With the recent rainfall received in the state, conditions have become more favorable for some foliar corn diseases that are now being observed. UK Plant Pathology interns have been scouting corn fields in western Kentucky for the past few weeks, and they have been observing northern leaf blight, gray leaf spot, common rust, and Diplodia leaf streak. As of July 12, southern rust has not yet been identified by UK Plant Pathology personnel in the state.
Below are my thoughts about these diseases and the use of foliar fungicides to manage these diseases.
Northern Leaf Blight
Northern leaf blight (Figure 1) can be a major yield robber in susceptible corn hybrids. Symptoms of this disease are observed on leaves as elliptical (“cigar-shaped”) lesions that have a light to tan-colored center. Some hybrids may have excellent resistance to northern leaf blight, while others may be extremely susceptible. It is important to know how susceptible a hybrid is to this disease to be able to determine the risk of disease occurrence and likelihood of a profitable foliar fungicide application.
This disease may be easily confused with Diplodia leaf streak (Figure 2), which also may appear as elliptical-shaped lesions on leaves. Lesions of Diplodia leaf streak will contain black pin-sized structures known as “pycnidia” inside lesions, which can help distinguish it from northern leaf blight. Diplodia leaf streak appears to still be a minor disease in Kentucky, and management of this disease is likely not needed at this time.
Gray Leaf Spot
Gray leaf spot (Figure 3) is likely the most commonly-observed foliar disease of corn in Kentucky. It is observed as rectangular lesions that occur on leaves. Hybrids can vary greatly in their level of susceptibility to this disease, but no hybrid is completely resistant. Conditions have been very favorable for gray leaf spot over the last several days. On susceptible hybrids, I expect that gray leaf spot will be very apparent within the next few days. Disease progress can be “sluggish” with this disease at first, since it can take several days between the time of initial infection and the appearance of symptoms; however, severity can increase suddenly when conditions are favorable.
Common rust has been widely observed in the state for several weeks now. In general, most yellow dent corn hybrids have an adequate level of resistance to common rust, and management of common rust with foliar fungicides in yellow dent corn hybrids is not generally needed.
Although southern rust has not yet been confirmed by UK Plant Pathology personnel in Kentucky for the 2016 season, it has been moving northward. To date and to my knowledge, Arkansas is the furthest north that this disease has been confirmed during the 2016 season. Since most hybrids are susceptible to southern rust, it is important to be able to be able to differentiate southern rust from common rust (Figure 4). A free webcast explaining how to differentiate these two diseases is available here.
The timing of southern rust’s appearance in Kentucky relative to corn growth stage is critically important in regards to risk of yield loss. From my program’s 2015 fungicide trials in Princeton, KY and in prior years when I was at the University of Illinois, research results showed the following (in general): if southern rust begins to appear after the R3 (milk) stage, yield loss caused by southern rust will be negligible (although disease levels can still appear to be very severe).
Foliar Fungicide Decision Making
When making decisions about applying a foliar fungicide to corn, the following are all important factors to consider: scouting observations, disease risk assessments, and economical considerations.
My research at the University of Illinois showed, when tested under 45 different environments, foliar fungicide applications made between tassel and silking stages to corn were most likely to be profitable when foliar diseases ended up affecting at least 10% of the ear leaf area in the non-treated checks. In Figure 5, the mean yield responses (relative to the non-treated checks) and the percent frequency of achieving a certain yield response (relative to the non-treated checks, from at least 3 bu/A to at least 11 bu/A) are shown. These are shown under different scenarios: overall – the 45 different environments, under low disease pressure (when final disease severity was less than 10% in the non-treated checks – 28 of the environments), and under moderate to high disease pressure (when final disease severity was at least 10% in the non-treated checks – 17 of the environments). These results clearly demonstrate that the odds of achieving a profitable fungicide application are greatly improved when foliar disease was present at a significant level.
By Carl A. Bradley, Extension Plant Pathologist