Showing papers in "Plant Disease in 2003"

Mechanisms Employed by Trichoderma Species in the Biological Control of Plant Diseases: The History and Evolution of Current Concepts.

Abstract: Fungal species belonging to the genus Trichoderma are worldwide in occurrence and easily isolated from soil, decaying wood, and other forms of plant organic matter. They are, for the most part, classified as imperfect fungi, in that they have no known sexual stage. Rapid growth rate in culture and the production of numerous spores (conidia) that are varying shades of green characterize fungi in this genus. The reverse side of colonies is often uncolored, buff, yellow, amber, or yellow-green, and many species produce prodigious quantities of thick-walled spores (chlamydospores) in submerged mycelium (8). The potential of Trichoderma species as biocontrol agents of plant diseases was first recognized in the early 1930s (31), and in subsequent years, control of many diseases has been added to the list (1,3,5,7,9,11,19, 23,29,34,37,40). This has culminated in the commercial production of several Trichoderma species for the protection and growth enhancement of a number of crops in the United States (24), and in the production of Trichoderma species and mixtures of species in India, Israel, New Zealand, and Sweden (D. R. Fravel, personal communication). One of the most interesting aspects of the science of biological control is the study of the mechanisms employed by biocontrol agents to effect disease control. Past research indicates that the mechanisms are many and varied, even within the genus Trichoderma. In order to make the most effective use of biocontrol agents for the control of plant diseases, we must understand how the agents work and what their limitations are. We can then develop effective means of culturing, storing, applying, and utilizing biocontrol agents so that we harness their best effort for disease control. The selected research papers cited in this article were chosen because they illustrate what has been learned about mechanisms involved in biocontrol with Trichoderma species.

Fire Blight Management in the Twenty-first Century: Using New Technologies that Enhance Host Resistance in Apple

Abstract: Fire blight has been known as a destructive disease of apple and pear for over 200 years (3). The disease is caused by the bacterium Erwinia amylovora, which is capable of infecting blossoms, fruits, vegetative shoots, woody tissues, and rootstock crowns (Fig. 1). There are several distinct phases of the disease including blossom blight, shoot blight, and rootstock blight. The diversity of host tissues susceptible to infection, combined with the limited number of management tools available to control the disease, has made it difficult to stop or slow the progress of fire blight epidemics. Effective management of fire blight requires an integrated approach of several practices that are aimed at (i) reducing the amount of inoculum that is available to initiate new infections, (ii) imposing barriers to successful establishment of the pathogen on the host, and (iii) reducing host susceptibility to infection (1,55). Most fire blight management strategies developed during the twentieth century focused on the reduction of inoculum in the orchard and the use of antimicrobial treatments to prevent infection. Although increasing host resistance has been recognized as an important component of fire blight management, its application has been limited by a lack of resistant cultivars suited to commercial needs and by a lack of management practices that could effectively increase resistance. Recent advances have made it feasible to change this paradigm in the twenty-first century. First, apple rootstock breeding programs have developed size-controlling (often dwarfing) rootstocks that are resistant to fire blight and are currently becoming available for commercial use (43). Second, genetic engineering of commercial apple cultivars for increased fire blight resistance has been demonstrated, and transgenic apple plants are now undergoing field trials (2). Third, chemical treatments that enhance host resistance have been demonstrated to be useful in the control of fire blight (9,33,61). Although these technologies are at the early stages of development and are either not available or not proven in the marketplace, incorporating the use of host resistance into fire blight management strategies has become a realistic goal in the twenty-first century. This article describes recent progress in the development of new fire blight control technologies that enhance host resistance by chemical or genetic means.

Influence of Wheat Cultivar, Species of Fusarium, and Isolate Aggressiveness on the Efficacy of Fungicides for Control of Fusarium Head Blight

Abstract: Mesterhazy, A., Bartok, T., and Lamper, C. 2003. Influence of wheat cultivar, species of Fusarium, and isolate aggressiveness on the efficacy of fungicides for control of Fusarium head blight. Plant Dis. 87:1107-1115. Attempts to control Fusarium head blight (FHB) with fungicides have been highly variable. Variability is caused by cultivar resistance, fungicide efficacy, fungicide coverage, timing, and pathogen aggressiveness. In this research, fungicides were tested on winter wheat cultivars with different levels of resistance to FHB using different isolates of Fusarium graminearum and F. culmorum to evaluate the role of host resistance and isolate aggressiveness on severity of FHB. Fungicides were applied to groups of wheat heads to provide full coverage. Incidence and severity of FHB was measured by the severity of head symptoms, percentage of Fusariumdamaged kernels (FDK), yield loss, and deoxynivalenol (DON) contamination. Development of FHB was affected by fungicides, cultivars, fungal isolates, and most of the two-way interactions of these variables. Among the fungicides tested, those containing tebuconazole tended to be more effective in reducing FHB. Reduction of disease in susceptible cultivars may not be adequate to produce marketable yields under conditions of high disease pressure. In most cases, if a fungicide reduced FHB visual symptoms, similar decreases were detected in yield loss, DON concentration, and FDK reaction. In 1998, an increase in DON contamination compared with the Fusarium check was observed with azoxystrobin and carbendazim on the more susceptible cultivar. This increase in DON with some fungicide requires additional research. Research to develop more resistant cultivars, better spraying technology, and more effective fungicides is also needed.

