Which Will It Be?

Sclerotinia blight or Southern stem rot — depending on the weather, one of these is likely in Virginia fields.

• By Amanda Huber •

Virginia producers find primarily three diseases in their peanut fields, says David Langston, plant pathologist at Virginia Tech’s Tidewater Agricultural Research and Extension Center. “Most fungicide sprays are targeted to leaf spot, Sclerotinia blight or Southern stem rot.”

However, based on weather conditions during the season, producers are likely to only encounter leaf spot and one of the other two. If weather is favorable for Sclerotinia blight, then it is not the hotter sometimes dryer weather preferred by Southern stem rot.

Sclerotinia blight is known by the tan lesions and the cottony white mycelium that puff out on the stems.
Photo by Damon Smith, University of Wisconsin-Madison, Bugwood.org

Record Disease Levels

Langston says, “Typically, severe outbreaks of stem rot and Sclerotinia blight don’t co-exist.

“In 2020, we saw a good bit of late leaf spot and Sclerotinia blight in our plots.

“Cooler-than-average temperatures beginning Labor Day created very favorable conditions for Sclerotinia blight. The Virginia Sclerotinia Blight Advisory indicated that the risk of disease development was high from Aug. 2 until Oct. 17.

Although few growers reported severe epidemics of Sclerotinia blight last year, field trials at the Tidewater AREC exhibited levels of disease that had not been observed in 10 or more years.”

Only one of these two diseases, Sclerotinia blight or Southern stem rot, is likely to be found based on weather conditions, each one produces a white mycelium growth on the stems but in different ways.

Sclerotinia blight is known by the tan lesions and the cottony white mycelium that puff out on the stems. With Southern stem rot, most of the coarse, white growth is closely pressed to the stems.”

Start With Rotation, Variety Selection

In contrast to Sclerotinia blight, the white mycelium growth of Southern stem rot, or white mold, is closely pressed to the stem.

Langston says the first line of defense against disease is the most basic of cultural practices.

“Crop rotation helps with all three diseases and is probably our biggest asset for management.

“Variety selection also helps provide tolerance to leaf spot and Sclerotinia blight. Fungicides are always the last line of defense and depend on field disease history, length of rotation and weather.

“Peanuts can be sprayed based on crop growth stage, days after planting, by advisories and through scouting. Early, preventative sprays work best.”

Fungicide Research

In 2020, fungicide trial plots were planted on May 15-16 to the cultivar Sullivan.

“We saw severe defoliation with late leaf spot in untreated checks as well as Sclerotinia blight.”

Langston’s field trial results demonstrated that a tankmix of Miravis at 3.4 fluid ounces per acre and Elatus at 9.5 ounces per acre, both in the Fungicide Resistance Action Committee Group 7, performed as well as programs receiving two applications of Omega 500 at 1.5 pints per acre for reducing Sclerotinia blight and improving yields.

“This information is significant to growers because control of peanut leaf spot, Southern stem rot and Sclerotinia blight can be achieved using the Miravis/Elatus two-spray program without additional fungicide inputs. However, I recommend applications of effective leaf spot fungicides that contain DMI (demethylation inhibitors) fungicides in FRAC Group 3 as well as chlorothalonil, especially later in the season, to manage leaf spot pathogen resistance to Miravis.

“In fields that have no history of losses to Sclerotinia blight or Southern stem rot, fungicide programs that include less expensive products of varying FRAC groups are sufficient for controlling leaf spot. Leaf spot is our most important disease that affects 100% of the peanut acreage while Sclerotinia blight and Southern stem rot cause problems on significantly less acreage.”

For the latest information on management of diseases and other field issues, visit the Virginia Ag Pest and Crop Advisory at http://blogs.ext.vt.edu/ag-pest-advisory.

Peanut/Cotton InfoNet

The Peanut/Cotton InfoNet provides crop management information updated daily during the growing season based on location. Data and information available on the advisory includes:

• Maximum, minimum, and average air temperatures.

• Average soil temperature at a 4-inch depth.

Starting May 1:

• Daily and accumulated peanut heat units.

• Daily and accumulated cotton degree-days.

• Daily and total seasonal rainfall.

• Last effective spray date for peanut leaf spot.

Sclerotinia blight risk.

• Frost advisory from Sept. 25 to completion of harvest.

A Look At Varietal Resistance

The Peanut Variety and Quality Evaluation is a multi-state testing program for varieties grown in Virginia, North Carolina and South Carolina. Maria Balota, Virginia Extension peanut specialist, serves as the leader of the PVQE program in addition to her research and Extension work with the Virginia Tech Tidewater AREC in Suffolk, Virginia.

Among the entries in PVQE for 2020 were a couple of Virginia-type varieties with good resistance to Sclerotinia blight and other diseases. One of these is available for planting and the other, which is in seed increase, will not be available for a couple of years.

Bailey II is a 2017 release by North Carolina State University and is the high-oleic version of Bailey.

“Bailey II has good resistance to leaf spot, Sclerotinia blight and tomato spotted wilt virus, but it should be considered susceptible to Cylindrocladium black rot,” Balota says.

This varieties matures at about 145 days and seed should be available for commercial production this year.

Looking Ahead

A 2020 released by NCSU is the variety NC 20, which was tested as N14023. This Virginia-type cultivar has high oleic oil chemistry and was selected from a program to develop varieties with multiple disease resistances.

“This line exhibits a disease resistance package similar to Bailey II, including moderate-to-high levels of leaf spot, TSWV and Sclerotinia blight resistance. NC 20 is a higher-yielding line than others previously released from the NCSU peanut breeding program, and it maintains its yield under heavy leaf spot pressure,” she says.

This variety will be in seed increase for a few years and should be available after the 2023 growing season.