild field strain of ZYMV (ZYMV-WK). In this study, exchange of fragments and site-directed mutagenesis within the HC-Pro gene in an infectious clone of ZYMV enabled the effects of the mutations on symptom expression to be mapped. The substitution of Ile for Arg at position 180 in the conserved motif Phe-Arg-Asn-Lys (FRNK) of potyviruses was found to affect symptom expression. Infection of cucurbits with the engineered ZYMV (ZYMV-AG) that contained this mutation caused a dramatic symptom change from severe to mild in squash and to a symptom-free appearance in cucumber, melon, and watermelon. The Ile to Arg mutation was found to be stable, and no revertant virus was found after several passages through plants after long incubation periods. The AG strain was detected 4 days...

Genetic diversity among Banana streak virus isolates from Australia

Abstract: Banana streak virus (BSV) is an important pathogen of bananas and plantains (Musa spp.) throughout the world. We have cloned and sequenced part of the genomes of four isolates of BSV from Australia, designated BSV-RD, BSV-Cav, BSV-Mys, and BSV-GF. These isolates originated from banana cvs. Red Dacca, Williams, Mysore, and Goldfinger, respectively. All clones contained a sequence covering part of open reading frame III and the intergenic region of the badnavirus genome. The sequences were compared with those of other badnaviruses, including BSV-Onne, a previously characterized isolate from Nigeria. The BSV-RD sequence was virtually identical to that of BSV-Onne, differing by only two nucleotides over 1,292 bp. However, BSV-Cav, -Mys, and -GF were divergent in nucleotide sequence. Phylogenetic analyses using conserved sequences in the ribonuclease H domain revealed that all BSV isolates were more closely related to each other than to any other badnavirus. BSV-Cav was most closely related to BSV-Onne, and there was 95.1% identity between the two amino acid sequences. Other relationships between the BSV isolates were less similar, with sequence identities ranging from 66.4 to 78.2%, which is a magnitude comparable to the distance between some of the recognized badnavirus species. Immunocapture-polymerase chain reaction assays have been developed, allowing specific detection and differentiation of the four isolates of BSV.

Effect of the Indigenous Microflora on the Development of Root and Crown Rot Caused by Pythium aphanidermatum in Cucumber Grown on Rockwool.

Abstract: The capacity of the microflora in rockwool to suppress Pythium aphanidermatum, the causative agent of root and crown rot in cucumber, was assessed. Disease development of cucumber (Cucumis sativus) grown on rockwool was evaluated in an "ebb-and-flood" system with a recirculating nutrient solution after inoculation with P. aphanidermatum. In five independent experiments from 1995 to 1998, 11 batches of used rockwool were tested. All batches without P. aphanidermatum problems in the preceding cucumber crop had significantly lower numbers of diseased plants in nonautoclaved than in autoclaved used rockwool; the disease incidence was reduced by 52 to 100%. Suppressiveness also was present in rockwool previously used to grow other vegetable crops. Rockwool originating from a cucumber crop that was severely attacked by Pythium resulted in a high disease incidence. Previously unused (new) rockwool had higher or similar percentages of diseased plants than did nonsterilized used rockwool. Disease suppression in used rockwool could also be measured in a smaller test system. In both systems, autoclaved rockwool became suppressive to Pythium after recolonization with the indigenous microflora. Population sizes of total culturable aerobic bacteria as well as of fluorescent pseudomonads did not correlate with disease suppressiveness, as numbers of bacteria and pseudomonads were similar or lower in nonautoclaved (suppressive) than in autoclaved (nonsuppressive) rockwool. Differences in the structure of the bacterial populations could be visualized by using eubacterial polymerase chain reaction (PCR) followed by denaturing gradient gel electrophoresis (DGGE). Interestingly, the nonautoclaved and the recolonized used rockwool, which were both suppressive, showed different dominating bacterial groups as compared with the autoclaved rockwool. PCR-DGGE patterns obtained at different sampling times showed that the composition of the bacterial populations changed during plant growth. Fungal populations were present in the treatments that yielded suppressive rockwool, i.e., the nonautoclaved and the recolonized rockwool, but they were absent or present in low numbers in the autoclaved rockwool, which permitted a high disease incidence. Suppressiveness of rockwool to Pythium root and crown rot is a hitherto undescribed phenomenon, and knowledge of the mechanism and microorganisms involved will stimulate the development of microbially balanced soilless growing systems.

