Leguminous Green Manure Incorporation and Fusarium Wilt Suppression in WatermelonView ArticleHimmelstein et al. investigate how changes in the soil environment caused by incorporating cover crops suppress Fusarium wilt of watermelon. Specifically, increases in soil respiration following incorporation of legume cover crops suggest increases in overall soil microbial activity and consequently general suppression of Fusarium oxysporum f. sp. niveum. However, lab and greenhouse studies indicate that specific suppression may also play a role.
Blueberry necrotic ring blotch virus in Southern Highbush Blueberry in U.S. SoutheastView Article
Blueberry necrotic ring blotch, caused by Blueberry necrotic ring blotch virus (BNRBV), is a new disease that has recently been observed on southern highbush blueberry (SHB) cultivars in the southeastern United States. Robinson et al. conducted experiments to determine how the virus spreads in the field. Their findings suggest that BNRBV does not infect SHB plants systemically, that it is not transmitted through vegetative propagation, and that it does not likely persist in plants after natural defoliation in the fall.
Timing of Glyphosate Applications to Wheat Cover Crops to Reduce Onion Stunting Caused by Rhizoctonia solaniView Article
Cereal cover crops are used in agricultural production systems for a number of reasons, including to prevent wind erosion of soil. A cereal winter cover crop is planted in the fall and killed with an herbicide just before or after planting a cash crop. Soilborne pathogens, such as Rhizoctonia solani, are often more prevalent in crops planted soon after the cover crop is killed. Sharma-Poudyal et al. evaluated how the length of the interval between herbicide application and onion seeding affects onion stunting caused by R. solani. As the interval between herbicide application and onion planting increased, the number of patches of stunted onions decreased and the DNA concentration of R. solani in soil sampled from the dead cover crop declined. These data suggest that in fields with a high risk of onion stunt, cover crops should be terminated 3 weeks before onion seeding.
Patterns of Ergot and Quantification of Sclerotia in Perennial Ryegrass Seed FieldsView Article
Ergot, caused by Claviceps purpurea, is the most important disease affecting perennial ryegrass seed production in eastern Oregon. The disease results in economic losses due to reduced yield, the cost of increased seed cleaning to remove sclerotia from seed lots, and the production of seed that cannot be sold as feed because of alkaloid contamination. Dung et al. quantify and describe the spatial pattern of ergot severity in commercial perennial ryegrass fields. Significant autocorrelation and clustering were indicated by Moran’s I and spatial analysis by distance indices of aggregation.
Distribution of Agrobacterium vitis in Grapevines and Its Relevance to Pathogen Elimination View ArticleCrown gall of grape, caused by Agrobacterium vitis, is an economically important disease, and propagation of disease-free material could provide a means of management by preventing infection of vines during their first years of growth. Johnson et al. used magnetic capture hybridization and real-time PCR to monitor A. vitis in grapevines. They report that the pathogen is distributed nonuniformly on dormant and green plant tissues and that it colonizes shoot meristems. Consequently, multifaceted strategies may be required to produce clean plants.
Detection of Race 3 Biovar 2 (R3bv2) and Native U.S. Strains of Ralstonia solanacearum View Article
Tran et al. compare several published methods of detecting R3bv2 and native strains of R. solanacearum for sensitivity, speed, cost, and ease of use. Based on their data, the authors suggest a pipeline for screening geranium plants using strain-specific PCR on DNA immobilized on FTA cards. The authors also identify a reliable, sensitive, cost-effective assay to test for R3bv2 infection of field-grown tomato plants.
Genetic Diversity and Biocontrol of Rosellinia necatrix Infecting Apple in Northern ItalyView Article
Rosellinia necatrix causes white root rot disease on many economically important plants, and because this fungus survives in soil for long periods, it is difficult to manage. To provide apple growers with management tactics, Pasini et al. assessed the genetic diversity of R. necatrix to determine potential sources of inoculum. They also evaluated soil water content and temperature as factors that influence infection, along with certain production practices and a biocontrol agent.
Survival of Phytophthora infestans in Tomato Seed at Cold TemperaturesView Article
Using a cooling degree-day model, Frost et al. demonstrate that Phytophthora infestans, causal agent of potato and tomato late blight, can overwinter via asexual means in tomato seed in northern latitudes. Their data suggest that in the spring, volunteer seedlings can serve as a source of primary inoculum for late blight disease in northern regions.
The Effect of Delayed-dormant Chemical Treatments on DMI Sensitivity in a DMI-resistant Population of Ventura inaequalis View Article
Frederick and colleagues report results of New York field trials in which several chemicals were applied to apple trees during the period between bud break and appearance of green leaf tissue. They found that a spraying a copper material at that time resulted in significant reduction of isolate growth on DMI (myclobutanil)-amended media, suggesting that such treatments could be used to help manage proliferation of DMI fungicide resistance in apple orchards.
