The University of Georgia's Peanut Fungal Disease Risk Index is, by necessity, a continually changing tool for growers, says Bob Kemerait, University of Georgia Extension plant pathologist.
“This is an evolving work,” says Kemerait. “As we continue to get better information and new varieties, we'll look at the index each year and improve upon it.”
The fungal disease index, modeled after the University of Georgia Tomato Spotted Wilt Virus Index, assigns points to each production variable in the categories of leafspot diseases, white mold and Rhizoctonia limb rot. Growers then can evaluate the potential risk in a given field and use this information in preparing for disease in a coming season as “low,” “medium” or “high.”
Currently, the index is an excellent educational tool for the grower, says Kemerait. Not only does it help the grower predict the severity of disease in the field in the coming season, but it also allows him to assess how changes in a production program may affect risk.
The ultimate goal of the index, says Kemerait, is to make recommendations to peanut producers for acceptable fungicide programs, what he prefers to call “prescription programs.”
“Most growers follow a calendar schedule for spraying fungicides. But with a prescription program, a grower who hasn't had a problem with diseases and has a good rotation might could get by with fewer sprays, provided he controlled soil-borne diseases. That's the direction we'd like to take the index,” he says.
The fungal disease index was first released in 2003, says Kemerait, and for the sake of simplicity, white mold and Rhizoctonia limb rot were grouped together under the single category of “soil-borne diseases.”
“But in 2004, we found it necessary to estimate the risk for these two diseases independently because they are affected by field factors in different ways. For example, some new peanut varieties have resistance to white mold that is better than that found in Georgia Green. However, some of these same varieties are more susceptible to Rhizoctonia limb rot,” says Kemerait.
Another example, he continues, would be in the area of crop rotation. Crop rotation with cotton will decrease the risk of white mold, as cotton is not a host for the white mold fungal pathogen, he adds.
“However, both cotton and peanuts are hosts for Rhizoctonia solani, the causal agent of Rhizoctonia limb rot. Therefore, rotating peanuts with cotton will reduce the risk of white mold to a much greater degree than it will reduce the risk of limb rot.”
In addition, more peanut varieties have been evaluated and included for disease resistance in the 2004 version of the risk index.
Another change from last year, says Kemerait, is the leafspot risk assigned to twin-row peanuts versus single-row peanuts. “Last year, the index indicated there was a little more risk to leafspot in twin rows than in single rows.
We've looked at that further, and for growers on a fungicide program, or growers in general, there's no increased risk to leafspot whether you are on single or twin rows.”