Host Resistance and Chemical Control for Management of Sclerotinia Stem Rot of Soybean in Ohio

Molecular Targets of Herbicides and Fungicides─Are There Useful Overlaps for Fungicide Discovery?

New fungicide modes of action are needed for fungicide resistance management strategies. Several commercial herbicide targets found in fungi that are not utilized by commercial fungicides are discussed as possible fungicide molecular targets. These are acetyl CoA carboxylase, acetolactate synthase, 5-enolpyruvylshikimate-3-phosphate synthase, glutamine synthase, phytoene desaturase, protoporphyrinogen oxidase, long-chain fatty acid synthase, dihydropteroate synthase, hydroxyphenyl pyruvate dioxygenase, and Ser/Thr protein phosphatase. Some of the inhibitors of these herbicide targets appear to be either good fungicides or good leads for new fungicides. For example, some acetolactate synthase and dihydropteroate inhibitors are excellent fungicides. There is evidence that some herbicides have indirect benefits to certain crops due to their effects on fungal crop pathogens. Using a pesticide with both herbicide and fungicide activities based on the same molecular target could reduce the total amount of pesticide used. The limitations of such a product are discussed.

A Mechanistic Model Accounting for the Effect of Soil Moisture, Weather, and Host Growth Stage on the Development of Sclerotinia sclerotiorum

The fungus Sclerotinia sclerotiorum causes serious losses to several agricultural crops worldwide. By using systems analysis, we retrieved the available knowledge concerning S. sclerotiorum from the literature and then analyzed and synthesized the data to develop a mechanistic, dynamic, weather-driven model for the prediction of epidemics on different crops. The model accounts for i) the production and survival of apothecia; ii) the production, dispersal, and survival of ascospores; iii) infection by ascospores; and iv) lesion onset. The ability of the model to predict the occurrence of apothecia was evaluated for epidemics observed with different climates, soil types, and host crops (soybean, white bean, and carrot) using independent data obtained from trials conducted in Ontario (Canada) in 1981, 1982, and from 1999 to 2002; in Michigan (U.S.A.) in 2015 and 2016; and in Wisconsin (U.S.A.) in 2016. The model showed 0.82 accuracy and 0.73 specificity in predicting the presence of apothecia, with a posterior probability of correctly predicting apothecia to be present or absent of 0.804 and 0.876, respectively. The model was also validated for its ability to predict disease progress on soybean and sunflower in Ontario in 1981 and 1982, in Manitoba (Canada) in 2001 and 2002, and in Michigan in 2015 and 2016. Comparison of model output with observations showed a concordance correlation coefficient of 0.948, and a root mean square error of 0.122. The model represents an improvement of previous S. sclerotiorum models and could be useful for making decisions on disease control.

Transcriptome Profiling Reveals Molecular Players in Early Soybean-Sclerotinia sclerotiorum Interaction

Sclerotinia sclerotiorum causes Sclerotinia stem rot on soybean. Using RNA sequencing, the transcriptomes of the soybean host and the S. sclerotiorum pathogen were simultaneously determined at 4 and 8 h postinoculation (hpi). Two soybean genotypes were involved: a resistant oxalate oxidase (OxO)-transgenic line and its susceptible parent, AC Colibri (AC). Of the 594 genes that were significantly induced by S. sclerotiorum, both hosts expressed genes related to jasmonic acid, ethylene, oxidative burst, and phenylpropanoids. In all, 36% of the differentially expressed genes encoded genes associated with transcription factors, ubiquitination, or general signaling transduction such as receptor-like kinases, mitogen-activated protein kinase kinases, and hormones. No significant differentially expressed genes were identified between genotypes, suggesting that oxalic acid (OA) did not play a differential role in early disease development or primary lesion formation under the conditions used. Looking at pathogen behavior through its gene expression during infection, thousands of genes in S. sclerotiorum were induced at 8 hpi, compared with expression in culture. Many plant cell-wall-degrading enzymes (PCWDEs), sugar transport genes, and genes involved in secondary metabolism were upregulated and could contribute to early pathogenesis. When infecting the OxO plants, there was a higher induction of genes encoding OA, botcinic acid, PCWDEs, proteases, and potential effectors, revealing the wealth of virulence factors available to this pathogen as it attempts to colonize a host. Data presented identify hundreds of genes associated with the very early stages of infection for both the host and pathogen.