Barberry as an Alternate Host for Wheat Rust Pathogens View Article
Wang and colleagues present evidence that in the U.S. Pacific Northwest, barberry is the alternate host of the wheat stem rust pathogen (Puccinia graminis f. sp. tritici) under natural conditions but not the wheat stripe rust pathogen (P. striiformis f. sp. tritici).
Leaf Doctor: A New Portable Application for Quantifying Plant Disease Severity View Article
Pethybridge and Nelson describe a new mobile app, available free from the iTunes Store, that streamlines the process of disease severity assessment. The app offers advantages of greater speed and accessibility over Assess, which is the current standard software for automated severity assessment. In their tests, Leaf Doctor performed well in assessing severity of foliar diseases of six different crops and commodities.
Efficiencies of Plum pox virus eradication programs in Pennsylvania and Ontario View Article
Gougherty and colleagues take the unusual path of evaluating how methodological differences between large-scale virus eradication programs carried out in two locations impacted the probability of detecting the virus in plum trees. Using a modeling approach, they determined that the efficiency of detection of plum pox virus was 72% in Pennsylvania vs. 41% in Ontario and pinpointed which components of the PPV eradication programs affected detection probability.
Fungicide Sensitivity of Botrytis cinereaView Article
Applying site-specific fungicides is the primary strategy for controlling gray mold of strawberry in Germany. In the August issue, Grabke and Stammler report on resistance of the gray mold fungus, Botrytis cinerea, to multiple fungicides. They found a wide diversity of fungicide sensitivity among nearly 200 isolates obtained from strawberry fields on a single day. Although many isolates were sensitive to single fungicides, a substantial number were resistant to as many as six fungicides.
Organic Mineral Seed Coating for Control of Seedling Diseases of Alfalfa View Article
Most alfalfa seed is coated with the fungicide mefenoxam to protect against seedling diseases caused by oomycetes. However, this treatment is ineffective against Aphanomyces euteiches and unacceptable for organic production. In the May issue, Samac and colleagues report on testing natural zeolite as a seed coating against both pathogen groups. In seed inoculation and infested soil trials in growth chambers, the zeolite coating provided significantly higher percentages of healthy seedlings than either mefenoxam or no treatment and did not affect in vitro growth of the nodule-forming bacterium Sinorhizobium meliloti. The authors conclude that zeolite has potential value as a seed coating for alfalfa.
Heart Rot of PomegranateView Article
In the April issue, Ezra and colleagues pinpoint the fungus Alternaria alternata as the causal organism of heart rot, a major disease of pomegranate in Israel, and explain how it causes internal rot of fruit. The investigators isolated A. alternata from the pistils of more than 85% of open flowers and tracked its path through the style and into the lower loculus of the fruit, where it can remain as a latent infection for up to 4 months until ripening begins.
Inhibition of the Butternut Canker Pathogen by a Bark ExtractView Article
In the March issue, Moore and colleagues describe development of a new technique to identify butternut (Juglans cinerea) trees with resistance to butternut canker—a fatal disease that has wiped out most native butternut in North America. Levels of the biochemical juglone in bark correlated directly with the results of a bark extract assay for suppression of the fungal pathogen (Ophiognomonia clavigignenti-juglandacearum). This assay may prove useful in selecting resistant butternut for conservation and forest restoration.
Influence of Open Alleys in Field TrialsView Article
In the February issue, Vincelli and Lee tackle a seldom-addressed but important question in field studies of fungicide impact: Can plot design bias results? The authors tracked yield as a function of subplot position with and without application of Headline AMP (pyraclostrobin plus metconazole). Although some interaction between yield and subplot position was noted, the overall impact of subplot position on yield was negligible. Thus, the authors conclude that the open-alley design typical of many corn fungicide trials does not bias assessment of treatment effects.
Forecasting of Rhizoctonia Web Blight Development on AzaleaView ArticleIn the January issue, Warren Copes reports on a disease risk assessment model based on weather measurements from 11 site-year datasets. Using receiver operating characteristic (ROC) curve analysis, Copes identified a model that is a strong predictor of epidemic development on container-grown azalea. The model awaits field testing to validate its usefulness for controlling Rhizoctonia web blight, a key disease in the nursery industry.
A Novel Plant Family-Specific Root PathogenView Article
In the December issue, Stanghellini and colleagues present evidence for a new species of Pythium with a host range that appears to be restricted to plants in the family Brassicaceae. The authors propose that the species formerly designated as P. jasmonium, which was never formally described, be renamed P. brassicum in recognition of the pathogen’s close association with the host family.
