These lesions, mostly found on upper leaf surfaces, are irregularly circular and consist of red-brown to purple colored borders.

Oval and circular lesions that form on the pods and stems have dark-colored margins and light gray to reddish-brown color on the inside.

With this range of susceptibility, the disease can occur several times throughout the plant canopy if the environment is ideal for the pathogen at each leaf layer emergence.

During Stages R1-R6 in soybeans and after a period of frequent rainfall is the best time to scout on frogeye leaf spot.

Cercospora sojina, the causal fungal agent, and Frogeye Leaf Spot, the resulting disease, have a polycyclic infection cycle.

In the winter, the pathogen is present as mycelium in crop residue and in leftover soybeans from the previous harvest.

Managing the pathogen is most successful when the disease is recognized close to flowering time and before growth stage R5 or at the beginning of seed development.

[4] Cercospora sojina is a fungal pathogen that varies in colony color, growth rate, and spore formation in culture.

Certain gene clusters in the fungus genome encode for secondary metabolites, such as mycotoxins and pigments, and virulence effectors.

The production of these metabolites is elevated during early infection and most likely play a key role in the fungus and plant interaction (Luo et al., 2017[5]).

More specifically, when comparing nonresistant and resistance strains of the pathogen from China, researchers observed 5 candidate genes that are linked to virulence.

The gene's function was related to metabolic mechanisms and the production of metabolites that can cause reduction in host resistant soybean plants (Gu et al., 2020).

If a producer has a field with continuous production of soybeans, there is a higher chance of frogeye leaf spot typically present.

If a large amount of lesions are present on the leaves of a soybean there is a loss of leaf area index which in turn results in less photosynthesis.

Rotation with crops not susceptible to the disease, like corn and small grains, can be a form of cultural control of frogeye leaf spot.

In the United States, Frogeye leaf spot has been reported to have resistance to Quinone outside inhibitors (strobilurins).

None of these methods have been proven to be better than others but by using multiple of these management tools a grower will help reduce the amount of inoculum available in the field and assist in protecting the plants from infection.

Since 2000, the fungus has been present in the northern and southern U.S. states as well as 27 other countries spanning North and South America, Europe Africa and Asia (Lin and Kelly 2018[2]).

The fungus is very prevalent in the southern U.S., but in previous years it has spread to northern soybean fields (Smith, 2020[8]).

New versions of the fungal genome have also been observed across the U.S., Brazil, China and other regions where soybeans are grown that have varying virulence and resistance levels (Lin and Kelly, 2018; Gu et al., 2020[6][2]).

Warmer winter temperatures have been suggested as a possible contributor to an increase in Frogeye Spot disease as well as susceptible soybeans and conservation tillage.

[2] If leftover residue from infected plants is not removed and crop rotation does not occur, the fungus will continue to develop into the next growing season.

3 Mueller, Daren, Kiersten Wise, Adam Sisson, Damon Smith, Edward Sikora, Carl Bradley, and Alison Roberstson, eds.