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Peanut disease resistance probably won't cut costs

Peanut growers across the Southeast generally produced good yields with good quality scores in 2008. Contract prices were higher than in the past few years, but for many farmers profit didn’t match production because of skyrocketing input costs.

As growers cautiously eye the 2009 season, most are looking for ways to cut input costs without negatively affecting yield.

Varietal selection is always a good place to start, but don’t count on disease resistant characteristics of different varieties being a good way to cut input costs.

“When we talk about disease resistance in Virginia-type peanuts, we are looking at relative differences. Because a variety is sold as resistant to a particular disease doesn’t mean it is completely clear of that disease. The best resistance we have is moderate resistance to moderate levels of a given disease,” says Barb Shew, a plant pathologist at North Carolina State University.

“For foliar diseases we don’t have in one variety a level of resistance for all three of our most common diseases — early leafspot, late leafspot, and web blotch — to warrant any cutback in fungicides.

“First, the levels of resistance aren’t high. And, foliar fungicides are controlling all three common foliar diseases. To cut back on fungicides a grower would need a variety with at least moderate resistance to all three diseases and none of our varieties have that,” says Shew.

Perry, for example, is a relatively new variety with plenty of good characteristics, including moderate resistance to early leafspot and web blotch. However, this variety is highly susceptible to late leafspot.

“A similar situation exists for our two longest running and among our most popular varieties NC-V11 and VA98R. Both are moderately resistant to late and early leafspot, but highly susceptible to web blotch,” she adds.

“If either of these varieties is planted late, and gets a late season tropical front that brings long periods of wet weather, they will be hammered by web blotch. Growers just can’t afford to take a chance on cutting back full-season fungicide programs for these popular varieties,” Shew stresses.

In soilborne diseases the level of resistance in some varieties is high enough to impact management decisions.

With CBR (cylindrocladium black rot)-resistant varieties, like Perry, growers may be able to avoid soil fumigation. The rule of thumb is: If there was less than 10 percent CBR in the field the last time peanuts were grown there, use a resistant variety and don’t fumigate.

By comparison, VA 98R is highly susceptible to CBR. If there is any history of the disease in the field, it is a good management decision to fumigate, Shew says.

Proline is a new fungicide that is expected to gain label clearance in the spring of 2009 for CBR control. In tests at several sites in North Carolina, Shew says the material has performed well and may provide some management options that could significantly reduce costs in fields with a history of CBR problems.

“Proline is another case in which the history of the field is going to be important. Most likely for fields with less than 10 percent CBR, Proline, plus a resistant variety, like Perry will be adequate. However, based on our data, Proline will not be a substitute for fumigation for fields with a 10 percent or higher level of CBR,” Shew says.

With schlerotinia blight there is some variability in resistance. VA 98R, Perry and possibly Champs will get a little less schlerotinia blight than some other popular varieties. In fields with a history of light infestations of schlerotinia blight, using these varieties may allow growers to use one versus two or three sprays for more susceptible varieties.

Bailey, named for long-time North Carolina State researcher Jack Bailey, is a promising new variety that does have true resistance to a number of peanut diseases. When it becomes available, it may provide growers with some options for cutting fungicide costs, Shew says.

The rapid encroachment of tomato spotted wilt virus into the Virginia-Carolina peanut belt adds another dimension and may significantly impact a growers’ options for choosing resistant varieties as a means of lowering input costs.

No peanut variety is immune to TSWV. However, a few varieties have consistently demonstrated moderate levels of resistance. In addition to resistance (reduced disease incidence), some varieties appear to have some degree of tolerance (reduced severity in infected plants) as well.

Because TSWV is vectored by thrips, growers need to pay special attention to both planting date and how it corresponds to thrips population buildups and how early, mid- and late-planted varieties respond to fungicides.

With peanut contracts expected to remain close to 2008 contracts, a number of growers are looking at the crop as an alternative because of lower fertilizer input costs. However, pathologists across the Southeast caution growers to understand the relationship between varietal resistance and susceptibility and fungicide use.

In short, because a peanut variety has resistance to a particular disease may not be reason enough to cut back on fungicide use.


TAGS: Peanuts
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