Field Application of Wuyiencin Against Sclerotinia Stem Rot in Soybean

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a devastating disease of soybean. Biological control is a potential alternative to chemical fungicides for disease management, and provides broad benefits to the environment, farmers and consumers. Herein, we established a field application technique for biocontrol of Sclerotinia stem rot in soybean using wuyiencin, expanding on a previous study showing biocontrol potential. We used wuyiencin to reduce sclerotia in soybean seed, and disease incidence analysis by seed bioassay revealed an optimal wuyiencin seed soaking concentration of 12.5 μg/mL. We found that different application methods had different effects on soybean plant growth. Soybean pot experiments showed that 100 μg/mL wuyiencin was obtained a significant disease protection effect and promote soybean growth through root irrigation, and the optimal concentration for wuyiencin spraying was 100–200 μg/mL. We tested the efficacy of applying wuyiencin under field conditions, and the protection effect of 200 μg/mL wuyiencin sprayed three times was the best (64.0%), but this was slightly inferior to the protection effect of 200 μg/mL dimethachlon (77.6%).

Cosmos white rot: First characterization, physiology, host range, disease resistance, and chemical control

A new disease of Cosmos sulphureus Cav. causing external and internal stem discoloration, premature death, and wilting was observed in 27.8% of plants with an average disease severity rating of 4.4 in Gazipur, Bangladesh. Morphological, pathological, and molecular analyses identified the isolated fungus as Sclerotinia sclerotiorum (Lib) de Bary, the causative agent of white rot disease. The optimum growth and sclerotium formation of S. sclerotiorum occurred at 20°C and pH 5.0, while glucose, peptone, yeast extract, casein, and ascorbic acid were the appropriate nutrient sources. Furthermore, mycelial growth and sclerotial development were favored in media containing potassium, magnesium, calcium, and sodium. As many as 20 plant species of 10 families; Calendula officinalisi, Chrysanthemum indicum, Catharanthus roseus, Solanum tuberosum, S. lycopersicum, S. melongena, Capsicum annum, Lablab purpureus, Phaseolus vulgari, Lens culinaris, Vigna radiata, Vigna mungo, Daucus carota, Raphanus sativus, Brassica juncea, Punica granatum, Spinacia oleracea, Ipomoea batatas, Ipomoea aquatica, and Elaeocarpus serratus were identified as the new hosts of the pathogen in Bangladesh. None of the C. sulphureus and Cosmos bipinnatus germplasms screened were genetically resistant to the pathogen. Among the tested fungicides, Autostin 50 WDG (carbendazim) and Rovral (Dicarboxamide) were most inhibitory to the fungus, while Autostin 50 WDG provided an efficient control of the pathogen in vivo up to 15 days after spray. The acquired results on characterization, physiology, host range, resistance, and fungicidal control of the pathogen could be valuable for effectively managing cosmos white rot in the field.

Evaluation of the Inhibitory Effects of Wuyiencin, a Secondary Metabolite of Streptomyces albulus CK-15, Against Sclerotinia sclerotiorum In Vitro

