One hundred fifty-two Diaporthe isolates were recovered from symptomatic soybean (Glycine max) stems sampled from the U.S. states of Iowa, Indiana, Kentucky, Michigan, and South Dakota. Using morphology and DNA sequencing, isolates were identified as D. aspalathi (8.6%), D. caulivora (24.3%), and D. longicolla (67.1%). Aggressiveness of five isolates each of the three pathogens was studied on cultivars Hawkeye (D. caulivora and D. longicolla) and Bragg (D. aspalathi) using toothpick, stem-wound, mycelium contact, and spore injection inoculation methods in the greenhouse. For D. aspalathi, methods significantly affected disease severity (P < 0.001) and pathogen recovery (P < 0.001). The relative treatment effects (RTE) of stem-wound and toothpick methods were significantly greater than for the other methods. For D. caulivora and D. longicolla, a significant isolate × method interaction affected disease severity (P < 0.05) and pathogen recovery (P < 0.001). Significant differences in RTEs were observed among D. caulivora and D. longicolla isolates only when the stem-wound and toothpick methods were used. Our study has determined that the stem-wound and toothpick methods are reliable to evaluate the three pathogens; however, the significant isolate × method interactions for D. caulivora and D. longicolla indicate that multiple isolates should also be considered for future pathogenicity studies.

Fusarium root rot of corn (Zea mays L.) is yield-limiting in the United States, but there is no information available on the disease in South Dakota. In 2015, corn seedlings with discolored roots were arbitrarily sampled from 50 South Dakota fields, and 198 isolates were recovered. Eight species (F. acuminatum, F. boothii, F. equiseti-incarnatum complex, F. graminearum, F. oxysporum, F. proliferatum, F. solani, and F. subglutinans) were identified by morphology and translation elongation factor 1-α gene sequencing. F. graminearum (26.8%) was the most common fungus, and F. boothii (0.5%) was the least recovered. Fourteen isolates, representing the eight species, were evaluated for their pathogenicity on 2-week-old seedlings of inbred ‘B73’ using the inoculum layer method in the greenhouse. Fourteen days postinoculation, root rot severity was evaluated on a 1-to-5 rating scale and expressed as relative treatment effects (RTEs). F. proliferatum isolate P2 caused significantly greater RTE (based on 95% confidence intervals) on seedlings than the other isolates and the noninoculated control, except F. graminearum isolate FG23. This study indicates that the eight species of Fusarium are aggressive root rot pathogens of corn in South Dakota, and this information will help evaluate strategies for producers to manage these pathogens in their fields.

Diaporthe caulivora and D. longicolla are the causal agents of stem canker of soybean (Glycine max L.). Accurate identification of stem canker pathogens upon isolation from infected soybean plants is difficult and unreliable based on morphology. In this study, two TaqMan probe-based quantitative polymerase chain reaction (qPCR) assays were optimized for detection of D. caulivora and D. longicolla in soybean plants. The assays used previously reported D. caulivora-specific (DPC-3) and D. longicolla-specific (PL-3) probe/primer sets. The sensitivity limit of the two assays was determined to be over a range of 100 pg to 10 fg of pure D. caulivora and D. longicolla genomic DNA. The qPCR assays were validated with plant samples collected from commercial soybean fields. The PL-3 set detected D. longicolla in soybean plants collected from the fields (quantification cycle value <35), which was confirmed by isolation on potato dextrose agar (PDA). D. caulivora was detected only in low levels (quantification cycle value <40) by DPC-3 set in a few of the symptomatic field samples, although the pathogen was not isolated on PDA. The qPCR assays were also useful in quantitatively phenotyping soybean plants for resistance to D. caulivora and D. longicolla under greenhouse conditions.

Diaporthe seed decay can compromise seed quality in soybean [Glycine max (L.) Merr.] in the warm and humid production areas of the United States during crop maturation. In the current study, 45 isolates of Diaporthe were recovered from seed sampled from soybean fields affected by Diaporthe-associated diseases in eight U.S. states in 2017. The isolates obtained belonged to 10 species of Diaporthe based on morphology and phylogenetic analyses of the internal transcribed spacer, the partial translation elongation factor 1-alpha, and beta-tubulin gene sequences. The associated species included D. aspalathi, D. caulivora, D. kongii, D. longicolla, D. sojae, D. ueckerae, D. unshiuensis and three novel fungi, D. bacilloides, D. flavescens and D. insulistroma. One isolate each of the 10 species was examined for pathogenicity on seed of cv. ‘Sava’ under controlled conditions. Seven days post-inoculation, significant differences in the percentages of decayed seeds and seedling necrosis were observed among the isolates and the non-inoculated control (p<0.0001). While the isolates of D. bacilloides, D. longicolla, and D. ueckerae caused significantly greater percentage of decayed seeds (p<0.0001), the isolate of D. aspalathi caused the greatest seedling necrosis (p<0.0001) when compared to the non-inoculated control. The observation of new fungi causing Diaporthe seed decay suggests the need for a more comprehensive survey in the U.S. soybean producing areas since members of the genus Diaporthe appear to form a complex that causes seed decay.

No abstract