Showing papers in "Plant Disease in 2000"

Detection of Virulence to Wheat Stem Rust Resistance Gene Sr31 in Puccinia graminis. f. sp. tritici in Uganda.

Abstract: In much of the world, resistance to stem rust in wheat, caused by Puccinia graminis f. sp. tritici, is based at least in part on the gene Sr31. During February 1999, high levels of stem rust infection were observed on entries in wheat (Triticum aestivum) grown in a nursery at Kalengyere Research Station in Uganda. Because several of the rusted entries were known to carry the 1BL-1RS chromosome translocation containing the Sr31, Lr26, and Yr9 genes for rust resistance, virulence to Sr31 was suspected. Urediniospores, collected in bulk from rusted stems of seven entries containing Sr31, were suspended in light mineral oil and sprayed on primary leaves of 7-day-old seedlings of South African wheat cv. Gamtoos (=Veery #3, pedigree: Kvz/Buho‘S’//Kal/BB). Plants were kept overnight at 19 to 21°C in a dew chamber before placement in a greenhouse at 18 to 25°C. After ≈14 days, urediniospores were collected from large, susceptible-type stem rust pustules and subsequently increased on Gamtoos, which served as a sel...

The effect of previous crop residues and tillage on fusarium head blight of wheat

Abstract: Effects of previous crop residues and tillage practices on Fusarium head blight (FHB) of wheat were examined. Fusarium head blight was monitored in plots of the FHB-susceptible spring wheat cultivar Norm following crops of corn, wheat, and soybeans in 1995, 1996, and 1997. Moldboard plow, chisel plow, and no-till treatments were imposed perpendicular to crop strips to establish a range of residue levels in each of the previous crop residues. Fusarium head blight incidence and severity were greatest when wheat followed corn and least when wheat followed soybeans. Incidence and severity were lower in moldboard plowed plots than in either chisel plowed or no-till plots, although differences among chisel plow and no-till treatments were not apparent. Yields of wheat were approximately 15% lower in plots where wheat followed corn or wheat than in wheat following soybeans and were 10% greater in moldboard plowed plots than in either chisel plowed or no-till treatments. The deoxynivalenol (DON) content of harvested grain was significantly correlated with FHB incidence and severity. The DON level in wheat following soybeans, averaged across tillage treatments, was 25% lower than in wheat following wheat and 50% of the level in wheat following corn. These findings suggest that changes in regional tillage practices, principally the move toward conservation tillage and reduced-till systems, contributed to the recent FHB epidemics in the Upper Midwest. Because differences in the type and quantity of crop residues in small plots affected disease development, it is likely that local sources of inoculum, such as those within a grower's field, contribute directly to the inoculum load and disease potential. The implication of these findings is that selection of cultural practices aimed to reduce inoculum-borne residues will assist in the control of FHB.

Rice Pest Constraints in Tropical Asia: Quantification of Yield Losses Due to Rice Pests in a Range of Production Situations

Abstract: A series of experiments was conducted where a range of injuries due to rice pests (pathogens, insects, and weeds) was manipulated simultaneously with a range of production factors (fertilizer input, water supply, crop establishment method, variety) in different seasons and years. These factors were chosen to represent lowland rice production situations characterized in surveys conducted in tropical Asia and their corresponding range of attainable yield. Experiments complemented one another in exploring the response surface of rice yields to yield-limiting and yield-reducing factors. The resulting experimental data base consisted of 445 individual plots and involved 11 manipulated injuries in a range of attainable yields of 2 to 11 t ha-1. A first, nonparametric, multivariate analysis led to a hierarchy of potential injuries, from marginally (e.g., bacterial leaf blight) to extremely harmful (e.g., rice tungro disease). A second, parametric, multivariate approach resulted in a multiple regression ...

Plant growth-promoting rhizobacterial mediated protection in tomato against Tomato mottle virus

Abstract: Tomato plants treated with plant growth-promoting rhizobacteria (PGPR), applied as an industrially formulated seed treatment, a spore preparation mixed with potting medium (referred to as powder), or a combined seed-powder treatment, were evaluated under field conditions for induced resistance to Tomato mottle virus (ToMoV). The PGPR strains used, based on their ability to induce resistance in previous experiments, included Bacillus amyloliquefaciens 937a, B. subtilis 937b, and B. pumilus SE34. Experiments were conducted in the fall of 1997 and the spring and fall of 1998 at the University of Florida's Gulf Coast Research & Education Center, Bradenton. All plants were rated for symptoms and analyzed for the presence of ToMoV DNA at 40 days after transplant (dat). Whitefly densities were determined on individual plants in each trial, and marketable fruit yields were determined at least two times during each trial. The highest level of protection occurred in the fall 1997 trial when, at 40 dat, ToMoV disease severity ratings were significantly less in all PGPR powder-based treatments than in either of the seed or control treatments. Detection of viral DNA using Southern dot blot analyses correlated with symptom severity ratings, as did fruit yields. A reduction in ToMoV symptom severity ratings and incidence of viral DNA were also observed for some PGPR treatments in the spring 1998 trial, although corresponding yield responses were not apparent. Little or no resistance was observed in the fall 1998 trial. No differences in disease severity, detection of ToMoV DNA, or yield occurred among treatments in any of the trials at 80 dat. These data show that up to 40 dat under natural conditions of high levels of vector-virus pressure, some PGPR treatments resulted in reduced ToMoV incidence and disease severity and, in some cases, a corresponding increase in fruit yield. The use of PGPR could become a component of an integrated program for management of this virus in tomato.