Sclerotinia sclerotiorum (Lib.) de Bary, a destructive fungal pathogen with an extensive host range, causes various diseases with the potential to cause huge economic losses to crops worldwide. Streptomyces species produce secondary metabolites with variable structures and biological activities that offer possible control methods for crop diseases. Herein, we evaluated the inhibitory effects of wuyiencin, a secondary metabolite of Streptomyces albulus CK-15, against S. sclerotiorum. The results showed that wuyiencin markedly inhibited mycelial growth and germination and the formation of sclerotia. It also increased cell membrane permeability, resulting in leakage of intracellular substances in pathogen mycelia. Wuyiencin markedly decreased oxalic acid content and the activities of polygalacturonase and pectin methyl-galacturonic enzymes. Moreover, it downregulated Nox1, ITL, pph1, Caf1, and sca1, all genes related to growth and infection. Lesions were smaller and less pronounced on soybean (Glycine max [L.] Merr.) leaves pretreated with wuyiencin in vitro, and the inhibition rate reached 78.36%. The results suggest that wuyiencin holds promise for the management of diseases caused by S. sclerotiorum, and the findings provide clues on the mechanism of action.

Biocontrol Potential of Sclerotinia sclerotiorum and Physiological Changes in Soybean in Response to Butia archeri Palm Rhizobacteria

Sclerotinia sclerotiorum is a necrotrophic parasitic fungus that causes Sclerotinia stem rot (SSR), which is currently one of the most difficult agronomic crop diseases to control. A number of plants of the Brazilian Cerrado biome have been shown to be important sources of symbiotic microorganisms with biotechnological potential, so we decided to test the potential of bacteria isolated from the dwarf jelly palm, Butia archeri (Arecaceae) for the control of the pathogenic effects provoked by S. sclerotiorum. For this, we bioprimed seeds and evaluated the effects of this biopriming on the OJIP transient patterns prior to and following infection by the phytopathogen. Plants treated with the BA48R strain of Enterobacter sp., and in particular, those treated with the BA88R strain of Bacillus cereus presented the best results in terms of the loss/gain of the physiological and symptomatological variables evaluated. The plants bioprimed with BA88R presented high post-infection levels of total chlorophyll (33.35 FCIs) and chlorophyll a (26.39 FCIs), maintained a high Nitrogen Balance Index (NBI = 18.87), and synthesized low concentrations of flavonoids (1.39). These plants also maintained high levels of PI_ABS (1.111) and PI_TOTAL (1.300) following infection, and low levels of Di₀/RC (0.602), which indicates that, in the presence S. sclerotiorum, the efficiency of the photosynthesis in the plants treated with these bacteria was less affected in the reaction centers, as confirmed by the negative amplitude recorded in the L band. The present study reconfirms the importance of the use of chlorophyll fluorescence for the diagnosis of disease and conditions of stress in crop plants, in addition to demonstrating the effectivenesss of the BA48R bacterial strain and, in particular, the BA88R strain on systemic resistance induction and suppression of S. sclerotiorum in Glycine max plants, with enormous potential for the development of more sustainable agricultural processes.

Effect of Seed Treatment and Foliar Crop Protection Products on Sudden Death Syndrome and Yield of Soybean

Sudden death syndrome (SDS), caused by Fusarium virguliforme, is an important soilborne disease of soybean. Risk of SDS increases when cool and wet conditions occur soon after planting. Recently, multiple seed treatment and foliar products have been registered and advertised for management of SDS but not all have been tested side by side in the same field experiment at multiple field locations. In 2015 and 2016, seed treatment fungicides fluopyram and thiabendazole; seed treatment biochemical pesticides citric acid and saponins extract of Chenopodium quinoa; foliar fungicides fluoxastrobin + flutriafol; and an herbicide, lactofen, were evaluated in Illinois, Indiana, Iowa, Michigan, South Dakota, Wisconsin, and Ontario for SDS management. Treatments were tested on SDS-resistant and -susceptible cultivars at each location. Overall, fluopyram provided the highest level of control of root rot and foliar symptoms of SDS among all the treatments. Foliar application of lactofen reduced foliar symptoms in some cases but produced the lowest yield. In 2015, fluopyram reduced the foliar disease index (FDX) by over 50% in both resistant and susceptible cultivars and provided 8.9% yield benefit in susceptible cultivars and 3.5% yield benefit in resistant cultivars compared with the base seed treatment (control). In 2016, fluopyram reduced FDX in both cultivars by over 40% compared with the base seed treatment. For yield in 2016, treatment effect was not significant in the susceptible cultivar while, in the resistant cultivar, fluopyram provided 3.5% greater yield than the base seed treatment. In this study, planting resistant cultivars and using fluopyram seed treatment were the most effective tools for SDS management. However, plant resistance provided an overall better yield-advantage than using fluopyram seed treatment alone. Effective seed treatments can be an economically viable consideration to complement resistant cultivars for managing SDS.

