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Date: 25 Jul 2007
Source: JournalGazette Times-Courier [edited]
<http://www.jg-tc.com/articles/2007/07/25/features/farm/doc46a6ba3c57af4646062985.txt>

Illinois farmers report cases of gray leaf spot infestation in some corn crops

Gray leaf spot (GLS) of corn is being reported in several areas across the state. The fungus _Cercospora zeae-maydis_ causes this common leaf blight. Gray leaf spot has easily identifiable symptoms in a susceptible variety. The lesions are rectangular, which is unusual for fungal leaf blight, and not very large ranging from about
3 mm [0.12 inches] by 1-6 cm [0.39 - 2.36 inches] and a light tan color. Lesions can turn a slight grayish color as they mature and the tissue becomes fully necrotic but this is really a fairly vague difference in color. The disease probably would have been a lot more aptly named as tan rectangle blight.

The most severe gray leaf spot epidemics usually occur in continuous corn production fields where there is also a substantial amount of corn debris that was infected in the previous season and remains on the soil surface. Not to suggest the spores of the fungus are not blown by the wind into fields that don't have this type of production, but earlier and more severe infections are much more likely under the situation of high infested residue and continuous corn. Something to think about with the recent penchant for continuous corn.

Spore production from previously infested residue starts usually in mid to late June depending on environmental conditions. Spores landing on leaves require relative humidity of 95 percent for successful infection. This disease does not have a rapid turn-around from spore to lesion to new spore production like common rust that has been our biggest foliar problem to date this year [2007]. For gray leaf spot you will be looking at about an 18-day turn around.
This is good in the sense that during the most critical time for leaf blights to cause yield loss -- 2 weeks before tasseling to 2 weeks after tasseling -- what you see is what you get. But the long turnaround time can be very problematic if the lesions all appear right about the time that it's too late to do anything to reduce yield loss.

Progress has certainly been made in the past several years in developing hybrids with some level of resistance to the disease. As with all resistant hybrids, keep in mind that resistance does not mean the hybrid will not be infected by the disease. Resistance in a resistant hybrid can be expressed in a number of ways. Commonly the effect of resistance will be expressed something along the lines of smaller lesions produced, fewer spores subsequently produced from those lesions, longer time for lesions to develop or fewer overall lesions produced. The resistant hybrids will have lesions develop but the lesions will not 'look' the same as a characteristic lesion on a susceptible hybrid.

Fungicides are useful when conditions favor diseases, especially if you are producing hybrid seed. Apply during the 14-day period before and after tasseling if disease is present at about 15 percent whole plant infection in field corn. Multiple applications may be necessary when disease pressures are high for seed production fields. However, during periods of high temperatures or dry conditions, disease pressure will be low and additional applications may not be necessary.

[Byline: Suzanne Bissonnette]

--
Communicated by:
J. Allan Dodds
Former ProMED-mail plant disease moderator <dodds@ucr.edu>

[_C. zeae-maydis_ is now recognised as one of the most significant yield-limiting diseases of maize worldwide. It is a threat to commercial maize production, but it also reduces yields of small-scale farmers in developing countries affecting food security.
It causes brownish gray lesions on leaves which may coalesce resulting in blighting of the whole leaf and the whole plant may die before the crop reaches maturity. Blighting seriously reduces the photosynthetic ability required by the host for grain production.
Under these conditions, the maize plant may also be pre-disposed to stalk-rotting fungal attacks adding to the yield losses. Infection is favoured by extended warm, wet, humid weather. The fungus survives on plant material between cropping seasons. Spores are spread by wind and splashing rain.

Disease management includes fungicides and cultural practices such as removal of inoculum, moderate plant density, crop rotation and timing of irrigation if applicable. Hybrid resistance is considered the most cost-effective strategy of managing GLS. Resistance is due to several genes which are additive in effect. The level of resistance required needs to be balanced against other characteristics such as yield potential and length of growing season when developing new cultivars.
A slight degree of tolerance in a genotype can already contribute to a substantial increase in yield.

Map of United States:
<http://www.census.gov/geo/www/us_regdiv.pdf>

Picture gallery of GLS symptoms:
<http://www.ent.iastate.edu/imagegal/plantpath/corn/grayleafspot/>

Links:
Disease description with pictures:
<http://agriculture.kzntl.gov.za/portal/Publications/TechnicalInformation/GreyLeafSpotofMaize/tabid/280/Default.aspx>
Information on GLS and other maize diseases:
<http://www.btny.purdue.edu/Extension/Pathology/CropDiseases/Corn/corn1.html#grayleafspot>
GLS control in Africa:
<http://www.cabi.org/ProjectsDetail.asp?ProjectID=130> and <http://www.arc.agric.za/home.asp?pid=637>
_C. zeae-maydis_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=161090>
- Mod.DHA]

[see also in the archive:
2002
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Gray leaf spot, maize, first conf'd rept - Canada 20020430.4060
1999
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Grey leaf spot, maize - Kenya (Rift Valley) 19991212.2151]

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