Fitness of Erysiphe necator Against Quinone Outside InhibitorsView ArticleRallos and colleagues report that strains of the grape powdery mildew pathogen (Erysiphe necator), isolated from Virginia and nearby states, that are resistant to quinone outside inhibitor (QoI) fungicides demonstrated fitness and persistence in mixed sensitive and resistant populations in the absence of QoI applications. This result suggests that QoI-resistant isolates will likely remain in vineyards even after use of these fungicides has been discontinued.
A Latent Period Duration Model for Wheat Stem RustView Article
In the October issue, Hernandez Nopsa and Pfender report on predicting duration of the period from infection initiation to pustule eruption by Puccinia graminis subsp. graminis, the causal agent of wheat stem rust. Experiments on wheat under controlled conditions found significant differences in latent period duration across a range of temperatures (5–33°C), and a mathematical model was developed to express the response pattern. Field experiments confirmed the predictive value of the model, which may help in estimating the rate of epidemic development.
A Sheepish Way to Spread DiseaseView ArticleThe September issue presents an unusual study carried out by Markakis and colleagues to determine the survival, persistence, and infection efficiency of Verticillium dahliae passed through the digestive tracts of sheep. In the study, which was conducted in Greece, the authors documented that when vegetable crops infected with V. dahliae were fed to sheep, the pathogen survived in the animals' digestive tracts up to 5 days. Interestingly, eggplant seedlings that were transplanted into composted manure from these sheep developed disease symptoms caused by V. dahliae—the first experimental evidence that sheep can function as a vector for dissemination of this important soilborne fungal pathogen.
Fungicide Resistance Profiles and Evidence for Stepwise Accumulation of Resistance in Botrytis cinereaView Article
In the August issue, Li and colleagues present statistical evidence—based on assessment of resistance of several hundred isolates obtained in conventionally managed blackberry and strawberry fields in North and South Carolina—that Botrytis cinerea acquires resistance to different fungicide classes in a stepwise manner. Specifically, isolates resistant to thiophanate-methyl were more likely to acquire resistance to pyraclostrobin than to boscalid and to fenhexamid when compared with random chance. The authors make a strong case that multifungicide resistance to B. cinerea has evolved via a stepwise accumulation of single-fungicide resistances.
En la edición agosto, Li y colegas presentan evidencia estadística—basado en evaluación de resistencia de varios cientos de aislados obtenidos en campos de zarzamora y fresa que fueron dirigidos de manera convencional en Carolina del Norte y Carolina del Sur—que Botrytis cinerea adquiere resistencia a diferentes clases de fungicidas en una manera constante. Específicamente, los aislados resistentes a thiophanate-methyl fueron mas probables adquirir resistencia a pyraclostrobin que boscalid y fenhexamid cuando comparado al azar. Los autores hacen un caso fuerte que la resistencia de amplio espectro a B. cinerea ha evolucionado por una constante acumulación de resistencias de fungicidas solas.
Evaluation of Atoxigenic Aspergillus flavus Strain AF36 in Pistachio OrchardsView Article
In the July issue, Doster and colleagues report success in altering the population balance of Aspergillus flavus in California pistachio orchards in favor of atoxigenic strain AF36 after applying AF36-infested grain to orchard floors. The researchers also report that this practice reduced the incidence of aflatoxin-contaminated nuts at harvest by 20–45% in treated orchards compared with untreated orchards. This reduction represents a meaningful advance in managing aflatoxin contamination, which is difficult to control.
En la edición de junio, Doster y colegas reportan éxito alterando la balanza de la populación de Aspergillus flavus en los huertos de pistacho en California a favor de la cepa atoxigénica AF36 después de aplicar granos infestados con AF36 a los suelos de los huertos. Los investigadores también reportan que esta práctica redujo la incidencia de nueces contaminados con aflatóxinas a cosecha entre 20 por ciento y 45 por ciento en los huertos tratados comparado con los huertos no tratados. Esta reducción representa un avance significante en el manejo de contaminación de aflatóxinas, las cuales sean difíciles controlar.
Cassava Frogskin Disease in Brazil View Article
In the June issue, de Souza and colleagues report progress in identifying the causal agent or agents of cassava frogskin disease (CFSD), a devastating disease of this major food crop in tropical regions of the world. By analyzing DNA extracts from symptomatic and asymptomatic plants using PCR, RFLP, and sequencing, the authors identified a phytoplasma belonging to group 16SrIII-A—the first report of this group associated with cassava. The authors also identified a dsRNA virus that may co-infect cassava plants with CFSD symptoms.
En la edición de junio, de Souza y colegas reportan progreso con la identificación del agente causal o los agentes del Cuero de Sapo de la yuca (CFSD), una enfermedad devastadora de este principal cultivo en regiones tropicales del mundo. Mediante el análisis de extractos de ADN de plantas sintomáticas y asintomáticas usando la reacción en cadena de la polimerasa (PCR), los polimorfismos de longitud en fragmentos de restricción (RFLP), y sequenciando, los autores identificaron una fitoplasma la cual pertenece al grupo 16SrIII-A—el primer reporte de este grupo asociado con yuca. Los autores también identificaron un virus de ARNcd que podría coinfectar las plantas de yuca con síntomas de CFSD.
