Frogeye Leaf Spot of Soybean
Symptoms and Signs:
- Small, gray spots with reddish-brown to purple borders (top of leaf)
- Underside of leaf, lesions are brown to gray with tiny dark “hairs” or conidia
- Premature leaf drop
- Infection of stems and pods can also occur
- Overwinters on crop residue left on the soil surface.
- Spores disperse by splashing rain onto young leaves.
- In 7 to 12 days, spots begin to appear depending on temperature.
- Conidia develop in primary lesions to infect new leaves throughout the growing season.
- If this cycle begins early in the season, disease severity may impact yield and defoliate the plant prematurely.
Host resistance: Plant varieties that are resistant to Frogeye leaf spot. This disease is effectively managed via single genes (Rcs genes), in which Rcs3 is still effective against all U.S. populations, including Ohio.
Identification: Scout susceptible varieties for the presence of frogeye leaf spot. The scale on figure 4 may be used to estimate the percent leaf area affected. Fungicides have been shown to be highly effective when 1 or 2 lesions were found every 25 feet at soybean growth stage R2.
Cultural Practices: Burying residue and/or crop rotation is very important. Crop residue should be fully buried. If residue cannot be buried then crop rotation to non-host crop is the next best step. C. sojina can overwinter in Ohio and planting soybeans back into infested residue increases the chance of an epidemic occurring the following season. Soybeans should not be planted for at least one year followed by planting resistant variety.
Fungicide applications: If lesions are found prior to growth stage R3, it may be wise to use chemical applications when using susceptible varieties. Applications may not be necessary if hot, dry weather is in the forecast, as this will arrest development of the disease. Fungicide resistance to Quinone outside inhibitors (QoI) in several areas in Ohio (see fact sheet).