Effect of Botanical Extracts on the Population Density of Fusarium oxysporum in Soil and Control of Fusarium Wilt in the Greenhouse.

Abstract: Several commercial formulations of botanical extracts and essential oils are being investigated as possible alternatives to soil fumigation for control of Fusarium wilt diseases. Soil infested with Fusarium oxysporum f. sp. chrysanthemi was treated with 1, 5, and 10% aqueous emulsions of formulated extracts of clove (70% clove oil), neem (90% neem oil), pepper/mustard (chili pepper extract and essential oil of mustard), cassia (extract of cassia tree), and Banrot (a standard fungicide applied at different labeled rates) in separate experiments. Population densities of F. oxysporum f. sp. chrysanthemi were determined at 0 (before treatment), 1, 3, 7, 14, and 21 days after treatment. Treatment of the soil with 5 and 10% aqueous emulsions resulted in significant (P < 0.05) differences among treatment means at each assay date. After 3 days, pepper/mustard, cassia, and clove extracts added as 10% aqueous emulsions reduced the population density of F. oxysporum f. sp. chrysanthemi 99.9, 96.1, and 97.5%, respectively, compared with the untreated control. Neem oil extract increased the population density of F. oxysporum f. sp. chrysanthemi at all concentrations tested. Banrot did not reduce the population density of F. oxysporum f. sp. chrysanthemi in any experiment. In a second, related experiment, soil infested with Fusarium oxysporum f. sp. melonis also was treated with 1, 5, and 10% aqueous emulsions of formulated extracts, incubated in closed plastic bags for 1 week, and planted with muskmelon seeds (cv. Gold Star) in the greenhouse. Treatment of infested soil with 5 and 10% aqueous emulsions of the botanical extracts resulted in differences among treatments after 5 to 6 weeks. The pepper/mustard, cassia, and clove extracts suppressed disease development in repeated experiments (80 to 100% healthy plant stand) compared with the untreated infested soil (<20% stand). The observed reduction in the pathogen population and increased healthy plant stand in the greenhouse indicates that these extracts could have important roles in biologically based management strategies for control of Fusarium wilt diseases.

Postharvest Biological Control of Rhizopus Rot of Nectarine Fruits by Pichia membranefaciens.

Abstract: A new yeast antagonist, Pichia membranefaciens, isolated from wounds of peach fruit, was evaluated for its biocontrol capability against Rhizopus stolonifer on nectarine fruits at different temperatures and with other treatments. P. membranefaciens at 5 × 108 CFU/ml of washed-cell suspension completely inhibited Rhizopus rot in nectarine wounds artificially inoculated with 5 × 104 spores per ml at 25, 15, and 3°C. A culture filtrate of the yeast antagonist failed to provide any protection against Rhizopus rot in nectarine fruits. The yeast mixed with iprodione at 100 μg a.i./ml gave better control of R. stolonifer than either yeast or iprodione alone. A solution of 20 g CaCl2 per liter enhanced the efficacy of P. membranefaciens (107 to 108 CFU/ml) as an aqueous suspension. Rapid colonization of the yeast in wounds was observed during the first 48 h at 25 and 15°C. P. membranefaciens at 5 × 108 CFU/ml was effective when applied 0 to 72 h before the pathogen, while at 1 × 108 CFU/ml, its efficacy was best when applied 24 to 48 h prior to inoculation with R. stolonifer. However, its efficacy was significantly reduced when the yeast was applied simultaneously with the pathogen, with disease incidence of 60% and lesion diameter of 37 mm.

Factors Affecting the Transmission and Spread of Sugarcane yellow leaf virus.