Relationship Between Biological Control, Incidence of Hypovirulence, and Diversity of Vegetative Compatibility Types of Cryphonectria parasitica in France.

Abstract: In France, chestnut blight, caused by Cryphonectria parasitica, has been controlled since 1974 in orchards, but never in coppice forests, by releasing hypovirulent strains infected with CHV1 hypovirus. We tested the hypothesis that this biological control (BC) has lead to a decrease in blight severity, spread of hypovirulence, and change in C. parasitica populations. The low severity of chestnut blight was confirmed in the six regions studied (subdivided into zones). The remission of cankers was associated with the presence of white isolates presumed to be hypovirulent. These two parameters were also correlated, at the zonal level, to the frequency of sites where BC was used. However, the estimates of the natural background level of hypovirulence, independent of BC, ranged from 4% in forests in Dordogne to 60% in orchards in Lozere. Differences in the rate of hypovirulent isolates among regions were consistent with the diversity of vegetative compatibility (VC) types in populations of C. parasiti...

Genetics of host-pathogen relationships between Venturia inaequalis races 6 and 7 and Malus species.

Abstract: Resistance to scab originating from Malus floribunda clone 821 is the most widely form of resistance used in apple breeding programs A dominant gene, named Vf, was introgressed from this clone into recent cultivars, although the genetic determinants of the resistance of M floribunda 821 are apparently more complex than a single gene The appearance of new races overcoming the resistance of cultivars with the Vf gene, the parental clone, or both made it possible to undertake a genetic analysis of host-pathogen interactions The segregation of resistance in progenies of crosses from ‘Golden Delicious’ × M floribunda 821 and ‘Golden Delicious’ × ‘Idared’ into five strains of Venturia inaequalis—races 1 (strains 104, 1093, and 301), 6 (strain 302), and 7 (strain 1066)—demonstrated the existence of a second dominant gene in M floribunda 821 This gene, independent of Vf, was named Vfh because it seemed to induce a hypersensitive reaction The results obtained with strain 1066, virulent to M flori

Panama Disease: Cell Wall Reinforcement in Banana Roots in Response to Elicitors from Fusarium oxysporum f. sp. cubense Race Four

Abstract: The biochemical basis of tolerance in banana to Fusarium wilt, caused by the pathogen Fusarium oxysporum f. sp. cubense race four, was investigated. Tissue culture banana plants from tolerant cv. Goldfinger and susceptible cv. Williams were maintained in a hydroponic system and inoculated with conidial suspensions to evaluate the degree of tolerance to susceptibility between the two clones and to investigate the effectiveness of this technique as a potential tool for early screening for resistance in breeding programs. Similarly, defense responses were induced by treatment of the plants with an elicitor preparation from the mycelial cell walls of the pathogen. Differences in the induction of lignin and callose deposition, phenolics, and the enzymes involved in cell wall strengthening; phenylalanine ammonia lyase, cinnamyl alcohol dehydrogenase, peroxidase, and polyphenol oxidase were determined. Root tissue of the tolerant cv. Goldfinger responded to the fungal elicitor through the strong deposition of lignin, preceded by the induction or activation of the enzyme activities involved in the synthesis and polymerization thereof, whereas only slight increases were observed for the susceptible cv. Williams. No increase in callose content was observed for either clone. These results indicate an important role for cell wall strengthening due to the deposition of lignin as an inducible defense mechanism of banana roots against F. oxysporum f. sp. cubense race four.

The Role of Chitinase Production by Stenotrophomonas maltophilia Strain C3 in Biological Control of Bipolaris sorokiniana.