Toxoflavin Produced by Burkholderia glumae Causing Rice Grain Rot Is Responsible for Inducing Bacterial Wilt in Many Field Crops

Abstract: Severe wilt symptoms similar to bacterial wilt caused by Ralstonia solanacearum were observed in tomato, hot pepper, eggplant, potato, perilla, sesame, and sunflower in 2000 and 2001 in Korea. From diseased crops at 65 different locations, we obtained 106 isolates that produced green pigment on CPG medium; 36 were isolated from discolored rice panicles. The causal pathogen was identified as Burkholderia glumae based on its biochemical characteristics, fatty acid methyl ester analysis, and 16S rRNA gene sequence. Nine representative isolates produced toxoflavin, as determined by electrospray ionization mass spectrometry using a direct inlet system and TLC analyses, and caused bacterial wilt on tomato, sesame, perilla, eggplant, and hot pepper. However, BGR12, a wild-type isolate lacking toxoflavin production and toxoflavin-deficient mutants generated by Tn5lacZ failed to cause bacterial wilt on those five field crops. Cells of B. glumae and synthetic toxoflavin caused wilt symptoms on field crops,...

Broad-Spectrum Protection Against Several Pathogens by PGPR Mixtures Under Field Conditions in Thailand.

Abstract: Jetiyanon, K., Fowler, W. D., and Kloepper, J. W. 2003. Broad-spectrum protection against several pathogens by PGPR mixtures under field conditions in Thailand. Plant Dis. 87:13901394. Prior greenhouse experiments showed that four mixtures of plant growth-promoting rhizobacteria (PGPR) strains (all Bacillus spp.) elicited induced systemic resistance in several plants against different plant pathogens. Based on these findings, we sought to determine if systemic resistance induced by these PGPR would lead to broad-spectrum protection against several pathogens under field conditions in Thailand. Experiments were conducted during the rainy season (July to October 2001) and winter season (November 2001 to February 2002) on the campus of Naresuan University, Phitsanulok, Thailand. The specific diseases and hosts tested were southern blight of tomato (Lycopersicon esculentum) caused by Sclerotium rolfsii, anthracnose of long cayenne pepper (Capsicum annuum var. acuminatum) caused by Colletotrichum gloeosporioides, and mosaic disease of cucumber (Cucumis sativus) caused by Cucumber mosaic virus (CMV). Results showed that some PGPR mixtures suppressed disease more consistently than the individual PGPR strain IN937a. One PGPR mixture, Bacillus amyloliquefaciens strain IN937a + B. pumilus strain IN937b, significantly protected (P = 0.05) plants against all tested diseases in both seasons. Further, cumulative marketable yields were positively correlated with some treatments.

Suppression of Bacterial Spot of Tomato with Foliar Sprays of Compost Extracts Under Greenhouse and Field Conditions

Abstract: The efficacy of foliar sprays with compost water extracts (compost extracts) in reducing the severity of bacterial spot of tomato caused by Xanthomonas vesicatoria was investigated. Extracts prepared from composted cow manure, composted pine bark, an organic farm compost, or composted yard waste, applied as foliar sprays on tomato transplants, resulted in a moderate but statistically significant reduction in the severity of bacterial spot. The population of X. vesicatoria in infected leaves was reduced significantly by extracts prepared from composted cow manure. Efficacy of the water extracts was not affected by oxygen concentrations in the suspension during extraction, compost maturity, or sterilization by filtration or autoclaving. The degree of control provided by foliar sprays with the most effective compost extracts did not differ from that obtained with the plant activator acibenzolar-S-methyl. In the field in two growing seasons, foliar sprays with compost water extracts did not reduce th...

Improved Efficacy of Newly Formulated Bacteriophages for Management of Bacterial Spot on Tomato

Abstract: Bacteriophages are currently used as an alternative method for controlling bacterial spot disease on tomato incited by Xanthomonas campestris pv. vesicatoria. However, the efficacy of phage is greatly reduced due to its short residual activity on plant foliage. Three formulations that significantly increased phage longevity on the plant surface were tested in field and greenhouse trials: (i) PCF, 0.5% pregelatinized corn flour (PCF) + 0.5% sucrose; (ii) Casecrete, 0.5% Casecrete NH-400 + 0.5% sucrose + 0.25% PCF; and (iii) skim milk, 0.75% powdered skim milk + 0.5% sucrose. In greenhouse experiments, the nonformulated, PCF-, Casecrete-, and skim milk-formulated phage mixtures reduced disease severity on plants compared with the control by 1, 30, 51, and 62%, respectively. In three consecutive field trials, nonformulated phage caused 15, 20, and 9% reduction in disease on treated plants compared with untreated control plants, whereas plants treated with PCF- and Casecrete-formulated phage had 27, 32, and 12% and 30, 43, and 24% disease reduction, respectively. Plants receiving copper-mancozeb treatments were included in two field trials and had a 20% decrease in disease in the first trial and a 13% increase in the second one. Skim milk-formulated phage was tested only once and caused an 18% disease reduction. PCF-formulated phage was more effective when applied in the evening than in the morning, reducing disease on plants by 27 and 13%, respectively. The Casecrete-formulated phage populations were over 1,000-fold higher than the nonformulated phage populations 36 h after phage application.

