Sclerotinia Blight Notes From Barbara Shew

— Written By

A busy disease year

A wise person told me many years ago that a good year for yield is a good year for diseases. In early August, North Carolina’s peanut crop was looking good and excellent yields were projected. If the rest of the year holds true to form, growers will need to stay on top of disease control for the crop to reach its full potential.

Leaf spot advisories have been very active so far this year. Regular rains, warm nights, and high humidity have meant that frequent sprays have been needed. To keep canopies in good shape through harvest, maintaining good leaf spot control in August and early September is crucial. Using a good soil fungicide is also important since stem rot is a threat as long as the weather is hot. This year, heavy vine growth and ample moisture has led to some problems with Rhizoctonia limb rot. Abound is very effective against Rhizoctonia; other soil fungicides are also effective. Avoid vine injury by limiting trips across the field, particularly when vines are heavy with water from rain or dew. It may help to spray at night when leaves are folded. Wrap up the season with a multi-site fungicide like Bravo to prevent build-up of fungicide resistant strains of leaf spot.

Growers who planted late may need to extend leaf spot control a bit later into the fall – it will all depend on the weather. In most years, mid-September brings dry weather and cool nights. In these years, leaf spot slows down so that sprays are not needed from early September until the crop is dug at the end of the month. In some years, warm nights and high humidity can continue into the fall. In those years, late-harvested peanuts may need continued fungicide protection. The best way to tell if you need to spray during this time is to follow leaf spot advisories, even if you use a calendar schedule the rest of the season.

The weather this year has been particularly favorable for Sclerotinia blight, and spray advisories have been frequent across the state. Fortunately, reports of outbreaks have been scattered so far. As we move into late summer, we can expect weather to become even more favorable. If this year is like most, night temperatures will begin to drop into the low 70’s and morning dews will be common. Dense canopies in late season stay cool and moist for several hours each day, setting up an ideal environment for disease.

The fungicides that control Sclerotinia blight, primarily Omega (fluazinam), must be used preventively, so it is important to control Sclerotinia blight at the first outbreak of disease. If you have applied Omega already, expect it to be effective for about three weeks. Continue to scout carefully and watch Sclerotinia advisories starting three weeks after the first fungicide application. If disease control has been good through the summer, fungicide sprays made within three or four weeks of digging give little added benefit. Remember that Omega cannot be applied within 30 days of harvest.

A few new fungicides – Quash (metconazole), Fontelis (penthiopyrad), and Propulse (fluopyram + prothioconazole) – have some activity against Sclerotinia blight. We applied these fungicides according to the Sclerotinia advisory in tests over the past few years. Our results from 2010 are shown below.

Sclerotinia pressure was extremely high in this trial and the full rate of Omega (1.5 pt) was necessary to control disease. The other fungicides suppressed Sclerotinia blight but did not control it. There may be better ways to use these fungicides to aid in Sclerotinia control than the ones we tried. For example, these fungicide may be helpful under lower pressure or when used as a follow-up to an Omega treatment. We will be looking at other approaches to using these fungicides against Sclerotinia blight in the future.

Table 1. Sclerotinia blight control at the Upper Coastal Plain Research Station in 2010.
Treatment* Sclerotinia %  Yield lb/a
Untreated 58 a 1451 c
Quash 3 oz 45 ab 1874 bc
Q8Y78 24 oz 53 a 2012 bc
Q8Y78 18 oz 39 ab 2019 bc
Omega 1 pt 23 ab 2038 bc
Quash 4 oz 39 ab 2087 bc
Fontelis 24 oz 32 ab 2138 bc
Fontelis 16 oz 36 ab 2213 b
Propulse 14 oz 34 ab 2244 b
Picoxystrobin 8 oz 31 ab 2421 b
Picoxystrobin 16 oz 31 ab 2525 ab
Omega 1.5 pt 11 b 3115 a
*All plots were cover sprayed with Bravo Weather Stik for leaf spot control. Sclerotinia fungicides were applied twice according to advisory.

Wrapping up

As disease control programs wind down for this year, it is not too soon to plan for upcoming seasons. There are fewer options now for CBR and nematode control, so keeping good field histories is more important than ever. CBR is easiest to diagnose late in the year. Look for roots that are blackened, brittle, and rotten, and for the brick-red fungus that may be present on the crown. Inspect freshly dug plants for root and pod galls caused by root-knot nematodes. Fields to be planted next year should be sampled for nematodes and the samples submitted to the North Carolina Department of Agriculture for assay and recommendations.

Article first appeared as North Carolina Peanut Note (PNNC-2012-052)