Abstract: The role of chitinase production by Stenotrophomonas maltophilia strain C3 in biological control of leaf spot on tall fescue (Festuca arundinacea), caused by Bipolaris sorokiniana, was investigated in vitro and in vivo. The filtrate of a broth culture of C3, with chitin as the carbon source, was separated into fractions. A high molecular-weight fraction (>8 kDa) was chitinolytic and more inhibitory than a low-molecular-weight, nonchitinolytic fraction to conidial germination and hyphal growth by B. sorokiniana and to leaf spot development. A protein fraction derived by ammonium sulfate precipitation and a chitinase fraction purified by chitin affinity chromatography also were chitinolytic and highly antifungal. The chitinolytic fractions caused swelling and vacuolation of conidia and discoloration, malformation, and degradation of germ tubes. When boiled, the chitinolytic fractions lost chitinase activity along with most of the antifungal properties. Two chitinase-deficient and two chitinase-reduced mutants of C3 were compared with the wild-type strain for inhibition of germination of B. sorokiniana conidia on tall fescue leaves and for suppression of leaf spot development in vivo. The mutants exhibited reduced antifungal activity and biocontrol efficacy, but did not lose all biocontrol activity. An aqueous extract of leaves colonized by wild-type C3 had higher chitinase activity than that of noncolonized leaves and was inhibitory to conidial germination. The addition of chitin to leaves along with the wild-type strain increased both chitinase and antifungal activity. The chitinase activity level of extracts from leaves colonized by a chitinase-deficient mutant of C3, with and without added chitin, was no higher than the background, and the extracts lacked antifungal activity. Chitinolysis appears to be one mechanism of biological control by strain C3, and it functions in concert with other mechanisms.

Effects of herbicides on Fusarium solani f.sp. glycines and development of sudden death syndrome in glyphosate-tolerant soybean.

Abstract: Sanogo, S., Yang, X. B., and Scherm, H. 2000. Effects of herbicides on Fusarium solani f. sp. glycines and development of sudden death syndrome in glyphosate-tolerant soybean. Phytopathology 90:57-66. Sudden death syndrome of soybean, caused by Fusarium solani f. sp. glycines, is a disease of increasing economic importance in the United States. Although the ecology of sudden death syndrome has been extensively studied in relation to crop management practices such as tillage, irrigation, and cultivar selection, there is no information on the effects of herbicides on this disease. Three herbicides (lactofen, glyphosate, and imazethapyr) commonly used in soybean were evaluated for their effects on the phenology of F. solani f. sp. glycines and the development of sudden death syndrome in four soybean cultivars varying in resistance to the disease and in tolerance to glyphosate. Conidial germination, mycelial growth, and sporulation in vitro were reduced by glyphosate and lactofen. In growth-chamber and greenhouse experiments, there was a significant increase in disease severity and frequency of isolation of F. solani f. sp. glycines from roots of all cultivars after application of imazethapyr or glyphosate compared with the control treatment (no herbicide applied). Conversely, disease severity and isolation frequency of F. solani f. sp. glycines decreased after application of lactofen. Across all herbicide treatments, severity of sudden death syndrome and isolation frequency were lower in disease-resistant than in susceptible cultivars. Results suggest that glyphosate-tolerant and -nontolerant cu ltivars respond similarly to infection by F. solani f. sp. glycines after herbicide application.

Enhanced Polymerase Chain Reaction Methods for Detecting and Quantifying Phytophthora infestans in Plants

Abstract: Sensitive and specific primer sets for polymerase chain reaction (PCR) for Phytophthora infestans, the oomycete that causes late blight of potato and tomato, were developed based on families of highly repeated DNA. The performance of these primers was compared to those developed previously for the internal transcribed spacer (ITS) of ribosomal DNA. The detection limit using the new primers is 10 fg of P. infestans DNA, or 0.02 nuclei. This is about 100 times more sensitive than ITS-directed primers. Nested polymerase chain reaction (PCR) allows the measurement of down to 0.1 fg of DNA using the new primers. To enhance the reliability of diagnostic assays, an internal positive control was developed using an amplification mimic. The mimic also served as a competitor for quantitative PCR, which was used to assess the growth of P. infestans in resistant and susceptible tomato. A key dimension of this study was that two laboratories independently checked the specificity and sensitivity of each primer ...

Transmission, characterization, and serology of a luteovirus associated with yellow leaf syndrome of sugarcane.