Abstract: Sugarcane, Saccharum spp hybrid, is widely infected in the United States and many other countries with a yellowing and stunting disease called sugarcane yellow leaf syndrome The causal agent, Sugarcane yellow leaf virus (ScYLV), is a Polerovirus of the Luteoviridae family In this study, it was transmitted by the sugarcane aphid, Melanaphis sacchari, and also by the corn leaf aphid, Rhopalosiphum maidis, and the rice root aphid, R rufiabdominalis Two other aphids that infest sugarcane in Hawaii did not transmit the virus Some Hawaiian sugarcane cultivars are susceptible to ScYLV, while others remain virus-free in the field The latter were not infected when inoculated with viruliferous M sacchari Virus-free plants of susceptible cultivars were produced through apical meristem culture and were readily reinfected by viruliferous M sacchari They were also quickly reinfected when planted in a field in proximity to other infected sugarcane naturally infested with M sacchari Sugarcane cultivars are hybrids of several Saccharum species In a field-grown collection of Saccharum and related species, 11 to 71% of the clones of four of the species were infected with ScYLV None of the related genus Erianthus plants were infected, but four clones were infected experimentally by aphid inoculation A low to moderate percentage of corn, rice, and sorghum seedlings became infected when inoculated with ScYLV, but barley, oats, and wheat proved to be very susceptible None of seven weeds common in sugarcane fields were infected with ScYLV

Rice Pest Constraints in Tropical Asia: Characterization of Injury Profiles in Relation to Production Situations.

Abstract: A protocol for characterizing patterns of rice cropping practices and injuries due to pathogens, insects, and weeds was developed and used in six sites in tropical Asia covering a wide range of environments where lowland rice is cultivated. The data collected in a total population of 456 individual farmers' fields were combined to site-specific weather data and analyzed using non-parametric multivariate techniques: cluster analyses with chi-square distance and correspondence analyses. The main results are: (i) patterns of cropping practices that are common across sites can be identified; (ii) conversely, injury profiles that are common across sites can be determined; (iii) patterns of cropping practices and injury profiles are strongly associated at the regional scale; (iv) weather patterns are strongly associated with patterns of cropping practices and injury profiles; (v) patterns of cropping practices and injury profiles allow for a good description of the variation in actual yield; and (vi) p...

Incidence and Impact of Swiss Needle Cast in Forest Plantations of Douglas-fir in Coastal Oregon

Abstract: An epidemic of Swiss needle cast, caused by the ascomycete Phaeocryptopus gaeumannii, is causing defoliation and growth reductions in Douglas-fir forest plantations along the Oregon Coast. The area of symptomatic plantations has been monitored annually since 1996 by aerial survey; in spring 1999, 119,500 ha were affected. Pathogen and symptom development have also been monitored on nine permanent plots in stands of differing disease severity. Infection levels and symptom severity are greatest in low elevation plantations close to the coast. In areas of severe disease, trees retain only current year needles. Defoliation is proportional to the number of stomata occluded by pseudothecia of the fungus, with needles being shed when about 50% of stomata are occupied, regardless of needle age. Fungus sporulation and premature needle abscission are greatest on the upper branches of trees. Annual application of fungicides increases needle retention significantly. Tree height and diameter growth and total tree volume are reduced by disease, and tree volume is significantly correlated with needle retention on our plot trees. The epidemic continues to be most severe in Douglas-fir plantations established on sites where Sitka spruce and western hemlock or red alder predominated in earlier times.

Biocontrol of Cucumber Diseases in the Field by Plant Growth-Promoting Rhizobacteria With and Without Methyl Bromide Fumigation.

Abstract: Field trials were conducted in 1996 and 1997 to determine the effect of plant growth-promoting rhizobacteria (PGPR) strains, which previously were found to induce systemic resistance in cucumber, on cucumber plant growth and on naturally occurring cucumber diseases with and without methyl bromide fumigation. Seven PGPR seed treatments included single-strain treatments and mixtures of Bacillus pumilus strain INR7, Curtobacterium flaccumfaciens strain ME1, and Bacillus subtilis strain GB03. In both years, in the absence of methyl bromide, all seven PGPR treatments significantly promoted plant growth, compared to the non-treated control, while with methyl bromide fumigation, only 3 and 1 of the same PGPR treatments promoted growth significantly in 1996 and 1997, respectively. In 1996, main runner length of plants in all seven PGPR treatments without fumigation was statistically equivalent to the main runner length of the nontreated control with methyl bromide fumigation. Naturally occurring foliar d...

Biological Control of Cyclamen Soilborne Diseases by Serratia marcescens Strain B2.

Abstract: Cyclamen plants were treated with a highly chitinolytic bacterium, Serratia marcescens strain B2, and then challenge inoculated with Rhizoctonia solani sclerotia or Fusarium oxysporum f. sp. cyclaminis conidia. The bacterium suppressed these fungal diseases of cyclamen plants, especially the damping off caused by R. solani, in a greenhouse. Strain B2 survived at approximately 106 to 107 CFU/g in soil for 4 months after the initial application under greenhouse conditions. Chitinolytic enzymes and antifungal low-molecular-weight compounds were present in filtrates of S. marcescens B2, which suppressed germination of R. solani sclerotia in vitro.

