by: Horacio Lopez-Nicora, Stephanie Karhoff, OSU Extension
In early August we recommended to start scouting fields for soybean diseases. At that time (two weeks ago), disease incidence across Ohio was very low to moderate. Conducive environmental conditions, however, are turning things around and more fields are developing disease symptoms.
Sudden Death Syndrome (SDS)
We are finding fields in Ohio severely affected by sudden death syndrome (SDS) [Fig.1 and Fig. 2]. SDS is caused by the fungal pathogen Fusarium virguliforme. This species is the most prevalent in the region, however, other Fusarium species can cause SDS. SDS above-ground symptoms can be confused with those produced by a different fungus (Cadophora gregata) that causes brown stem rot (BSR). To distinguish SDS from BSR, symptomatic plants should be dug out and stem cut open longitudinally. SDS-infected plants have white, healthy-looking pith, while BSR-infected plants present brown discoloration of the pith. Moreover, fields with severe SDS symptoms can also have high levels of soybean cyst nematode (SCN). Visit here for more information on SDS.
Figure 1. Soybean field in south Ohio severely affected by sudden death syndrome (SDS) with premature defoliation in the R5/R6 growth stage (A); symptoms begin with interveinal yellowing (chlorosis) of leaf (B); eventually leaf tissue dies and becomes brown but veins remain green (C). The fungus infects the root and produces toxins that are responsible for the above-ground symptoms.
If you have SDS, we encourage you to submit a sample to the Soybean Pathology and Nematology Laboratory in the Department of Plant Pathology at The Ohio State University in Columbus (see address below). We will confirm if it is SDS or BSR; additionally, if it is SDS, we want to determine what Fusarium species is the causal agent. To submit samples, dig out three to five symptomatic plants (including roots), placed them in a plastic bag, and submit them to our lab. Do not hesitate to contact your extension educator or us if you have any questions.
Bacterial Blight, White Mold, and Phytophthora Root and Stem Rot
Recent rainstorms with high winds and lower temperatures favored the development of bacterial blight (caused by Pseudomonas savastanoi pv. glycinea) in different parts of Ohio (Fig. 3). Angular brown lesions surrounded by chlorotic halo appear first in the upper canopy. Visit here for more information about bacterial blight of soybean.
We are also finding more fields in Ohio with white mold, a fungal disease caused by Sclerotinia sclerotiorum. To scout for this disease, we recommend walking soybean fields and looking in-between rows. A white fluffy mass of fungal mycelia will be observed in infected plants (Fig. 4). Black round sclerotia will be present amidst the white mycelia. Visit here for more information about scouting for white mold of soybean.
Figure 4. White mold of soybean in northeast (A, photo credit: Lee Beers) and south (B, photo credit: James Morris) Ohio. White, fluffy fungal mycelia (red circle) and sclerotia (yellow circle) on stem of infected soybean plants. Eventually, infected plants will wilt and die. Note how many soybean pods are lost when plants are affected by white mold compared to healthy plants (B).
We continue to receive samples with plants affected by Phytophthora root and stem rot. Commonly, these samples come from fields with poor drainage. Phytophthora root and stem rot can sometimes be confused with stem canker and white mold. You are welcome to submit samples to the Soybean Pathology and Nematology Lab for diagnosis. Visit here for more information about scouting for Phytophthora root and stem rot in soybean.
Frogeye Leaf Spot
We are finding frogeye leaf spot in our fungicide trials in north and south Ohio (Fig. 5). Frogeye leaf spot is caused by a fungal pathogen (Cercospora sojina) which can reduce yield if plants are severely affected between R3 to R5 soybean growth stage. We encourage growers to submit samples with frogeye leaf spot lesions to our lab. The fungus can develop resistance to fungicide, and we want to determine if these populations are present in Ohio. Best way to submit frogeye leaf spot samples to our lab is by placing symptomatic leaves in a plastic Ziploc bag and mail it to our lab as soon as possible. Keep samples in cool conditions and avoid exposure to sunlight and heat. Visit here for more information on frogeye leaf spot.
Figure 5. Frogeye leaf spot symptoms (A) in soybean plants at R3/R4 and R5 growth stage in north and south Ohio, respectively. Lesions (A) present conidiophores which produce conidia and look like whiskers (B). Spores (i.e., conidia) are club-shaped (C).
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