Abstract: A previously uncharacterized luteovirus was associated with one form of yellow leaf syndrome (YLS), a widespread disease of sugarcane (Saccharum sp.). The virus was named Sugarcane yellow leaf luteovirus (ScYLV), and was identified in major sugarcane-producing areas of the world. Typical disease symptoms were reproduced when ScYLV was transmitted by Melanaphis sacchari or Rhopalosiphum maidis from infected to healthy sugarcane, suggesting that this virus may be the causal agent of one form of YLS. The only known hosts of ScYLV are Saccharum and Erianthus spp. Virions of ScYLV were 24 to 29 nm in diameter in sodium phosphotungstate at pH 5.0, had a buoyant density of 1.30 g/cm3 in Cs2SO4, and contained a 5.8-kb genomic ssRNA. The capsid protein had an estimated relative molecular mass of 27 kDa and was not glycosylated. A polyclonal rabbit antiserum raised against ScYLV did not detect any of eight other luteoviruses by enzyme-linked immunosorbent assay or immunosorbent electron microscopy, but in ...

The Genetic Structure of Field Populations of Rhynchosporium secalis from Three Continents Suggests Moderate Gene Flow and Regular Recombination.

Abstract: Salamati, S., Zhan, J., Burdon, J. J., and McDonald, B. A. 2000. The genetic structure of field populations of Rhynchosporium secalis from three continents suggests moderate gene flow and regular recombination. Phytopathology 90:901-908. Restriction fragment length polymorphism (RFLP) markers were used to compare the genetic structure of field populations of Rhynchosporium secalis from barley. A total of 543 isolates representing 8 field populations were sampled from Australia, California, Finland, and Norway. Gene and genotype diversity were high in all populations. Nei’s average gene diversity across seven RFLP loci was 0.513. Hierarchical gene diversity analysis showed that 9% of the total genetic variability was distributed among continents, 4% was distributed among fields within continents, and 13% was distributed among collection stations within a field. The majority (74%) of genetic variability was distributed within collection areas of approximately 1 m 2 within fields. Gene flow appears to be significant on a regional scale but more restricted among continents. Allele frequencies were significantly different at several RFLP loci. Genetic distances were small among populations within regions and large between regions. Pairwise comparisons of genotype diversity in the populations revealed significant differences among populations that were related mainly to differences in sampling strategies. Isolates from Norway and Finland showed a lower copy hybridization pattern with probe pRS26. This probe functioned as a fingerprint probe for the California and Australian isolates. Seven out of the eight populations studied were at gametic equilibrium for RFLP loci, s uggesting that R. secalis populations in Norway, Finland, and Australia undergo regular recombination, although a teleomorph has not yet been recognized.

Mapping of quantitative trait loci for fusarium head blight resistance in barley.

Abstract: Ma, Z., Steffenson, B. J., Prom, L. K., and Lapitan, N. L. V. 2000. Mapping of quantitative trait loci for Fusarium head blight resistance in barley. Phytopathology 90:1079-1088. Fusarium head blight (FHB) is a devastating disease that causes significant reductions in yield and quality in wheat and barley. Barley grains infected with deoxynivalenol (DON), a vomitoxin produced by Fusarium graminearum, are rejected for malting and brewing. Among six-rowed barley cultivars tested thus far, only cv. Chevron exhibited resistance. This study was conducted to map genes and to identify DNA markers for marker-assisted breeding for FHB resistance in cv. Chevron with restriction fragment length polymorphism (RFLP) markers. A doubled haploid (DH) population was created from a cross between cv. Chevron and susceptible cv. Stander. Seven field experiments were conducted in four different locations in 2 years. A RFLP map containing 211 loci and covering over 1,000 centimorgans (cM) of the genome was used to map quantitative trait loci (QTL) associated with relatively low FHB severity and DON concentration. Morphological traits differing between the parents were also measured: heading date, plant height, spike angle, number of nodes per cm of rachis in the spike, and kernel plumpness. Many of the QTL for FHB and DON coincided with QTLs for these morphological traits. The “fix-QTL” algorithm in Mapmaker QTL was used to remove the part of the variance for FHB resistance that may be explained by heading date or plant height. Results from this study suggest that QTLs with major effects for FHB resistance probably do not exist in cv. Chevron. Three QTL intervals, Xcmwg706-Xbcd441 on chromosome 1H, Xbcd307b-Xcdo684b on chromosome 2H, and Xcdo959b-Xabg472 on chromosome 4H, that are not associated with late heading or height may be useful for markerassisted selection.