Implications of Sexual Reproduction for Phytophthora infestans in the United States: Generation of an Aggressive Lineage.

Abstract: Phytophthora infestans isolates (n = 26) collected in the Columbia Basin of Oregon and Washington in 1993, which had been characterized previously for mating type, metalaxyl sensitivity, and alleles at the glucose-6-phosphate isomerase locus, were analyzed for nuclear restriction fragment length polymorphism (RFLP) bands detected by probe RG57 and mitochondrial haplotype. Analyses involving the larger set of markers indicated that this group of isolates satisfied expectations of a sexual progeny: they contained much greater genetic diversity than has been reported for most other epidemic populations of P. infestans in the United States and Canada (16 unique multilocus genotypes); both mating types were present in proximity; all possible combinations of alleles occurred at many pairs of polymorphic loci; and two distinct mitochondrial haplotypes were distributed among the isolates. An in vitro laboratory cross involving the putative parents (US-6 and US-7) as parental strains produced progeny with the same general characteristics as the field isolates. Among the field progeny were two genotypes, US-11 and US-16, that had been described previously but from subsequent and largely clonal collections. Isolates obtained from tomatoes (n = 40) and potatoes (n = 7) in 24 counties in California in 1998 were analyzed as described above, and all except one US-8 isolate from potatoes were of the US-11 clonal lineage, consistent with the hypothesis that the US-11 lineage is an especially fit clonal lineage that has survived over time and can dominate pathogen populations over a large area. We conclude that the 1993 Columbia Basin collection represents a sexual progeny that generated the US-11 lineage, and that this lineage is particularly fit when tomatoes are part of the agroecosystem.

First Report of Pepino Mosaic Virus on Tomato

Abstract: Early in 1999 a new viral disease occurred in protected tomato (Lycopersicon esculentum) crops in the Netherlands. Infected plants showed yellow leaf spots and mosaic. Transmission electron microscopic analysis revealed particles typical of potexviruses. Only three potexviruses have been reported to infect solanaceous crops: Pepino mosaic virus (PepMV), Potato aucuba mosaic virus (PAMV), and Potato virus X (PVX). Inoculation of test plants and serological tests showed that the new virus clearly differed from PAMV and PVX. Immuno-electron microscopy with antiserum to PepMV (1), the original PepMV isolate, and the virus from tomato showed decoration titers of 1:800 (homologous) and 1:400, respectively. Neither virus reacted with antiserum to PVX, nor did PVX react with antiserum to PepMV. Results of host plant analysis with 17 plant species mostly resembled those expected for PepMV. Nucleotide sequence alignment of DNA fragments obtained by reverse-transcriptase polymerase chain reaction with a specific primer set for potexviruses, directed against the RNA polymerase region, showed 93% identity between PepMV and the virus from tomato, while homologies with PVX, PAMV, and other potexviruses were <60%. Results indicate that the potexvirus in tomato is PepMV. PepMV was first found in pepino (Solanum muricatum) in Peru in 1974 and described by Jones et al. in 1980 (1). This is the first report of a natural infection of tomato by PepMV. Reference: (1) R. Jones et al. Ann. Appl. Biol. 94:61, 1980.

Detection of Acidovorax avenae subsp. citrulli in Watermelon Seed Using Immunomagnetic Separation and the Polymerase Chain Reaction

Abstract: An immunomagnetic separation and polymerase chain reaction (IMS-PCR)-based assay was developed for detecting Acidovorax avenae subsp. citrulli in watermelon seed. IMS yielded a 10-fold increase in recovery of A. avenae subsp. citrulli over direct spread-plating on King's Medium B; however, the presence of seed debris reduced IMS efficiency. Synthetic oligonucleotide primers were designed based on the 16S rRNA gene of a known A. avenae subsp. citrulli strain and tested for specific DNA amplification by PCR. The primers amplified DNA from all A. avenae subsp. citrulli strains tested but also yielded amplicons with several closely related bacteria. IMS-PCR resulted in a 100-fold increase in A. avenae subsp. citrulli detection sensitivity over direct PCR and was unaffected by PCR inhibitors in watermelon seed. The threshold of A. avenae subsp. citrulli detection for IMS-PCR was 10 CFU/ml in watermelon seed wash, and seedlots with 0.1% infestation were consistently detected. IMS-PCR represents an efficient and sensitive approach to detecting A. avenae subsp. citrulli in watermelon seedlots.

