Gray Leaf Spot Severity Increasing Rapidly
July 10, 2009
Figures 1 and 2. Gray leaf spot lesions on corn.
Gray leaf spot severity increased rapidly this week and is earlier than normal, indicating the potential for a severe gray leaf spot epidemic in 2009 if conducive conditions persist. Foliar fungicide applications will likely be required in many high risk fields.
In southeast and south central Nebraska lesions have been found as high as the ear leaf, where as many as 30 small lesions were evident. In northeast Nebraska some of the affected corn fields have not yet begun to tassel.
Conducive Conditions and Life Cycle
|It is especially concerning that the disease has progressed to the ear leaf this early in the season. The ear leaf and those above it contribute to about 70% of the yield.|
This disease is an annual problem to some extent in Nebraska corn. It is caused by the fungus Cercospora zeae-maydis which overwinters very effectively in the residue from previously infected corn crops. Fungal spores, called conidia, are produced in the crop residue and splash up to the lower leaves of plants. If rain or irrigation water is available or there is high canopy humidity (>90%) for at least 12 hours, these spores may germinate and infect the plant.
The optimal temperature range for infection is 70-90°F, which is consistent with recent weather. After infection, lesion development and further spore production on the leaf takes 14-28 days, depending on conditions and hybrid resistance. The infection cycle repeats as long as favorable conditions persist and new spores are splashed further up the plant, causing new infections. Severe infections eventually can result in severely blighted leaves which can reduce grain fill by up to 50%.
Two to three weeks ago there was frequent rain and several consecutive days of high humidity which likely led to this seemingly 'overnight' development of gray leaf spot lesions. The disease is likely to worsen after this past week's showers, fog, overcast conditions, and mild temperatures.
Resistant hybrids and ratings for gray leaf spot reaction are available from more than 60% of the companies in Nebraska. Although resistant hybrids may still develop disease, resistance slows disease progression by limiting lesion development and slowing advancement up the plant. Resistant hybrids may provide enough protection to delay fungicide applications or, in some cases, to avoid them all together.
Since the ear leaf and those above it contribute the most to yield (approximately 70%), it is especially concerning that the disease has already progressed to them this early in the season. It will be important to protect these leaves. Fungicide applications may be necessary to slow disease progression and should be seriously considered where the disease has reached the ear leaf.
Scout corn regularly to monitor for the development and spread of gray leaf spot and other diseases. Accurate disease diagnosis is critical as other diseases, such as Goss's bacterial wilt and blight and holcus bacterial leaf spot, are also present at this time and cannot be managed with fungicides.
Prioritize fields in the greatest need of fungicide applications based on
- disease severity,
- hybrid susceptibility,
- weather conditions,
- disease history, and
- other high risk factors, such as continuous corn.
Cost Recovery. Consider whether the application cost can be recovered given current corn prices. Fungicide application costs have increased to approximately $25/acre (including the application), requiring an increase of at least 7-8 bushels per acre to cover the cost.
Timing. Most fungicide applications have historically been made to corn at tasseling (VT) or later, so there is limited data on pre-tassel applications to corn. Previously, there was some evidence of phytotoxic injury when fungicides tank-mixed with other products were applied to corn prior to tassel emergence, particularly when used with non-ionic surfactants. Due to these occurrences, some companies altered their recommendations.(See Foliar Fungicides On Corn: Label Changes And Potential Phytotoxic Effects in the March 7, 2008 CropWatch.)
The protection provided by most fungicides labeled for this treatment will likely wear off within 21 days, leaving the crop vulnerable to disease at the critical grain fill period. It is possible to have late season onset of other diseases, such as southern rust, as happened in 2006 and 2007 in parts of Nebraska, or even further progression of gray leaf spot, both of which can be severe later in the season and require a fungicide application. At this point, however, the likelihood of severe southern rust development is probably less than that of having yield-limiting levels of gray leaf spot.
As always, read and follow product label guidelines prior to fungicide use and consult the most current application recommendations from the manufacturer to minimize the risk for potential phytotoxic effects to corn.
I'd like to thank crop consultants from across the state for contributing their observations.
For more information about gray leaf spot and management with foliar fungicides, see:
- Gray Leaf Spot of Corn, UNL Extension NebGuide G1902
- Plant Disease Central, the UNL Extension Plant Pathology team Web site, which includes the results of recent foliar fungicide trials in Nebraska.
Extension Plant Pathologist