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Gray Leaf Spot
Compliments of
Maynard Ochs, Agronomic Services
With the onset of warm, humid weather, conditions are favorable for gray leaf spot (GLS)
to appear in your corn crop. In fact GLS has already been confirmed across much of the
cornbelt and is especially heavy in the western area where rains have been frequent and
high humidity prevalent. You should be scouting your cornfields for signs of this disease.
GLS is a fungal disease that infects the leaf tissue of corn plants.
During the early stages of infection, the affected areas will appear as small pinpoint
lesions surrounded by a yellow halo when held up to light. As these lesions enlarge, they
become rectangular in shape, with individual lesions restricted in width by the leaf
veins. This is different from diseases such as southern leaf blight and Stewart's wilt,
which can cross over veins and form somewhat oval shaped lesions. Warm humid temperatures
and 12 or more hours of leaf wetness each night favor the development of GLS. The optimum
temperature range for this disease is 70-85F. The leaf surface has to be wet in order for
the disease spores to germinate and infect the plant. 14-21 days from the time of
infection, the lesions will produce new spores. The newly produced spores are dispersed by
the wind and dispersal is favored by periods of dry leaf surfaces throughout the day.
With the early season onset of GLS, yield losses can be significant
depending on the amount of leaf area lost. Yields generally are hurt when GLS lesions grow
together or coalesce, killing large portions or the entire leaf. If a hybrid is very
susceptible, all of the leaves may become completely blighted. The plants will then be
prone to early senescence, stalk lodging and low testweights.
If the disease doesn’t reach the upper leaves on a corn plant
which contribute the most to grain fill or is a late season infection, GLS usually
doesn’t result in significant yield losses. Early planted corn, which is nearly
mature before any serious level of the disease is present on the upper leaves, will
generally escape economic yield losses. However, late-planted corn is at a greater risk
for significant yield losses from gray leaf spot.
As far as your particular fields are concerned, if you carefully
observed your crops last year and did not find GLS, your corn may escape the disease this
season. However, GLS usually enters non-infected fields by wind and if the disease entered
the field early in this growing season, you should monitor disease progression in your
field and possibly treat with a fungicide.
Several fungicides are currently registered for corn and if disease
lesions reach the third leaf below the ear before tasseling, there is a good chance a
fungicide application could be warranted.
It is important to continue to monitor and record disease
progression in both treated and untreated fields. This information will help you make
solid choices for your fields the following year. GLS overwinters on corn residue and the
heaviest disease potential exists in fields of continuous corn under no-till or
minimum-till practices, which had the disease the previous season. Once the disease is in
a field, inoculum builds up in the residue. Tillage, which buries the debris, speeds
decomposition and will decrease the severity of infection. Crop rotation and hybrid
selection are other viable methods of controlling GLS in a field. Even though total
resistance does not yet exist in corn hybrids, they do differ in their level of resistance
to GLS.
Several Mycogen hybrids are available that give very good levels of
protection. We offer hybrids with levels of resistance or tolerance to GLS, which protects
the plants through the grain fill period. Mycogen hybrids 2677, 2757IMI, 2815, 2828, 2853
and 2888IMI exhibit above average levels of resistance to GLS. Our popular hybrids
2544IMI, 2545, 2545IMI, 2717, 2717IMI 2722, 2799, 2799IMI, 7250, 2801 and 2832IMI express
good field tolerance to GLS. Your Mycogen Seeds sales rep can help you select a GLS
tolerant hybrid that's right for your fields.
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