Disease Thresholds Barbara Shew Peanut Notes No. 145 2019

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Disease Thresholds

Barbara Shew

Department of Plant Pathology and Entomology

NC State Extension

In most cases, the timing of disease control measures is based on date or favorable weather and not thresholds. This is because control depends on prevention – once disease becomes visible, it usually is too late for effective control.

In the case of leaf spots (early or late), prevention is absolutely necessary. In well rotated fields, start leaf spot sprays by R3 (very early pod), which usually happens in early July. Some growers may feel reassured starting leaf spot sprays a bit earlier than R3, but spraying earlier than 45 DAP is not justified in our area, except with very poor rotations (no rotation, peanuts grown in alternate years, or very heavy volunteer populations).

After the first spray, advisories can be used to determine whether sprays can be delayed due to weather that is not favorable for leaf spot. Alternatively, growers can continue to spray on a 14-day schedule for most fungicides. Growers should avoid spraying in very hot, dry weather but check advisories to confirm that weather is unfavorable – don’t guess!

Likewise, a preventative approach to stem rot control works best. A stem rot fungicide should be applied by 60 DAP. At this time the canopy will be close to closing and the fungus often will be active. Unfortunately, no one has been able to work out a good stem rot advisory because the fungus is so sneaky (in very technical terminology). It’s best to assume stem rot is active from about mid-July to mid-August in our area. After the first spray, a second or third spray may be beneficial on susceptible cultivars, in fields with a history of stem rot problems, or when signs of the fungus are apparent.

Sclerotinia blight control usually is determined by weather/advisory. It’s favored by cool weather with rain or long periods of high humidity. Sclerotinia blight advisories also take canopy size into account. Risk increases when rows are within 6 inches of touching and becomes higher when the canopy closes. Since current cultivars stand up pretty well against a little Sclerotinia blight and infestations vary from field to field, a spray threshold of 1% can be used. However, very intensive scouting is required to check for threshold levels of this disease. The scout will need to look inside the canopy in 100-ft sections in several spots in the field. Sclerotinia blight is very patchy so the field must be covered thoroughly. Deciding when to spray requires a combination of advisory, scouting, and knowledge of past damage. We’d probably just recommend preventative sprays in fields with a history of disease if Sclerotinia fungicides were not so expensive.

TSWV control is entirely preventative and is accomplished at planting based on the TSWV index.

At this point, we have very little CBR in NC. You can check Peanut Information or the Peanut Risk Management tool for information about risk of diseases found in NC, including CBR.