Sweetpotato Virus Disease (SPVD): Distribution, Incidence, and Effect on Sweetpotato Yield in Peru

Abstract: Sweetpotato virus disease (SPVD), the most important disease affecting sweetpotato (Ipomoea batatas (L.) Lam), is caused by the synergistic interaction of the aphid-transmitted Sweetpotato feathery mottle virus (SPFMV) and whitefly-transmitted Sweetpotato chlorotic stunt virus (SPCSV). In this study, SPVD was the main disease in the Canete Valley, the major sweetpotato-producing area in Peru. Studies on virus incidence showed that SPCSV and SPFMV were the most frequently identified viruses in Canete Valley. Symptoms of different severity were associated with isolates of both viruses involved in the SPVD. Over 80% of plants infected with both SPFMV and SPCSV showed the symptoms (leaf reduction and deformation, vein clearing or mosaic, and stunting) typically attributed to SPVD elsewhere. SPFMV did not significantly affect the yield of the sweetpotato cultivars Jonathan and Costanero, but infection of these cultivars by SPCSV was associated with significant yield reduction. Double infection by the two viruses resulted in SPVD and greater yield reduction than for either alone. These results demonstrate that SPFMV and SPCSV interact synergistically and that the severity of SPVD symptoms also depends on the particular isolate of each virus.

Effects of Plant Essential Oils on Ralstonia solanacearum Population Density and Bacterial Wilt Incidence in Tomato.

Abstract: Greenhouse experiments were conducted to determine the effectiveness of plant essential oils as soil fumigants to manage bacterial wilt (caused by Ralstonia solanacearum) in tomato. Potting mixture (“soil”) infested with R. solanacearum was treated with the essential oils at 400 mg or μl and 700 mg or μl per liter of soil in greenhouse experiments. R. solanacearum population densities were determined just before and 7 days after treatment. Populations declined to undetectable levels in thymol, palmarosa oil, and lemongrass oil treatments at both concentrations, whereas tea tree oil had no effect. Tomato seedlings transplanted in soil treated with 700 mg/liter of thymol, 700 ml/liter of palmarosa oil, and 700 ml/liter of lemongrass oil were free from bacterial wilt and 100% of plants in thymol treatments were free of R. solanacearum. Soil amendment with fresh leaves of essential oil-producing plants did not reduce bacterial wilt incidence compare to untreated inoculated control. Some thyme oil-pro...

Isothiocyanates Produced by Brassicaceae Species as Inhibitors of Fusarium oxysporum

Abstract: Glucosinolates contained in members of the Brassicaceae release isothiocyanates potentially useful in controlling Fusarium oxysporum pathogens in conifer seedling nursery soils. Our objective was to determine the toxicity of individual isothiocyanates to different growth stages of the fungus. Bioassays with four F. oxysporum isolates were conducted using sealed containers in which 0.3 μl of 2-propenyl, ethyl, buty, phenylethyl, benzyl, or phenyl isothiocyanate was allowed to volatilize. Propenyl and ethyl isothiocyanates were the most fungistatic of those compounds tested. The same concentrations of propenyl and ethyl isothiocyanates that inhibited mycelial growth completely suppressed conidial and chlamydospore germination of all isolates. Other isothiocyanates including ethyl, benzyl, and phenethyl were also fungitoxic to F. oxysporum conidia and chlamydospores. Reduction in pathogen populations resulting from a green-manure crop are likely achievable since chlamydospores are sensitive to isothiocyanate. Pathogenic F. oxysporum isolates infesting nursery soils would likely be most suppressed by species of plants such as Brassica carinata, B. nigra, and B. juncea, which contain glucosi-nolates that release high concentrations of propenyl isothiocyanate.

Near-Harvest Applications of Metschnikowia fructicola, Ethanol, and Sodium Bicarbonate to Control Postharvest Diseases of Grape in Central California

Abstract: The yeast Metschnikowia fructicola, ethanol, and sodium bicarbonate (SBC), alone or in combinations, were applied to table grapes on vines 24 h before harvest to control the incidence of postharvest diseases. In four experiments, all significantly reduced the total number of decayed berries caused by Botrytis cinerea, Alternaria spp., or Aspergillus niger after storage for 30 days at 1°C followed by 2 days at 20°C. In three experiments, a mean gray mold incidence (caused by B. cinerea) of 34.2 infected berries per kilogram among untreated grape was reduced by Metschnikowia fructicola at 2 × 107 CFU/ml, ethanol at 50% (vol/vol), or SBC at 2% (wt/vol) to 12.9, 8.1, or 10.6 infected berries per kilogram, respectively. Ethanol, SBC, and SO2 generator pads were similarly effective. M. fructicola effectiveness was not improved when combined with ethanol or SBC treatments. Ethanol and yeast treatments did not harm the appearance of the grapes. M. fructicola and SBC left noticeable residues, and SBC caus...

Incidence of Viruses and Virus like Diseases of Sweetpotato in Uganda

Abstract: Mukasa, S. B., Rubaihayo, P. R., and Valkonen, J. P. T. 2003. Incidence of viruses and viruslike diseases of sweetpotato in Uganda. Plant Dis. 87:329-335. Sweetpotato plants were surveyed for viruslike diseases and viruses in the four major agroecological zones of Uganda. Testing of 1,260 sweetpotato plants, of which 634 had viruslike symptoms, showed that virus disease incidence ranged from 2.7% (Soroti district, short grassland–savannah zone) to 20% (Mukono district, tall grass–forest mosaic zone). Sweet potato chlorotic stunt virus (SPCSV), Sweet potato feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), and sweet potato chlorotic fleck virus (SPCFV) were serologically detected and positive results confirmed by immunocapture reverse transcriptase polymerase chain reaction (IC-RT-PCR) and subsequent sequence analyses of the amplified fragments, except SPCFV, which lacked sequence information. SPCSV and SPFMV were detected in all the 14 districts surveyed, whereas SPMMV and SPCFV were detected in 13 and 8 districts, respectively. Logistic regression analysis revealed that SPCSV and SPFMV, SPFMV and SPMMV, and SPFMV and SPCFV more frequently occurred together than any other virus combinations or as single virus infections. Co-infections of SPCSV with SPFMV and/or SPMMV were associated with more severe and persistent symptoms than infections with each of the viruses alone. Several plants (11%) displaying viruslike symptoms did not react with the virus antisera used, suggesting that more viruses or viruslike agents are infecting sweetpotatoes in Uganda.