Survival and Growth of Listeria monocytogenes on Fresh-Cut Apple Slices and Its Interaction with Glomerella cingulata and Penicillium expansum

Abstract: The food-borne human pathogen Listeria monocytogenes survived and its populations increased on cv. Delicious apple slices at 10 or 20°C in air or controlled atmosphere of 0.5% O2 and 15% CO2, but did not grow at 5°C. Controlled atmosphere had no significant effect on the survival or growth of L. monocytogenes. The pathogen populations declined over time when grown in various concentrations of apple juice and the decline was greater as the concentration of the juice decreased. Populations of L. monocytogenes inoculated into decayed apple tissue continually increased on fruit decayed by Glomerella cingulata but did not survive after 5 days on fruit decayed by Penicillium expansum. The pH of the decayed area declined from pH 4.7 to 3.7 in the case of P. expansum, but in the case of G. cingulata the pH increased from pH 4.7 to 7.0. This pH modification may be responsible for affecting the growth of the food-borne pathogen. Storage temperature, as well as the absence of postharvest pathogens such as G. cingulata, is important for maintaining the safety of fresh-cut apples.

Impact of Azoxystrobin, Dimethomorph, Fluazinam, Fosetyl-Al, and Metalaxyl on Growth, Sporulation, and Zoospore Cyst Germination of Three Phytophthora spp.

Abstract: In vitro activity of azoxystrobin, dimethomorph, and fluazinam on growth, sporulation, and zoospore cyst germination of Phytophthora capsici, P. citrophthora, and P. parasitica was compared to that of fosetyl-Al and metalaxyl. The 50% effective concentration (EC50) values for)inhibition of mycelial growth of the three pathogens usually were lowest for dimethomorph and (metalaxyl, ranging from 3,000 μg/ml. Reduction of sporangium formation by P. capsici, P. citrophthora, and P. parasitica in the presence of dimethomorph at 1 μg/ml was significantly greater than that recorded for the same concentration of azoxystrobin, fluazinam, and fosetyl-Al. For the three species of Phytophthora, zoospore motility was most sensitive to fluazinam (EC50 and EC90 values of 1,000 μg/ml, respectively) and metalaxyl (EC50 and EC90 from 32 to 280 and 49 to 529 μg/ml, respectively), and lowest in sensitivity to azoxystrobin and fosetyl-Al (EC50 and EC90 from 256 to >1,000 μg/ml). The activity of azoxystrobin, dimethomorph, and fluazinam on one or more stages of the life cycle of P. capsici, P. citrophthora, and P. parasitica suggests that these compounds potentially could provide Phytophthora spp. disease control comparable to that of the established fungicides fosetyl-Al and metalaxyl.

Effect of nitrogen fertilization on disease progress of rice blast on susceptible and resistant cultivars

Abstract: The effects of three nitrogen fertilization treatments on the development of rice blast were studied on eight cultivars under field conditions in Arkansas in 1995 and 1996. The eight cultivars (Kaybonnet, Cypress, Lacassine, Mars, Adair, Alan, Newbonnet, and RT7015) ranged from resistant to susceptible to blast according to previous field observations. The recommended nitrogen levels for the eight cultivars varied from 123 to 168 kg/ha/year. Three treatments, consisting of different rates and timing of nitrogen applications, were tested over 2 years at one location. The first treatment consisted of a single nitrogen (N) application applied to plots at the recommended rate at preflood during the midtillering stage. The second treatment consisted of applying nitrogen as a single preflood application but at 1.5 times the recommended N rate used in treatment one. The third treatment (control) consisted of applying the recommended amount of nitrogen fertilizer used in treatment one, but in a three-way-split application with 56 to 100 kg/ha (depending on the cultivar) of urea applied at preflood followed by the application of 34 kg/ha of N applied approximately 10 and 20 days after the panicle differentiation (PD) growth stage. Inoculated spreader plots were used to initiate rice blast epidemics in the test plots. The results indicate that the disease progress for rice blast, regardless of N treatments, followed a unimodal curve, whereby disease incidence and total lesion area per plant reached a maximum near midseason (PD growth stage) and then gradually declined. This decline in disease was attributed to adult resistance, leaf senescence, and the formation of new (noninfected) leaves. Application of nitrogen above the recommended rate for any given cultivar significantly increased disease incidence and total lesion area per plant on all cultivars except Kaybonnet, a highly resistant cultivar. Furthermore, a differential cultivar response to nitrogen was observed when measuring both disease incidence and total lesion area per plant. Leaf blast was significantly more severe on the susceptible and very susceptible cultivars when N fertilizer was applied as a single application at preflood than in the split application treatment. Nitrogen treatments did not significantly affect the incidence of collar rot or neck blast.

