Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
The UD Field Crop Pathology team surveyed numerous fields of full season and double cropped soybeans this week. Most full season beans were past R3 and starting the critical grain fill process. The most common diseases encountered were brown spot (Figure 1) and Downy mildew (Figure 2). Remember that brown spot is a fungal disease that overwinters in the soil and develops under very wet conditions. As a result, it is often only observed in the lower canopy in most fields and seasons. Symptoms include small, angular, brown lesions with yellow halos. Over time the leaves may turn yellow and defoliate. Brown spot typically is not yield-limiting, but if you think it may be impacting your yield in some full season fields, rotation and variety selection will help to reduce the initial amount of inoculum in these fields. In general, varieties with good levels of frogeye leaf spot resistance also have decent brown spot resistance.
Figure 1. Brown spot on soybeans is common, and often observed in the lower canopy. Seldom does it reach the upper third of the canopy, which contributes the majority of carbohydrates for grain fill. When you open the canopy, you may observe leaves with small, angular lesions with bright yellow boarders as well as yellow senescing leaves.
The second most common disease we have observed, particularly in areas with heavier soils or irrigated beans, is soybean downy mildew. This is not the same pathogen as the downy mildew we encounter in lima beans or cucurbits and is not in the same zip code in terms of ability to damage the crop. Growers likely will observe symptoms in the mid-canopy at this point in time where more moisture may be held for longer periods and temperatures may be slightly cooler. Symptoms include small, light green/yellow colored spots or flecks on the upper leaf surface that will have a fuzzy grey/white growth underneath. No management for downy mildew should be needed.
Figure 2. Small yellow flecks characteristic of downy mildew on soybeans. On the underside of leaves you should notice fuzzy growth. This is not the same as the downy mildew in cucurbits, which is caused by a complete different organism.
We observed only light frogeye leaf spot in a couple of fields. This disease can be problematic if it starts early and makes its way into the upper canopy during grain fill. Over the last 3 years, we have seen few instances where chemical intervention might have been required. Lesions of frogeye leaf spot are irregular and have a purplish hue (Figure 3). The centers of the lesions are grey and when flipped over you will see dark fuzzy patches at the center of the lesions. Developing tissues are the most susceptible to frogeye leaf spot, so you likely will see it in layers on plants. Frogeye leaf spot can be confused with herbicide damage that you can observe in some double crop beans at this time.
Things you should ask yourself if you see symptoms that look like frogeye leaf spot in your double crop beans: 1) Has the canopy closed? Frogeye needs extended periods of free water on the foliage to grow and develop. 2) Are the lesions small or are some large and diffuse? Large, spreading lesions are not characteristic of frogeye leaf spot. 3) Do I see fuzzy growth coming from the lesions on the leaf underside, or are lesions white and papery? 4) Are symptoms spreading to new tissues over time?
Figure 3. Frogeye leaf spot lesions in a soybean field. Note the purple margins and purplish centers. If the leaf is flipped over, particularly early in the morning when humidity is high, you may observe fuzz growing from the centers of the lesions. It is easy to confuse frogeye leafspot for other issues, especially when plants are small and can show symptoms of herbicide damage. Consider the environment, patters, and lesion size/shape. If you are not positive, send samples to the UD Plant Diagnostic clinic for a professional assessment.
As with brown spot, if frogeye leafspot has been an issue for you, select varieties rated well for resistance to this pathogen and rotate problematic fields to a non-host such as corn for at least one full season. Practices that help bury or decompose old soybean residue will also be beneficial. Fungicide sprays at R3 have been shown to be the most economical where Frogeye leaf spot has reached potentially yield-limiting levels. Unfortunately we do not have any thresholds for this disease in our area because it does not cause enough damage on a yearly basis to allow for this at the current point in time.
Lastly, we have had a couple of reports of charcoal rot in soybeans over the past 10 days. Charcoal rot is a root/stem disease caused by a fungus with a wide host range. When plants are stressed, particularly water or nutrient-stressed, the fungus is able to colonize roots and grow into the stems. The first symptoms are chlorotic or wilted plants, often in the drier parts of the field (compacted or sandier areas). Typically symptoms are not observed until the plant has put on enough foliage to significantly increase water demand, so affected plants often are not observed until after R3. If you scrape the base of the stem, you may observe small black dots with the aid of a hand lens (Figure 4). If the lower stem is sliced it likely will have a grey color to it and be speckled with these same small black dots. These dots, called microsclerotia, are characteristic of the fungus. Black lines in the stem are caused by another fungus and are not due to charcoal rot, despite what is stated in older diagnostic guides.
As far as management is concerned, double cropped beans should have fewer issues with charcoal rot as they are typically smaller and in the vegetative stages of growth during the hottest months of the season. Irrigation will minimize problems in full season beans. In dryland fields, no till may improve soil moisture and minimize stress. Other practices, such as avoiding excessive seeding rates and rotation to crops such as small grains and corn can help reduce initial inoculum present in the soil. Fungicides will not help with charcoal rot management.
Figure 4. Symptoms of charcoal rot in a soybean stem. Note the black specks. Image from https://ag.purdue.edu
Factsheets on frogeye leaf spot and other information on soybean disease management, such as the Delaware Field Crop Disease Management Guide, can be found on the University of Delaware Field Crops Pathology website located at http://extension.udel.edu/ag/plant-pathology-and-diseases/commercial-field-crops/. Other information on soybean diseases in Delaware can be found at the Field Crop Disease Management Blog http://extension.udel.edu/fieldcropdisease/category/soybean/. If you need help identifying diseases on your plants, send a sample to Nancy Gregory at the University of Delaware Plant Diagnostic Clinic.