Efficacy of Biological and Chemical Treatments for Control of Fusarium Root and Stem Rot on Greenhouse Cucumber

Abstract: Potential disease control methods were evaluated against root and stem rot of cucumber (Cucumis sativus) caused by Fusarium oxysporum f. sp. radicis-cucumerinum. Crab/shrimp shell chitin; three composted media; the biological control agents Pseudomonas chlororaphis strain 63-28, Trichoderma harzianum (RootShield Drench), Streptomyces griseoviridis (Mycostop), Gliocladium catenulatum (Prestop WP, Prestop Mix), and Trichoderma (Gliocladium) virens (SoilGard); and the fungicides thiram or benomyl were added at seeding time followed by inoculation with the pathogen. The addition of chitin (4%, vol/vol) to a peat-based medium significantly (P ≤ 0.05) enhanced seedling growth, increased soil pH, and reduced F. oxysporum f. sp. radicis-cucumerinum populations, but the severity of disease was increased. The addition of composted media (greenhouse compost, windrow composted dairy solids, and vermi-composted dairy solids) to the seeding cavity in a rock wool block medium, followed 48 h later by inoculation with F. oxysporum f. sp. radicis-cucumerinum, reduced seedling mortality when measured after 37 days. Greenhouse compost was significantly (P ≤ 0.05) more suppressive than the other two composts, and the suppression was partially eliminated by sterilization of the compost. The biological control agent G. catenulatum (formulated as Prestop WP and Prestop Mix) significantly reduced seedling mortality when it was applied at seeding 24 h prior to inoculation with the pathogen in the rock wool block medium. None of the other biological control agents reduced disease incidence when compared with control plants under these experimental conditions. Pseudomonas chlororaphis and the fungicide thiram both significantly reduced plant mortality at 17 and 24°C when pathogen-infested seed was treated, or when bacteria-treated and fungicide-treated seed were planted into pathogen-infested peat medium at 24°C. Under semicommercial propagation conditions, treatments consisting of Prestop WP, RootShield Drench, My-costop, and windrow composted dairy solids reduced the severity of disease caused by F. oxysporum f. sp. radicis-cucumerinum in two out of three trials. The efficacy of the biological control agents was affected by seasonal differences in growing conditions, which affected the incidence and severity of the disease. The results from this study indicate that several different approaches can be used at seeding to control Fusarium root and stem rot on greenhouse cucumber.

Population Race Structure of Pyrenophora tritici-repentis Prevalent on Wheat and Noncereal Grasses in the Great Plains

Abstract: The fungus Pyrenophora tritici-repentis, cause of tan spot of wheat, is an important foliar pathogen worldwide. Genetic variation in the fungal population prevalent in the Great Plains was studied by analysis of 270 single-spore isolates of P. tritici-repentis recovered from wheat, durum, and 10 noncereal grasses: Alti wild rye, barnyard grass, crested wheatgrass, intermediate wheatgrass, needle and thread grass, quackgrass, smooth bromegrass, sand reedgrass, slender wheatgrass, and wild barley. The isolates were grouped into five known races based on necrosis and/or chlorosis induction on standard differentials with two additional wheat genotypes ND495 and M-3. The isolates recovered from wheat were races 1, 2, and 4, while those from durum were races 1 and 5. Isolates from noncereal grasses were all race 4, except for the recovery of two isolates of race 1 from smooth bromegrass. Race 3 was not found in this study. This is the first record of barnyard grass and slender wheatgrass as alternative hosts for P. tritici-repentis. The recovery from noncereal grasses suggests that the fungus has a fairly wide host range; however, predominance of a race that is avirulent on wheat on these grasses tends to eliminate their significance in the disease epidemiology of wheat. The results indicate that P. tritici-repentis has a diverse population on wheat and noncereal grasses. For durable resistance, wheat lines should be tested against all virulent races found in the field.

Effect of Partial Resistance on Phytophthora Stem Rot Incidence and Yield of Soybean in Ohio

Abstract: Phytophthora root and stem rot of soybean commonly causes losses in both stand and yield in Ohio. Environmental conditions which favor the pathogen typically occur in many areas of the state during late spring and summer. This study examined the performance of 12 soybean cultivars with partial resistance, with or without Rps genes, to different populations of Phytophthora sojae and various levels of disease pressure. The soybean cultivars were evaluated in seven field environments with and without metalaxyl over 4 years. There was a highly significant genotype-environment interaction which was due in part to variable disease pressure. The incidence of Phytophthora stem rot in subplots ranged from 0 to 10 plants in the most susceptible cultivar, Sloan, while significantly less stem rot developed in cultivars with high levels of partial resistance or partial resistance combined with an Rps gene in three of the seven environments. Metalaxyl applied in-furrow had a significant effect on early and fin...