Application of Candida saitoana and Glycolchitosan for the Control of Postharvest Diseases of Apple and Citrus Fruit Under Semi-Commercial Conditions.

Abstract: The efficacy of the combination of Candida saitoana with 0.2% glycolchitosan (the bioactive coating) as a biocontrol treatment of postharvest diseases of apple and citrus fruit was evaluated in tests with natural inoculations that simulated commercial packinghouse conditions. The growth of C. saitoana in apple wounds and on fruit surfaces was not affected by glycolchitosan. The bioactive coating was more effective in controlling decay of several cultivars of apples (Red Delicious, Rome, Golden Delicious, and Empire) than either C. saitoana or 0.2% glycolchitosan alone. Depending on the apple cultivar used, the bioactive coating was comparable or superior to thiabendazole in reducing decay. The bioactive coating was also superior to C. saitoana in controlling decay of oranges (cvs. Washington navel, Valencia, Pineapple, and Hamlin) and cv. Eureka lemons, and the control level was equivalent to that with imazalil. The bioactive coating and imazalil treatments offered consistent control of decay on Washington navel oranges and Eureka lemons in early and late seasons, while C. saitoana or 0.2% glycolchitosan were most effective on early-season fruit. The combination of C. saitoana with 0.2% glycolchitosan also reduced the incidence of stem-end rot of cv. Valencia oranges, but control was less effective than treatment with imazalil.

Assessments of Fusarium head blight of wheat and barley in response to fungicide treatment.

Abstract: Benomyl and tebuconazole reduced head blight incidence, head blight severity, and the percentage of visually scabby kernels (VSK) in harvested grain when applied to the susceptible spring wheat cv. Norm at anthesis. Deoxynivalenol (DON) concentration was also reduced. Fungicide treatment increased head weight, test weight, thousand kernel weight, and yield. Fludioxonil reduced certain parameters associated with the disease in barley, including incidence, severity, and deoxynivalenol concentration, while increasing the percentage of plump kernels and yield. In vitro, isolates of Fusarium graminearum were sensitive to fludioxonil, benomyl, tebuconazole, and mancozeb (concentrations that give 50% growth inhibition of 7.4 × 10-3, 3.5 × 10-1,9.9 × 10-1, and 7.3 mg a.i./liter, respectively). No evidence for insensitivity (resistance) was found. Prospects for chemical control of Fusarium head blight (FHB), however, remain limited. The repeatability of assessment methods used in evaluating host response to fungicides and to the damage caused by F. graminearum is discussed. Disease incidence, disease severity, VSK, and DON were identified as key variables that best measure the effects of FHB.

Improved Control of Apple and Citrus Fruit Decay with a Combination of Candida saitoana and 2-Deoxy-D-Glucose.

Abstract: A combination of Candida saitoana with 0.2% 2-deoxy-D-glucose to control decay of apple, lemon, and orange fruit was evaluated. Growth of C. saitoana in vitro was reduced by 2-deoxy-D-glucose; however, in apple wounds, the yeast grew as well in the presence of 2-deoxy-D-glucose as in its absence. When applied to fruit wounds before inoculation, the combination of C. saitoana with 0.2% 2-deoxy-D-glucose was more effective in controlling decay of apple, orange, and lemon caused by Botrytis cinerea, Penicillium expansum, and P. digitatum than either C. saitoana or the application of a 0.2% solution of 2-deoxy-D-glucose alone. Increasing the concentration of 2-deoxy-D-glucose from 0.2 to 0.5% did not improve control significantly. The combination of C. saitoana with 0.2% 2-deoxy-D-glucose was also effective against infections established up to 24 h before treatment. When applied within 24 h after inoculation, the combination of C. saitoana with 0.2% 2-deoxy-D-glucose was very effective in controlling...

Toward Integrated Management of Monosporascus Wilt of Melons in Israel.

