Tomato spotted wilt calculations

Few changes made in peanut index Following the old adage, "if it ain't broke, don't fix it," University of Georgia researchers have made few changes to a tomato spotted wilt virus (TSWV) risk index that has saved growers millions of dollars since its inception.

The index combines what is known about individual risk factors into a comprehensive but simple estimate of TSWV risk for a given field, says Steve L. Brown, University of Georgia Extension entomologist. It assigns a relative importance to each factor so that an overall level of risk can be estimated, he adds.

The first version of the index was developed in 1996 and was based on available research data, notes Brown. "Small plot studies and on-farm observations have been used to evaluate index performance each year since the release of the first version," he says.

In research plots where multiple TSWV management practices were used, as little as five percent of the total row feet were severely affected by the virus compared to more than 60 percent in high-risk situations, says Brown. Yield differences were more than 2000 pounds per acre in some cases.

Results of these and other validation studies have been used, he says, to make modifications in the 2001 version of the index. "Keep in mind that the risk levels assigned by this index are relative. In other words, if this index predicts a low level of risk, we would expect that field to be less likely to suffer major losses due to TSWV than a field that is rated with a higher level of risk.

"A low index value does not imply that a field is immune from TSWV losses. Losses due to the virus vary from year to year. In a year where incidence is high statewide, even fields with a low risk level may experience significant losses.

One change in the 2001 index is that the location factor has been removed, says Brown. "We took out the location factor because it had diminished over time. There were strong differences between the southwest corner of Georgia and other parts of the state. There's still some geographical variations, but it's difficult to nail down, and we don't feel as though it was significant enough to continue as a component of the index."

The peanut variety section also has been refined, says the entomologist. A Virginia variety - Gregory been added to the index, he says.

The variety C99R still is given the top rating in the index. "It's still our best variety for TSWV resistance. We didn't see any justification for changing that."

Planting dates and seeding rates also remained the same in the 2001 index, says Brown. "We have some pretty good data that has held up over the years, and we didn't see a reason to change what was working."

The incidence of TSWV wasn't as severe this past year as it has been in recent years, he says. "In some places, certain varieties were hit harder than others. All in all, the situation was better in 2000. In most cases, the virus came later in the season. We saw some late-season yellowing in some fields. The virus just doesn't damage yields as severely when it comes later in the season."

A majority of Georgia's peanut producers, says Brown, now are following multiple components of the index. "The use of twin rows has risen to about 40 percent of the total acreage grown in Georgia. This is the result of a steady increase in recent years. In addition, Georgia Green was the dominant variety again in 2000.

"Data also shows that planting dates shifted more this past year than in other years.

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