Characterization of Verticillium dahliae Isolates and Wilt Epidemics of Pepper

Abstract: Epidemics of Verticillium wilt in pepper fields of the central coast of California and isolates of Verticillium dahliae associated with these epidemics were characterized. The mean incidence of wilted plants per field ranged from 6.3 to 97.8% in fields with Anaheim, jalapeno, paprika, or bell peppers. In general, incidence of wilt in jalapeno and bell pepper crops was lower than in crops of other types of pepper. Inoculum density of V. dahliae in the surveyed pepper fields ranged from 2.7 to 66.6 microsclerotia g-1 dry soil, and the correlation between disease incidence and density of microsclerotia was high (r = 0.81, P < 0.01). Distribution of Verticillium wilt was aggregated in a majority of the pepper fields surveyed, but the degree of aggregation varied. Vegetative compatibility group (VCG) characterization of 67 isolates of V. dahliae indicated that 67% belonged to VCG 2, 22% to VCG 4, and 11% to a new group, designated VCG 6. The pathogenicity of isolates of V. dahliae from bell pepper and...

Breakdown of a Brassica rapa subsp. sylvestris Single Dominant Blackleg Resistance Gene in B. napus Rapeseed by Leptosphaeria maculans Field Isolates in Australia.

Abstract: Blackleg, caused by Leptosphaeria maculans, is a major disease of oilseed rape (Brassica napus) grown in Canada, Europe, and Australia. Cv. Surpass 400 was released in Australia in 2000 as the most resistant cultivar to L. maculans. It carries a single dominant resistance gene from B. rapa subsp. sylvestris. This cultivar usually shows a hypersensitive response to L. maculans characterized by small, dark brown lesions that are necrotic, localized, and without pycnidia on cotyledons, leaves, and stems. However, in 2001 on a Western Australian experimental farm, a small proportion of the lesions on the lower stem and crown region of cv. Surpass 400 were typical of those observed in susceptible cultivars, which were brown, necrotic lesions with a darker margin, but they contained fewer pycnidia. Forty seedlings of cv. Surpass 400 and susceptible cv. Westar were inoculated with pycnidiospore suspensions (106/ml) of each of 18 isolates taken from lesions on cv. Surpass 400. All 18 isolates caused collapse of cotyledons of susceptible cv. Westar. Four of these isolates caused large cotyledon lesions with some pycnidia on cv. Surpass 400. Three of these four isolates were subsequently inoculated onto 60 seedlings per isolate, at each of the four cotyledon lobes of each seedling of the two cultivars. Inoculated plants were assessed for disease severity on cotyledons and transplanted to the field 14 days after inoculation. The cotyledons of inoculated cv. Surpass 400 showed characteristic large, necrotic lesions with pycnidia, while the cotyledons of cv. Westar had collapsed and contained a mass of pycnidia. Blackleg disease severity in the crown region of the stem was assessed at 2 weeks before harvest. Fifty-four percent of the cv. Surpass 400 transplanted inoculated plants subsequently developed susceptible symptoms of crown cankers on stems. These symptoms were deep, girdling, brown lesions on the plant crowns with some pycnidia. One hundred percent of cv. Westar plants were infected and dead at this stage. This confirmed the ability of these field isolates to overcome the single dominant resistance gene present in cv. Surpass 400. To our knowledge, this is the first report of breakdown of a single dominant B. rapa subsp. sylvestris gene based resistance to blackleg in oilseed rape in the field.

Fluctuations of Phytophthora and Pythium spp. in Components of a Recycling Irrigation System.

Abstract: Bush, E. A., Hong, C. X., and Stromberg, E. L. 2003. Fluctuations of Phytophthora and Pythium spp. in components of a recycling irrigation system. Plant Dis. 87:1500-1506. Stringent standards of water quality have prompted many horticultural enterprises to limit pollutant discharge associated with nutrient and pesticide applications. Collecting and recycling effluent is a method that has been implemented by many operations to contain pollutants; however, plant pathogens may be spread through recycled effluent. In this study, Phytophthora and Pythium spp. present in a water-recycling irrigation system at a perennial container nursery in southwestern Virginia were characterized using filtering and baiting techniques with two selective media. Members of Phytophthora were identified to species, whereas Pythium spp. were identified to genus only. Pythium spp. were recovered more frequently and in greater numbers than Phytophthora spp. Phytophthora capsici, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, and P. nicotianae were recovered in filtering assays. Only P. cryptogea and P. drechsleri were identified from baits placed on the surface of the irrigation reservoir, whereas P. cactorum, P. capsici, P. citricola, P. citrophthora, P. cryptogea, and P. drechsleri were recovered at depths, specifically at 1 and 1.5 m. This research provides data for development of detection technology and management practices for plant pathogens in irrigation water and may lead to improvements in conventional assay protocols.

Simultaneous Detection of the Defoliating and Nondefoliating Verticillium dahliae Pathotypes in Infected Olive Plants by Duplex, Nested Polymerase Chain Reaction

Abstract: Pathogen-free certified planting material and accurate detection of Verticillium dahliae pathotypes infecting the plant are key components of successful management of Verticillium wilt of olive. Use of a nested-polymerase chain reaction (PCR) procedure developed in earlier studies for in planta detection of the defoliating (D) and nondefoliating (ND) V. dahliae pathotypes resulted in ambiguous detection of the pathogen in some cases, due to heterologous amplification of the D-associated marker in ND-infected olive plants. In the present study, an improved procedure was developed that eliminates ambiguity and reduces time and cost for detection of D and ND V. dahliae in olive. The improved procedure is based on the simultaneous amplification of both an ND- and a new D-specific marker by means of duplex, nested PCR. The procedure was effective in the rapid and unequivocal detection of the D and ND V. dahliae in both artificially inoculated, own-rooted olive plants and naturally infected adult olive...