Abstract: The phenomenon of melon wilting dueto Monosporascus infection (Fig. 1) isknown in many regions around the worldand has been referred to as melon collapse(13,27,32,38), sudden wilt (2,6,8,26,29),root rot (17,39), vine decline (1,3,36), androot rot and vine decline (21–23,41,42).This disease is known also in the ’AravaRift Valley of southern Israel (8,19,26,31;Fig. 2), and the major causal agent isMonosporascus cannonballus Pollack &Uecker (Fig. 3). This pathogen is commonin hot, semiarid melon-growing areas ofIndia (22), southern Spain (13), southwest-ern regions of the United States (23,24,36),Saudi Arabia (15), Central America (1),Japan (40), Taiwan (37), and Tunisia (21).This disease in the ’Arava can be verysevere, capable of destroying the entirecrop (26), and will be referred to here asMonosporascus wilt. To date, disease man-agement in the ’Arava (38) has beenmainly based on methyl bromide fumiga-tion of the soil prior to planting. Sincemethyl bromide use will be prohibited inthe near future (33), there is an urgent needto develop alternative strategies for diseasemanagement.Melon root rot and vine decline causedby M. cannonballus has been reviewed in afeature article by Martyn and Miller (22),which describes the biology, pathology,and epidemiology of the disease, as well asmolecular methods for detecting variationin the pathogen population. In this article,we discuss approaches for the control ofMonosporascus wilt, with an emphasis onthe potential for integrated management, inview of the coming phaseout of methylbromide. These approaches include breed-ing for resistance, grafting melon plantsonto resistant Cucurbita and melon root-stocks, changes in irrigation schemes, im-proved soil solarization, chemical controlwith fungicides, and the use of other fumi-gants, alone or combined with soil solari-zation, to improve disease control.In field trials conducted by Reuveni andKrikun in the Jordan Valley and southern’Arava region in Israel (Fig. 2) in the early1980s, it was shown that Monosporascuseutypoides (apparently synonymous withM. cannonballus) is the primary agent ofmelon collapse (19,31). Pathogenicity testsperformed in 1995 and 1996 suggested thatthe most virulent species involved in themelon collapse syndrome in the ’Arava(Fig. 2) is M. cannonballus, although otherpathogens might also be involved (26).Monosporascus appears to be adapted tohot climates. This can be inferred from theclimatic conditions in the areas in whichthe fungus has been found and by itsgrowth temperature optimum. Vegetativemycelial growth is extensive in the rangeof 25 to 35°C, and perithecia formation invitro is optimal at 25 to 30°C (22).In commercial fields in Israel, the meloncrop can be totally destroyed by Monospo-rascus wilt in the autumn cropping season,whereas disease incidence and severity in acrop raised in the same plot during thefollowing winter–spring season can bemuch lower (2). Differences in soil tem-perature between crop seasons have beensuggested as a possible cause for such aphenomenon (17,28). This idea has beensupported by enhanced wilting obtainedfollowing artificial heating of the soil dur-ing the winter–spring crop season (28).Soil fumigation with methyl bromidebefore planting is the most common ap-proach for controlling Monosporascus wiltof melons in Israel. Methyl bromide haslethal exposure periods as short as 2 daysand can be applied at relatively low tem-peratures. The aeration period to eliminatevolatile residues before planting is short inmost soils, 3 to 10 days, allowing plantingshortly after treatment (18). In fact, meloncultivation in the ’Arava region is ex-tremely risky without methyl bromidefumigation prior to planting, due to theubiquity of M. cannonballus in ’Aravasoils that results in severe yield losses. Thephaseout of methyl bromide in developed

First Report of Plum pox virus (Sharka Disease) in Prunus persica in the United States.

Abstract: Plum pox (Sharka) is the most important virus disease of Prunus in Europe and the Mediterranean region and is caused by Plum pox potyvirus (PPV). In September 1999, PPV-like symptoms were observed in peach fruit culls in a packinghouse in Pennsylvania. All symptomatic fruit originated from a single block of peach (P. persica cv. Encore) in Adams County. Trees in the block exhibited ring pattern symptoms on their leaves. A potyvirus was detected in symptomatic fruit using the Poty-Group enzyme-linked immunosorbent assay (ELISA) test from Agdia (Elkhart, IN). Reactions for symptomatic peach fruit and leaves also were positive using triple-antibody sandwich ELISA with the PPV polyclonal antibody from Bioreba (Carrboro, NC) for coating, the Poty-Group monoclonal antibody (MAb; Agdia) as the intermediate antibody, and double-antibody sandwich ELISA with PPV detection kits from Sanofi (Sanofi Diagnostics Pasteur, Marnes-La-Coquette, France) and Agdia and the REAL PPV kit (Durviz, Valencia, Spain) containing universal (5B) and strain typing (4DG5 and AL) PPV MAbs (1). PPV also was identified by immunocapture-reverse transcription-polymerase chain reaction (IC-RT-PCR) amplification and subsequent sequencing of the 220-bp 3' noncoding region (2) (>99% sequence homology to PPV) and by IC-RT-PCR amplification of a 243-bp product in the coat protein (CP) gene (1). The virus was identified as PPV strain D based on serological typing with strainspecific MAbs and on PCR-restriction fragment length polymorphism of the CP IC-RT-PCR product with Rsa1 and Alu1 (1). This is the first report of PPV in North America. References: (1) T. Candresse et al. Phytopathology 88:198, 1998. (2) L. Levy and A. Hadidi. EPPO Bull. 24:595, 1994.