Meta-Analytic and Economic Approaches for Evaluation of Pesticide Impact on Sclerotinia Stem Rot Control and Soybean Yield in the North Central United States

As complete host resistance in soybean has not been achieved, Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum continues to be of major economic concern for farmers. Thus, chemical control remains a prevalent disease management strategy. Pesticide evaluations were conducted in Illinois, Iowa, Michigan, Minnesota, New Jersey, and Wisconsin from 2009 to 2016, for a total of 25 site-years (n = 2,057 plot-level data points). These studies were used in network meta-analyses to evaluate the impact of 10 popular pesticide active ingredients, and seven common application timings on SSR control and yield benefit, compared with not treating with a pesticide. Boscalid and picoxystrobin frequently offered the best reductions in disease severity and best yield benefit (P < 0.0001). Pesticide applications (one- or two-spray programs) made during the bloom period provided significant reductions in disease severity index (DIX) (P < 0.0001) and led to significant yield benefits (P = 0.0009). Data from these studies were also used in nonlinear regression analyses to determine the effect of DIX on soybean yield. A three-parameter logistic model was found to best describe soybean yield loss (pseudo-R2 = 0.309). In modern soybean cultivars, yield loss due to SSR does not occur until 20 to 25% DIX, and considerable yield loss (-697 kg ha-1 or -10 bu acre-1) is observed at 68% DIX. Further analyses identified several pesticides and programs that resulted in greater than 60% probability for return on investment under high disease levels.

Spatiotemporal Distribution Pattern of Sclerotinia sclerotiorum Apothecia is Modulated by Canopy Closure and Soil Temperature in an Irrigated Soybean Field

Identifying the optimal timing for fungicide application is crucial in order to maximize the control of Sclerotinia stem rot (SSR), which is caused by Sclerotinia sclerotiorum. In this study, the impact of canopy closure and soil temperature on apothecia production was investigated to optimize fungicide application timing. Replicated soybean plots with a row spacing of 0.36 and 0.38 or 0.76 m were established in 2015 and 2016 in an irrigated soybean field at Michigan State University's Montcalm Research Center. The number of apothecia and ascospores and the incidence of SSR were monitored two times per week for 10 to 12 weeks. In both row-spacing trials, apothecia were observed earlier in 2016 (before the R1 growth stage) than in 2015 (between R1 and R2). The maximum number of apothecia was 50 times higher with the 0.36-m row spacing than with the 0.76-m row spacing in 2015 but was 2.5 times higher with the 0.76-m row spacing than with the 0.38-m row spacing in 2016, though the overall numbers were much lower in 2016. The apothecia distribution pattern was synchronized with the canopy closure pattern and the soil temperature profile. The peak number of apothecia was observed when canopy closure reached at least 50% and when average soil temperature in the row was between 21.5 and 23.5°C. In 91% of the cases, the presence of apothecia was observed when the percentage of light blocked was 70%, and no apothecia germinated in the absence of light or under full light exposure. During the first 50 days after plant emergence, the rate of canopy closure was higher in 2016 than in 2015, and the first diseased plant was observed earlier in 2016 (R2) than in 2015 (R5). Canopy closure and the distance of the sampling point from the soybean row explained much of the variability in the number of apothecia. These results can partially explain the inconsistent efficacy of fungicide applications based on the soybean growth stage and will be helpful for informing disease models and fine-tuning fungicide application strategies.