Effect of Organic Amendments on Soilborne and Foliar Diseases in Field-Grown Snap Bean and Cucumber

Abstract: Several paper mills in Wisconsin have programs for spreading paper mill residuals (PMR) on land. A growing number of vegetable farmers recognize the agronomic benefits of PMR applications, but there have been no investigations on the use of PMR for control of vegetable crop diseases. Our objective was to determine the effect of PMR amendments on soilborne and foliar diseases of cucumber and snap bean grown on a sandy soil. Raw PMR, PMR composted without bulking agent (PMRC), or PMR composted with bark (PMRBC) were applied annually in a 3-year rotation of potato, snap bean, and pickling cucumber. Several naturally occurring diseases were evaluated in the field, along with in situ field bioassays. All amendments suppressed cucumber damping-off and Pythium blight and foliar brown spot of snap bean. Both composts reduced the incidence of angular leaf spot in cucumber. In a separate field experiment planted with snap bean for two consecutive years, all amendments reduced common root rot severity in the second year. In a greenhouse experiment, the high rate of PMRBC suppressed anthracnose of snap bean. These results suggest that the application of raw and composted PMR to sandy soils has the potential to control several soilborne and foliar diseases.

Serratia marcescens, a Phloem-Colonizing, Squash Bug -Transmitted Bacterium: Causal Agent of Cucurbit Yellow Vine Disease.

Abstract: Cucurbit yellow vine disease (CYVD), which can inflict heavy losses to watermelon, pumpkin, cantaloupe, and squash in U.S. production areas from the midwest to northeastern states, causes phloem discoloration, foliar yellowing, wilting, and plant decline. Bacteria were cultured from the phloem of crown sections of symptomatic plants of Citrullus lanatas and Cucurbita pepo. Those bacteria testing positive in CYVD-specific polymerase chain reaction (PCR) were all gram negative and appeared morphologically identical, producing creamy white, smooth, entire, convex colonies on Luria-Bertani or nutrient agar. Characterized cucurbit-derived strains of Serratia marcescens were introduced into greenhouse-grown squash plants by puncture inoculation and into field-grown squash plants by enclosure with S. marcescens-fed squash bugs, Anasa tristis. Up to 60% of the bacteria-inoculated plants in the greenhouse and up to 17% of field plants caged with inoculative squash bugs developed phloem discoloration and tested positive for S. marcescens by CYVD-specific PCR. None of the controls developed phloem discoloration or tested positive by PCR. Of the diseased field plants, 12% (2 of 35) also yellowed, wilted, and collapsed, exhibiting full symptom development of CYVD. However, neither plant collapse nor decline was observed in the greenhouse-grown, puncture-inoculated plants. The morphology, growth habit, and PCR reaction of bacteria cultured from crown tissue of a subset of plants in each experimental group were indistinguishable from those of the inoculum bacteria. Evidence presented from our studies confirms that the squash bug can transmit S. marcescens, the CYVD causal bacterium. The S. marcescens-A. tristis relationship described here is the first instance in which the squash bug has been identified as a vector of a plant pathogen. Our experiments represent a completion of the steps of Koch's postulates, demonstrating that S. marcescens is the causal agent of CYVD and that the squash bug, A. tristis, is a vector of the pathogen.

