NPIPM:Frogeye leaf spot on soybean
Compiled by: Buyung Hadi, from the materials by: Dean Malvick and Lawrence Osborne
First reported in the United States in 1924, frogeye leaf spot is an annual disease problem of soybean production in the southeastern states. Outbreak of frogeye leaf spot was recorded in Central Iowa in 2001 and its incidence was reported for the first time in southern Wisconsin in 2002. The northward progression of frogeye leaf spot distribution may be facilitated by relatively warmer winter temperatures, usage of susceptible soybean varieties and widely practiced conservation tillage which may retain pathogen-infested plant debris as inoculum source in the field.
Causal Organism
Frogeye leaf spot is caused by Cercospora sojina. Twelve races of C. sojina have been reported in the United States, although more races are likely to exist. Each race of C. sojina expresses variable abilities to cause disease on different soybean varieties.
Symptoms and Signs
Frogeye leaf spot primarily infects soybean leaves although soybean stems, seeds and pods are also susceptible. The most common symptoms of frogeye leaf spot are circular spots on leaves up to 5 mm in diameter. Leaf spots are without halo, typically light gray in the center with purple to red-brown outer margin. Leaf spots may coalesce to form larger lesions with irregular margins.
Symptoms on stems usually appear late in the season under heavy foliar infection. On stems, lesions appear elongated with red-brown center that turns light gray with time. Infected pods develop lesions similar to stem lesions. Seeds in close contact with pod lesions can be infected, developing gray or brown discoloration. Symptomatic area on seed vary in size, covering the entire seed in severe cases.
Disease Impacts
Frogeye leaf spot reduces photosynthetically active area of the leaves. Leaves with large lesions of merging spots may drop prematurely. Infected seeds have poor germination. Even if the infected seed germinates, the resulting seedling suffers from reduced vigor. Yield losses seemed to be more severe in the southern states. Significant yield losses of 30-73% is reported from infected susceptible soybean varieties in the southern US. Reports from Indiana and Illinois mentioned 21% and 10% yield losses from frogeye leaf spot on susceptible varieties.
Life Cycle and Epidemiology
Cercospora sojina survives the winter in infected seeds and plant debris left in the field. No other plant has been confirmed as alternative host of the fungus. Transparent conidia (spores) are produced on infected plant residues and infected seeds. Conidia may travel in short distance by wind or rain splashes. Under the temperature range of 77-86˚F (25-30˚C) and in the presence of water, the fungal conidia can germinate within an hour upon arrival on healthy soybean tissue and start the infection process. Young leaves are more susceptible than older soybean leaves. In areas with warm (77-86˚F or 25-30˚C) and humid (>90% relative humidity) conditions, new spots may produce conidia rapidly exacerbating the extend of leaf spot epidemic. As the fungus requires water presence to germinate, areas or seasons with low rainfall may witness sporadic frogeye leaf spot incidence. On the other hand, frequent rainfall increases the incidence of frogeye leaf spot.
Management Approaches
Cultural Methods
To control frogeye leaf spot, pathogen-free seeds should be used. In areas with history of severe frogeye leaf spot infection, soybean residues should be buried before the next planting season. Two-year rotation with non-bean crops should also be considered.
Host Plant Resistance
Soybean varieties containing Rcs3 gene are currently resistant towards all known Cercospora sojina races in the US, although only a few are available for the northern states.
Chemical Control
In areas with severe frogeye leaf spot infection, foliar fungicide application from bloom to early pod set is reported to reduce disease severity and increase seed yield. Recently, strobiluron fungicide resistance for a C. sojina isolate collected from Tennessee was detected. The resistant C. sojina isolate required 200 to 7,000 times higher fungicide concentration than nonresistant pathogen isolate. To avoid development of fungicide resistance on frogeye leaf spot pathogen, it is important to limit the usage of fungicide to when it is really necessary.
