Frogeye Leaf Spot of Soybean
The causal agent of Frogeye leaf spot is the fungus Cercospora sojina. In the lab, the fungus sporulates best on V8 or lima bean agar, producing elongate to fusiform conidia. Conidia germinate when submerged in water for at least one hour, however, in culture, conidia will germinate readily without special conditions. In 2008, one study classified 93 isolates of C. sojina collected worldwide into 11 races. More diversity among races as well as more physiological races have been detected.
Symptoms and Signs:
Symptoms are commonly observed shortly after flowering to early maturity on leaves in the top canopy. Young leaves are extremely susceptible while older leaves are more resistant. Lesions appear as small, gray spots with reddish-brown to purple borders (see photo). On the underside of the leaf, the lesion appears brown to gray with tiny dark “hairs”. These hairs are the long conidia, or infective spores, of the fungus. Smaller lesions may coalesce to form larger, irregular spots on leaves. In severe cases, frogeye leaf spot can cause premature leaf drop and, if rainfall and humidity persist, stems and pods may also become infected. Lesions on pods are reddish brown, shrunken, and circular to elongate in shape. Older lesions on pods become brown to dark gray, usually with a narrow, dark brown border.
Extended periods of wet weather during the growing season will favor disease development. The pathogen overwinters on crop residue left on the soil surface. Rain splashing on the residue will carry spores to young leaves. It takes 7 to 12 days after spores infect the plant for symptom development, depending on the temperature. From these primary lesions, more conidia develop which can spread to new leaves on the plant throughout the same growing season. Spores can also be carried by strong winds to surrounding fields. Hurricane Dennis in 2005 is believed to have brought this fungus to Ohio based on sentinel plot observations that year. If the first symptoms of this disease are detected late in the season (at or after growth stage R4-R5) there is very little impact on the plant. However, if this cycle begins prior to or at flowering, then substantial amounts of disease can develop on plants that will impact yield.
Previously, this disease was believed to be a problem limited to Southern states due to low survival rate of inoculum in freezing temperatures. However, a 2007 study in Ohio monitored the residue from two soybean fields, which were heavily infected with C. sojina, throughout the winter. At both locations, conidia of this fungus were recovered throughout the winter of 2007 and, more importantly, into the spring. Thus, this fungus could indeed overwinter here in Ohio.
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 in the photo below 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: In fields where very high levels of disease develop, burying residue and/or crop rotation become 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: When frogeye lesions are found on plants prior to soybean growth stage R3, fungicide applications may be warranted on highly susceptible varieties. This is entirely dependent on the occurrence of weather conditions that continue to favor infection and lesion development. Hot, dry conditions will arrest disease development and prevent further spread of the disease in the crop canopy. Pathogen resistance to Quinone outside inhibitors (QoI) has been detected in Ohio.
Isolates of Cercospora sojina recovered from frogeye leaf spot samples were tested for strobilurin fungicide sensitivity by detecting the presence of the G143A mutation that confers QoI fungicide resistance. The presence of mutant results in several counties across Ohio suggests the continued development of strobilurin fungicide resistance in frogeye leaf spot populations.