Using Genetic Diversity to Achieve Sustainable Rice Disease Management

Abstract: Host plant resistance is an important tool for rice disease control and has played a key role in sustaining rice productivity, especially in tropical Asia. Deploying resistant varieties as a means of disease control is attractive because it requires no additional cost to farmers and is environmentally safe (62). Furthermore, resistant varieties can be easily disseminated as seeds, leading to wide adoption (12). These are important considerations, because for resource-poor rice farmers in developing countries, the options for managing diseases are few. For example, during the 1970s and 1980s, when epidemics of rice tungro were frequent in the Philippines and Indonesia, farmers expressed more confidence in using resistant varieties than in other control measures. Disease control using chemicals is more common in the temperate or subtropical production environments where farmers apply fungicides for controlling blast (caused by Pyricularia grisea) and sheath blight (caused by Rhizoctonia solani). Despite regional differences in control measures, planting resistant varieties is considered most effective by rice farmers. Hence, breeding for disease resistance has been a major objective in rice improvement programs conducted at international agricultural research centers, such as the International Rice Research Institute (IRRI), and at the national agricultural research systems (NARS) of developing countries. There are limitations, however, in using resistant varieties alone to manage rice diseases. Most varieties are resistant only to a few major diseases that are the subjects of intensive breeding efforts. The rice production environments, particularly in the tropics, are habitats of many rice pathogens causing varying degrees of damage. Even the “minor” diseases collectively could pose a significant threat to production (63). Thus, pathologists and breeders have to deal with yield loss caused by diseases of epidemic and endemic nature. Epidemic loss is dramatic but less frequent, whereas endemic loss is less obvious but pervasive in each cropping season. Recent surveys indicated that an estimated annual yield loss from 1 to 10% was due to a combination of different diseases (80). Thus, resistance against a few targeted diseases offers only a partial solution to rice disease problems. To those diseases caused by nonspecialized pathogens, such as sheath blight and false smut (caused by Ustilaginoidea virens), no useful source of resistance has been identified to improve the resistance of rice varieties. To achieve sustainability of rice production in Asia, we need a rice production system built upon effective resistant varieties with broad resilience to a range of diseases and insect pests. Broad-spectrum resistance at the genotypic level and sustainability at the cropping systems level are therefore complementary approaches in managing rice diseases. Although considerable progress has been made over the past decades, much more can be done to integrate these two approaches to achieve results in farmers’ fields. Modern agricultural development has transformed the diverse, traditional rice production system into a monoculture system that relies only on a few fertilizer-responsive and high-yielding varieties. Farmers’ preference to high yield has led to wide adoption of modern rice varieties cultivated in millions of hectares of rice land. Although most modern varieties have built-in resistance against multiple diseases, genetic uniformity inevitably predisposes the system to disease epidemics, and under certain circumstances can lead to serious yield loss caused by diseases and insect pests (43). Varieties carrying a few resistance genes in a uniform genetic background are vulnerable to rapid adaptation of pathogens and pose uncertainty to farmers. For instance, emergence of new pathogen races caused several blast epidemics in Korea in the 1970s, leading to yield losses of 30 to 40% (38). In the 1980s, other disease outbreaks on a regional scale included epidemics of bacterial blight in northern India and Southeast Asia, tungro in Southeast Asia, and bacterial blight and blast in Japan (38,61,89). Another impact of the monoculture system is the gradual decline in the diversity of varieties grown by farmers. As modern high-yielding varieties expand to millions of hectares, they also replace the traditional varieties. Although useful genes from these traditional varieties are being used in breeding for modern varieties, many unique attributes and gene combinations resulting from years of selection are difficult to reconstitute. To achieve the productivity needed, it is not possible to revert to planting diverse traditional varieties that are poor yielding. However, it is within our capacity to work toward disease management methods that sustain productivity yet maintain adequate diversity and resilience in the production systems. In the past two decades, IRRI has moved toward increasing genetic diversity of modern rice varieties through resistance breeding (12,39,43) and deployment of different resistance genes based on an unCorresponding author: Twng Wah Mew, Entomology and Plant Pathology Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines; E-mail: T.Mew@cgiar.org

Evaluation of Inoculation Techniques for Fusarium Ear Rot and Fumonisin Contamination of Corn.

Abstract: Fumonisins have been associated with potentially serious toxicoses of animals and humans. Prior to initiating a corn (Zea mays) breeding program for resistance to these mycotoxins, an efficient inoculation technique must be developed. Four inoculation techniques were evaluated on 14 commercial corn hybrids in Urbana, IL in 1999 and 2000. The techniques were: injection of inoculum through the ear husk leaves at R2 (blister); silks sprayed with inoculum at R2 and covered with a shoot bag until harvest; silks sprayed with inoculum at R2, covered with a shoot bag, reinoculated 1 week thereafter, and covered with a shoot bag until harvest; and insertion of six Fusarium-colonized toothpicks into the silk channel at R2. Only injection of inoculum through the husk leaves significantly increased the concentration of fumonisin in grain and severity of Fusarium ear rot compared with a control. This technique effectively differentiated hybrids previously identified as resistant or susceptible to Fusarium ear rot. The rank order of hybrids inoculated with this technique did not significantly change in the 2 years of this study. This technique is suitable for efficiently evaluating a large number of corn genotypes for resistance to Fusarium ear rot and fumonisin concentration.

Relative Longevity of Macrophomina phaseolina and Associated Mycobiota on Residual Soybean Roots in Soil

Abstract: Survival of the charcoal rot pathogen (Macrophomina phaseolina) in soybean (Glycine max) on residual root systems was studied over a 2-year period. Root segments colonized by M. phaseolina were placed into fiberglass-mesh bags and buried at depths of 0, 7.6, and 25.4 cm in a Marietta fine sandy loam soil in field microplots. Samples for year 1 and year 2 were buried in October 1999 and 2000, respectively, and sampled every 2 months over a 14-month period. Mean percent frequencies from both years for M. phaseolina showed a decreased linear trend over time from 91% before burial to 24.7% 2 months later, 10.3% at 6 months, and 0.3% after 14 months. Tissues were degraded in the June samplings of both years and this degradation was believed to be partially responsible for the reduced survival of M. phaseolina. Mean isolation frequencies were significantly (P ≤ 0.05) greater at the 0-cm depth than at 7.6 and 25.4 cm across all sampling dates. Other fungi which were present in the preliminary assay, including Fusarium spp. and those in the Phomopsis/Diaporthe complex, also declined by the end of the study. Trichoderma spp. was isolated at significantly (P ≤ 0.05) greater frequencies and a significant positive linear trend over time. It is possible that Trichoderma spp. were involved in direct tissue degradation and nutrient depletion of the root segments, or may have acted as a mycoparasite, reducing the survival of the pathogen M. phaseolina and other associated fungi. Results from this study indicate that farm practices which increase residue destruction immediately after harvest or those that enhance Trichoderma spp. populations may directly or indirectly lower the relative longevity of soilborne pathogens, including M. phaseolina.

Induction of growth promotion and resistance against downy mildew on pearl millet (Pennisetum glaucum) by rhizobacteria.

