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Date: 25 Jun 2007
Source: Stop Soybean Rust, Ohio State University news release [edited] <http://www.stopsoybeanrust.com/viewStory.asp?StoryID=1055>

Ohio State University's soybean sentinel plot program, established in 2005 to track soybean rust (still a no-show in Ohio), is fulfilling its purpose as the 1st line of defense against more immediate, threatening diseases.
One of those is frogeye leaf spot.

The foliar disease, rare in Ohio, was recently identified at one of the 45 sentinel plot locations. "This is one of the clear benefits of this sentinel plot program. We can find and detect a disease long before controls need to be implemented," said Anne Dorrance, an Ohio State University plant pathologist with the Ohio Agricultural Research and Development Center.

Dorrance, who also holds an Ohio State University Extension appointment, was one of several university researchers instrumental in establishing the national program of sentinel plots. The early detection of frogeye is helpful for growers who can now begin scouting their fields for any potential problems. Although uncommon in Ohio, the disease has been showing up more frequently the past several seasons, and can greatly impact on susceptible soybean varieties.

Last year [2006] was the 1st time growers suffered economic losses to frogeye -- anywhere from 10 per cent to 30 per cent yield losses on 300 000 acres (121 406 hectares). "The good news is we scored varieties in the Soybean Variety Performance Trials last year [2006], and over 90 per cent of the entries had high levels of resistance," said Dorrance. "We don't expect a similar economic problem this year [2007], but it is good to monitor the situation."

The frogeye pathogen survives in crop residue and makes its appearance in mid to late July, affecting leaves and sometimes spreading to stems and pods. Frogeye lesions are distinct from other soybean foliar diseases in that they have a gray-silver center surrounded by a cranberry-red border.

Growers can manage frogeye by
- practising tillage to bury crop residue and reduce the amount of disease inoculum,
- practising crop rotation, and by
- planting resistant varieties.

"Conditions for the next few weeks are predicted to be hot and dry. Frogeye requires moisture and tends to infect young new leaves as opposed to older leaves," said Dorrance. "For the next few weeks we will be surveying the state to determine how widespread this is and if fungicides will be needed."

Updates on frogeye and other soybean foliar diseases from Ohio State University are available at <http://agcrops.osu.edu>.

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

[Frogeye leaf spot is caused by the fungus _Cercospora sojina_, which can infect leaves, stems, and seeds of soybean. The pathogen is found worldwide but is only problematic under warm, humid conditions. Significant yield losses of 10 to 50 per cent are commonly associated with frogeye epidemics.
The fungus is spread via infected plant material and can also be seed borne leading to inferior seed quality. It is most likely to become a problem if infected seed is planted or if infected residue of a previous soybean crop serves as a source of inoculum. Disease management includes use of disease-free seed, planting of resistant varieties, crop rotation, tillage, and fungicide applications.

A close relative of the frogeye pathogen is _C. kukuchii_, which causes leaf blight and purple seed stain on soybean. The genus _Cercospora_ includes a number of species causing leaf spot diseases on crops such as grain legumes, carrot, eggplant, pepper, tomato, rice, corn, sorghum, oil palm, and coffee. The related genus _Pseudocercospora_ also causes similar leaf spot diseases on a different range of hosts, for example avocado and grapevine.

Sentinel plots are experimental field plantings of indicator varieties of the host crop and often include a number of cultivars with different degrees of susceptibility to the pathogen. The plots are grown under controlled conditions and with continuous observation. They are a useful tool for early diagnosis of emerging crop diseases, and to identify the need for appropriate disease management measures at a time when these would be most effective.

Map of US states:
<http://www.census.gov/geo/www/us_regdiv.pdf>
Pictures
Frogeye symptoms, upper leaf surface:
<http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy003/frogeye2.jpg>
Frogeye symptoms, leaf underside:
<http://extension.missouri.edu/explore/images/g04442frogeyebottom.jpg>
Links
Frogeye information:
<http://extension.agron.iastate.edu/soybean/diseases_frogeye.html> and <http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy003/soy003.htm>
_C. sojina_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=119516>
Overview of disease due to _Cercospora_ species:
<http://www.oisat.org/pests/diseases/fungal/cercospora_leaf_spot.html>. - Mod.DHA]

[see also in the archive:
Cercospora spot, avocado - Kenya 20070604.1803
2002
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Frogeye leaf spot, soybean - USA (Wisconsin) 20021028.5655 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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