在今年六月的期刊上， de Souza及其同事报道了鉴定木薯蛙皮病 （cassava frogskin disease CFSD）病原的进展。这是一种对全球热带地区的主要粮食作物有巨大威胁的病害。作者将有此症状和无此症状植物样本的DNA 进行PCR, RFLP以及序列分析后，鉴定其病原为16SrIII-A组植原体—这是首次在木薯上发现这一组植原体。作者同时发现一种双链RNA病毒有可能和木薯蛙皮病的病原协同侵染木薯。
Triazole Sensitivity in Fusarium graminearum from New York WheatView Article
In an article in the May issue, Spolti and colleagues describe the discovery in New York of the first tebuconazole-resistant field isolate of Fusarium graminearum reported in the Americas. The isolate was found to have the lowest sensitivity to tebuconazole ever documented for this pathogen, which causes Fusarium head blight (FHB). Following the application of tebuconazole, wheat plants inoculated with this isolate had more severe FHB and higher concentrations of the mycotoxin deoxynivalenol (DON) than plants inoculated with a tebuconazole-sensitive isolate. Moreover, preliminary assessments indicated that the resistant isolate was more competitive in these experiments.
En “Sensibilidad de Triazole en Una Población Contemporánea de Fusarium graminearum en Trigo de Nueva York y la Competitividad de Un Aislamiento Resistente a Tebuconazole,” Spolti y colegas describen el descubrimiento en Nueva York del primer aislamiento de campo de Fusarium graminearum reportado en las Américas resistente a tebuconazole. El aislamiento presentó la mas baja sensibilidad a tebuconazole antes documentada para este patógeno, el agente causal de Fusarium head blight (FHB). Seguida la aplicación de tebuconazole, las plantas de trigo inoculadas con el aislamiento exhibieron/presentaron FHB más severa y concentraciones más altas de la micotoxina deoxynivalenol (DON) que en las plantas inoculadas con un aislamiento sensible a tebuconazole. Las evaluaciones preliminares indicaron que el aislamiento resistente a tebuconazole fue más competitivo en estos experimentos.
Xylella fastidiosa and Bacterial Leaf Scorch View Article
Bacterial leaf scorch of blueberry, caused by Xylella fastidiosa, is a recently recognized disease of southern highbush blueberry in the southeastern United States. In the April issue of Plant Disease, Holland and colleagues report on the potential value of pruning for disease management—namely, by sampling various plant parts for the presence and titer of the pathogen in naturally infected plants displaying various levels of symptom severity. Using real-time PCR with specific primers, the researchers found that X. fastidiosa was consistently present in the sap of stem samples in the middle and basal portions of plants even at low symptom severity levels and in the roots at moderate to high symptom severity levels. They concluded that pruning was unlikely to be helpful in curing plants infected with the disease.
In the March issue of Phytopathology, Carr and Nelson assess how extracts of vermicompost affect the soilborne pathogen Pythium aphanidermatum. Using time-lapse photomicroscopy to monitor the responses of mature zoosporangia, the researchers found that both sterile and nonsterile vermicompost extracts inhibit indirect germination and production of zoospores; however, zoospores are able to germinate directly by producing germ tubes in the compost extracts. In bioassays, the compost extracts did not suppress germ tube growth or infection of cucumber seed. Carr and Nelson conclude that composts may be able to affect multiple stages of P. aphanidermatum development, perhaps by both microbial and nonmicrobial means.
Seedling Resistance to Sclerotinia sclerotiorumView Article
In the February issue of Plant Disease, Uloth and colleagues report differences in seedling resistance to Sclerotinia sclerotiorum based on an assay of 46 lines from 12 species of cruciferous crops and weeds. Lines of cabbage and broccoli were most resistant, whereas lines of various wild radish and mustard species were most susceptible. This article is the first to report high levels of resistance to this important soilborne pathogen in Brassica oleracea (cabbage and broccoli) at the cotyledon stage.
Control of Phytophthora nicotianaeView Article
In the January issue, Morales-Rodríguez and colleagues report on their examination of a set of isolates of Phytophthora nicotianae from tomato and pepper plants in southwestern Spain. Mefenoxam sensitivity was greater in the pepper isolates than in the tomato isolates, and inhibition of the pathogen by biofumigants was dosage dependent. A commercial form of pelletized Brassica was effective in suppressing the pathogen in greenhouse trials using pepper plants. The authors anticipate that Brassicaceae tissues can supplement mefenoxam in an integrated disease management program to forestall development of resistance to the fungicide.