Over the past few weeks, there have been increased reports of spotting on corn leaves across the state. Symptoms include tiny tan to brown spots with a darker brown margin (Figure 1). These spots can be just a few spots scattered on a leaf or can be clustered on the leaf and large sections of the leaf surface (Figure 2).
These symptoms are easily confused with the disease southern rust, or even insect damage. Symptoms are generally observed in the mid to lower canopy, and are more noticeable on field edges.
This phenomenon has been observed for the past two years in Kentucky, and in other states to our south. To date, no pathogen has been observed in conjunction with the symptomatic leaves, and attempts to isolate a pathogen from symptomatic leaves have been unsuccessful.
Because we have not been able to associate a pathogen with these symptoms yet, we are classifying these symptoms as abiotic in nature. It is possible that this is a physiological crop response to stress, such as heat or moisture stress. Each year the symptoms have appeared after tasseling and into grain fill, during the hottest part of the summer. We have also observed hybrid differences in symptom severity.
We are still learning about this disorder, but as of now, no management is necessary, and it is important to distinguish this disorder from foliar diseases, like southern rust, to prevent unwarranted fungicide applications.
More information about how to identify southern rust can be found in a previous article of Kentucky Pest News here.