Abstract: A series of laboratory, greenhouse, and field experiments were conducted to evaluate seven strains of plant growth-promoting rhizobacteria (PGPR). The PGPR were tested as suspensions of fresh cultures and talc-based powder formulations. Evaluations were conducted on pearl millet (Pennisetum glaucum) for growth promotion and management of downy mildew caused by Sclerospora graminicola. All treatments with fresh suspensions and powdered formulations showed enhancement in germination and vigor index over the respective untreated controls. With fresh suspensions, maximum vigor index resulted from treatments by Bacillus pumilus strain INR7 followed by B. subtilis strain IN937b (64 and 38% higher than the untreated control, respectively). With powdered formulation, treatment with strain INR7 also resulted in the highest germination and vigor indexes, which were 10 and 63%, respectively, over the untreated control. Under experimental plot conditions, prominent enhancement in growth also was observed in the disease tests. Yield was enhanced 40 and 37% over the untreated control by seed treatment with powdered formulations of strains INR7 and SE34, respectively. The same strains also increased yield by 36 and 33%, respectively, when applied as fresh suspensions. Studies on downy mildew management resulted in varied degrees of protection by the PGPR both under greenhouse and field conditions. With fresh suspensions, treatment with INR7 resulted in the highest protection (57%), followed by B. pumilus strain SE34 and B. subtilis strain GBO3, which resulted in 50 and 43% protection, respectively, compared with the untreated control. With powdered formulation, PGPR strain INR7 suppressed downy mildew effectively, resulting in 67% protection, while SE34 resulted in 58% protection, followed by GBO3 with 56% protection. Treatment with Apron (Metalaxyl) resulted in the highest protection against downy mildew under both greenhouse and field conditions. Thus, the present study suggests that the tested PGPR, both as powdered formulations and fresh suspensions, can be used within pearl millet downy mildew management strategies and for plant growth promotion.

Pathogenic Diversity of Phytophthora sojae in Ohio Soybean Fields

Abstract: Problems with early season soybean stand establishment, and an increase in incidence of Phytophthora root and stem rot caused by Phytophthora sojae, prompted a reassessment of the pathogen population in Ohio. Earlier studies had indicated a potential for pathogen adaptation to commonly deployed Rps genes in soybeans. Fifty-seven fields, part of an earlier study in 1990 and 1991, along with 29 additional fields were sampled in either 1997 or 1999. Two soybean cultivars, Sloan (rps) and Resnik (Rps1k), were used as bait in a seedling bioassay to isolate P. sojae from the soil samples. P. sojae was recovered from 82 of the 86 fields sampled. Of the 429 isolates recovered from these soils, 325 and 104 were baited with soybean cultivars Sloan and Resnik, respectively. The P. sojae population in Ohio increased in the number of pathotypes (races) as well as in complexity since the earlier surveys. There were 72 and 202 pathotypes identified on 8 and 13 Rps gene differentials, respectively, in the current study. When the data were compared by location, 96, 65, 73, 78, 51, and 52% of the locations had at least one isolate with virulences to Rps1a, Rps1b, Rps1c, Rps1k, Rps3a, and Rps6, respectively. The mean complexity, the number of susceptible interactions on 8 differentials, increased from 3.01 to 4.06 between 1991 and 1997/1999. In addition, the pathogenic diversity as measured by the Shannon index increased from 2.71 to 3.28 for isolates recovered from the 57 fields sampled in both surveys. Producers whose fields were sampled were surveyed to determine if changes in the P. sojae population could be linked with production practices. There was a significant association between (P ≤ 0.05) reduced tillage practices and the presence of isolates that had virulence to Rps1k; reduced tillage fields also had isolates with virulence to a greater number of differentials. Due to the percentage of isolates that have virulence to many of the Rps genes, it is questionable how long a single Rps gene or several stacked Rps genes will remain viable disease management tools for P. sojae, unless a novel Rps gene is identified.

Long-Term Monitoring for Resistance of Botryotinia fuckeliana to Anilinopyrimidine, Phenylpyrrole, and Hydroxyanilide Fungicides in Switzerland.

Abstract: In Switzerland, the use of phenylpyrrole, anilinopyrimidine, and hydroxyanilide fungicides for control of Botryotinia fuckeliana, causal agent of gray mold in grapes, has been restricted to one treatment per fungicide class per year as part of an anti-resistance strategy. Resistance development in B. fuckeliana was monitored from 1995 to 2001 for the anilinopyrimidine cyprodinil and the phenylpyrrole fludioxonil and from 1997 to 2001 for the hydroxyanilide fenhexamid in experimental vineyards in Richterswil and Stafa, Switzerland. In total, over 2,400 field isolates were tested. In 1996, the first case of field resistance to anilinopyrimidines was encountered in Richterswil. Efficacy of the anilinopyrimidine cyprodinil decreased significantly, and 54% of the isolates were resistant to anilinopyrimidines. During 7 years of monitoring, one field isolate was found that showed a slightly decreased sensitivity to the phenylpyrrole fludioxonil. Resistance to the hydroxyanilide fenhexamid was not found in 1997 and 1998. From 1999 to 2001, the level of fenhexamid-resistant isolates increased to 100% in Stafa. The analysis of monitoring and efficacy data showed that subpopulations of B. fuckeliana resistant to cyprodinil and fenhexamid have increased significantly; however, the efficacy of a mixture of fludioxonil and cyprodinil and of fenhexamid alone was still effective. The objective of this study was to initiate long-term monitoring in order to establish an early resistance-detection system as a tool to assess the effectiveness of the anti-resistance strategy used in Switzerland.