Fungal pathogens can be very damaging to soybean (Glycine max (L.) Merrill) production reducing both yield and quality. Environmental conditions can increase disease severity and favor the spread of certain pathogens. In the Mid-Atlantic, we continue to observe extreme weather events and periods of prolonged rainfall that can lead to widespread fungal infection and reduced seed quality. A survey was established in 2019 to document which soilborne pathogens are most commonly observed across the region. This project identified numerous fields with Charcoal rot caused by Macrophomina phaseolina and fields with various diseases caused by the Phomopsis/Diaporthe complex. This survey was continued in 2020 and in-season diseases caused by Diaporthe species were in over 60% of the surveyed fields. Symptoms from Diaporthe related diseases can resemble sudden death syndrome (SDS) and brown stem rot (BSR). It is likely Diaporthe has been an issue in the region, but has been mistaken as other diseases. Host resistance is available to certain Diaporthe species, but we do not have a baseline understanding of the differences in aggressiveness or yield
loss potential of the species that have been identified in the region. Through greenhouse screening and inoculated studies, this project seeks to characterize observed species to improve management recommendations.

1) Assess the agressiveness of Diaporthe isolates from Mid-Atlantic soybean field.
2) Estimate potential yield effects of Mid-Atlantic Diaporthe isolates.
3) Share research findings through extension events and use findings to inform future management trials.

Findings from this project will be shared through the University of Delaware’s Weekly Crop Update, which reaches over 700 growers, consultants, and stakeholders and provides a platform to discuss disease concerns and other production issues. Data will also be shared through training events and extension presentations such as Mid Atlantic Crop School, and the 2021 Delaware Ag week. Little is known about resistance to various Diaporthe species in currently available soybean cultivars. Improved understanding of variation in local isolate aggressiveness will inform viability of research to investigate resistance as a viable management option and if yield losses are significant enough to merit additional projects to expand the limited management options currently available.

Updated July 30, 2021:
From 2019 and 2020 surveys, D. longicolla, D. ueckerae, and D. aspalathi were identified as Diaporthe species impacting soybeans in DE/MD. Greenhouse trials were established to screen pathogen aggressiveness using an inoculated toothpick method. Briefly, pure culture plugs of each species were placed onto Petri dishes covered in sterilized toothpicks. The dishes of inoculated toothpicks were incubated for two weeks and colonized toothpicks were used to inoculate soybeans. Two 75-pot trials were conducted containing three soybean varieties, Hawkeye, GH3934X, and S39-G2X. Each variety was then inoculated as one of five treatments: no wound control, sterile toothpick control, D. longicolla toothpick treatment, D. ueckerae toothpick treatment, and D. aspalathi toothpick treatment, with five replications of each treatment. Plants were inoculated at the V3 growth stage and rated for disease at 37 days after inoculation. Ratings were based on incidence of disease or death, percentage of necrosis on the stem tissue, and number of pods formed. Microplot trials were established to look at inoculations in outdoor settings. Pots were seeded in early July, but replanted July 26 after drainage was improved.

Updated January 3, 2022:

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Soilborne pathogens can reduce soybean yield and quality. Field sites across DE and MD were surveyed during 2019-2020 where multiple species of the Diaporthe/Phomopsis disease complex were identified. Diaporthe longicolla was the dominant pathogen, but D. ueckerae and D. aspalathi were also collected. Little is known on the differences in relative aggressiveness or yield impacts among these species. Project objectives included: 1) Assess the aggressiveness of Diaporthe isolates collected from Mid-Atlantic soybean fields. 2) Estimate potential yield effects of Mid-Atlantic Diaporthe isolates. 3) Share research findings through extension events and use findings to inform future management trials. In this trial, D. longicolla was shown to have the highest disease severity on all varieties screened. The newly identified D. ueckerae had higher disease severity than D. aspalathi in two of the varieties screened. These findings highlight the importance of continuing to understand pathogen dynamics and distribution of Diaporthe species to inform management decisions.

Diaporthe sp. are an important group of fungal pathogens causing diseases such as seed decay, pod and stem blight, and stem canker that result in quality and yield losses in soybean production. Since these diseases affect different areas on the plant, it is possible for a single plant to be infected by more than one species at a time. While it is generally understood that these pathogens are widespread in the Mid-Atlantic, prior to 2019, little work had been done to understand the relative distribution or abundance in the region.