Severe Yellowing Outbreaks in Tomato in Spain Associated with Infections of Tomato chlorosis virus.

Abstract: Since 1997, yellowing disease outbreaks have occurred in tomato (Lycopersicon esculentum) crops in southern Spain. The outbreaks were associated with high populations of the whitefly Bemisia tabaci. Symptoms consisted mainly of interveinal yellowing that developed initially on lower leaves and then progressed to the upper part of the plant. Affected plants were less vigorous and yielded less due to reduced fruit growth and delayed ripening. During 1998 and 1999, the yellowing disease was widespread and occurred at high incidences in the Malaga province. The disease agent was readily transmissible from tomato to tomato by B. tabaci biotype Q. Samples from symptomatic tomato plants were analyzed and shown to be infected with Tomato chlorosis virus (ToCV) (genus Crinivirus, family Closteroviridae). This is the first report of ToCV epidemics in Europe.

Effect of silicon rate and host resistance on blast, scald, and yield of upland rice

Abstract: Blast-resistant, partially resistant, and susceptible cultivars of rice were planted in soil amended with Si at 0, 500, or 1,000 kg/ha at two locations in eastern Colombia to assess differential responses to leaf blast, neck blast, and leaf scald, and to examine the quantity and quality of grains harvested. Leaf and neck blast on partially resistant and susceptible cultivars were reduced by Si as the rate of Si was increased. Depending on the location, the level of severity of leaf and neck blast on partially resistant cultivars, when fertilized with Si at 500 or 1,000 kg/ha, was lowered to that of resistant cultivars without Si. At both locations, yields were increased by as much as 42%, depending on the cultivar, by Si applied at 1,000 kg/ha. In general, high rates of Si reduced the number of broken grains harvested. Grain discoloration, regardless of cultivar or location, was reduced by as much as 70% at the high rate of Si. The application of Si to complement host resistance to blast and scald appears to be an effective strategy for disease management in rice and provides the added benefit of improving the quantity and quality of rice yields.

Comparison of field, greenhouse, and detached-leaflet evaluations of tomato germ plasm for early blight resistance

Abstract: Twenty-nine tomato genotypes (cultivars, breeding lines, and plant introductions), representing three Lycopersicon species, were evaluated for resistance to early blight (EB) caused by the fungus Alternaria solani. Evaluations were conducted in replicated trials in multiple years under field and greenhouse conditions (with whole plants) and in growth chamber (with detached leaflets). In the field experiments, plants were evaluated for disease symptoms, and area under the disease progress curve (AUDPC) and final percent defoliation were determined. In the greenhouse experiments, plants were evaluated for percent defoliation following spray-inoculation with isolates of A. solani. In the growth chamber experiments, lesion radius, rate of lesion expansion, and final disease severity were determined for individual detached leaflets inoculated with isolates of A. solani. There were significant differences among genotypes in their response to A. solani infection in the field, greenhouse, and growth cham...

Effect of Irrigation and Soil Water Stress on Densities of Macrophomina phaseolina in Soil and Roots of Two Soybean Cultivars.

Abstract: The effects of irrigation and soil water stress on Macrophomina phaseolina microsclerotial (MS) densities in the soil and roots of soybean were studied in 1988, 1989, and 1990. Soybean cvs. Davis and Lloyd received irrigation until flowering (TAR2), after flowering (IAR2), full season (FSI), or not at all (NI). Soil water matric potentials at 15- and 30-cm depths were recorded throughout the growing season and used to schedule irrigation. Soil MS densities were determined at the beginning of each season. Root MS densities were determined periodically throughout the growing season. Microsclerotia were present in the roots of irrigated as well as nonirrigated soybean within 6 weeks after planting. By vegetative growth stage V13, these densities reached relatively stable levels in the NI and FSI treatments (2.23 to 2.35 and 1.35 to 1.63 log [microsclerotia per gram of dry root], respectively) through reproductive growth stage R6. After R6, irrigation was discontinued and root densities of microsclerotia increased in all treatments. Initiation (IAR2) or termination (TAR2) of irrigation at R2 resulted in significant changes in root MS densities, with densities reaching levels intermediate between those of FSI and NI treatments. Year to year differences in root colonization reflected differences in soil moisture due to rainfall. The rate of root colonization in response to soil moisture stress decreased with plant age. Root colonization was significantly greater in Davis than Lloyd at R5 and R8. This was reflected in a trend toward higher soil densities of M. phaseolina at planting in plots planted with Davis than in plots planted with Lloyd. Although no charcoal rot symptoms in the plant were observed in this study, these results indicated that water management can limit, but not prevent, colonization of soybean by M. phaseolina, that cultivars differ in colonization, and that these differences may affect soil densities of the fungus.