Disease Management Q & A

Sclerotinia blight, chlorothalonil shortages and fungicide resistance are a few issues producers may deal with this season.

Q: Sclerotinia blight has been found in Arkansas fields. How can I keep it from spreading into my fields?

A: Travis Faske, University of Arkansas, Extension plant pathologist

Preventing the spread of the pathogen is the best way to keep fields free of Sclerotinia blight. The pathogen can be transported in soil or peanut vines that hitch a ride on cultivation or harvesting equipment. It is a cool-season fungal disease that is challenging to manage because host resistance is moderate at best, fungicides are few and expensive, and finally, once the disease has been detected in the field there is no way to eradicate the pathogen.

Sclerotinia blight is caused by Sclerotinia minor and S. sclerotiorum.

Typically, this disease begins in mid- to late September, but if temperatures in August are cooler, it will develop earlier.

The first symptom is the “flagging” or wilting of infected branch tips and petioles. With Sclerotinia blight, the leaves are curled rather than clasped together as with southern blight.

With a closer inspection, the infected stems are often bleached with white fluffy hyphae surrounding the infested stem. Hyphae often become matted around the stem later in the afternoon, when conditions are warmer.

Black sclerotia can be found on and inside of infected stems.

If Sclerotinia blight is suspected, first confirm that the disease is Sclerotinia blight and document locations in the field where confirmed. Consider harvesting non-infested or non-diseased fields before harvesting diseased fields. Power wash equipment, tractor tires, tillage and harvesting equipment and remove as much soil as possible and all peanut stems before moving from infested or diseased fields to non-infested fields.

It is also best to develop a long rotation program and keep the field out of peanut for at least two years, although longer is better. Sclerotinia sclerotia can remain viable in the soil for several years.

Q: I’m having trouble locating my usual leaf spot fungicide with chlorothalonil. What are my options?

A: Barbara Shew, North Carolina State University, research assistant professor entomology and plant pathology

sclerotinia bmpsChlorothalonil is critical in leaf spot management programs because it helps to reduce the risk of resistance to site-specific fungicides, groups 3, 7, 11, in leaf spot populations. Growers are most familiar with Bravo Weather Stik but several other products and brands containing chlorothalonil are available. A table of products labeled on peanut and rates equivalent to 1.5 pints per acre Bravo can be found at http://go.ncsu.edu/readext?524210.

If you locate another product containing chlorothalonil, be sure to verify that it is labeled for use on peanut before purchasing.

Q: What can I do to stay on top of leaf spot and not compromise the fungicides used?

A: Bob Kemerait, University of Georgia Extension plant pathologist

Fungicide resistance, where a fungicide or class of fungicides is less effective now than in the past, continues to be of significant concern for peanut producers. Today, we are increasingly concerned for the management of leaf spot diseases using fungicides in the strobilurin class that includes popular chemistries such as azoxystrobin, pyraclostrobin and fluoxastrobin. As azoxystrobin is now available in numerous generic formulations, overall use of this class of fungicides will continue to increase.

Growers, especially those with increased risk to leaf spot diseases because of the variety they plant or peanuts planted in short rotation, should consider pre-mixes or tank-mixes of strobilurins with a fungicide of differing mode of action. Use of pre-mixes/tank-mixes has long been our recommendation for use of tebuconazole and some other triazole fungicides.

Leaf spot diseases were severe in many fields during the 2017 season, largely because of significant rainfall during the first half of the season. Risk to leaf spot is of increased concern in 2018 because of limited supply of some important fungicides and concerns over current efficacy of some of our once “better” products. Growers should develop a plan for best management of leaf spot diseases that includes variety selection, timing of application and fungicide selection.