Disease Information From Barbara Shew

— Written By

Peanut Diseases in the 2015 Season (so far)

Weather‐wise, it’s been a typically variable season so far. We started out with a very wet spring, movedon to some hot weather early, and then settled into an overall dry pattern but with rain amounts andtiming varying widely across the state. With variable weather we can expect variable maturity, although I think David Jordan will tell you that things tend to even out a bit as the season progresses. Be sure to read his article about digging dates. Maturity and especially late maturity affects disease controldecisions. Below are some thoughts about late‐season disease control for some important diseases.

Leaf spots

In spite of some dry weather, we have had sufficiently high humidity in most places for leaf spotadvisories to continue to recommend sprays though the season to date (August 1). Even so, I expect leafspot pressure to be on the low side unless weather patterns change a lot in August and September. If weather stays dry, watch the weather and leaf spot advisories before deciding to spray. Of course, things can change very quickly – all it takes is a little rain and humidity in August to get a leaf spot epidemic started. Cleaning up an active epidemic can be difficult, if not impossible, so maintain leaf spot control through early‐ to mid‐September in anticipation of digging 2 to 3 weeks after the last spray. As anticipated, we have had reports of limited supplies of chlorothalonil this year.

Typically I recommend applying chlorothalonil for the last spray. This reduces the risk of carrying over fungicide resistant leaf spot populations to following years. To quote from my last Peanut News article:“If you must use a product other than chlorothalonil [for the last spray], mixing fungicides from two different activity groups is strongly recommended. If possible, one of these fungicides should be from a group not previously used during the season.”

Spray decisions in September can be difficult if you plan to delay digging. In most years, September turns dry with lower humidity and cooler nighttime temperatures by mid‐month. Leaf spots will slow down and fungicide application is not necessary once dry weather with nights in the 50’s prevail. However, if warm and humid weather continues, or if there is a tropical storm, conditions are much more favorable for disease. In these cases, fungicide programs need to continue so that leaves, vines and pegs stay in good shape until harvest. Because September weather can be so unpredictable, the best approach is to follow disease advisories to make fungicide application decisions when digging late.

Stem rot

Any time we have a hot, dry summer, we are set up for stem rot outbreaks when rain finally comes. However, it also is very typical for stem rot to damage pods and pegs underground with no apparent symptoms above ground, especially in dry years. For these reasons, a preventive approach against stem rot is best. One application of a soil fungicide per season should be sufficient to keep this disease under control on Bailey, which handles stem rot very well. Two applications may be needed on more susceptible cultivars. Stem rot generally slows down in September due to cooler air and soil temperatures and additional control should not be necessary in most years.

Sclerotinia blight

In contrast to last year, pressure from Sclerotinia blight has been light so far, except in the few areas that have had ample or excess rains. Sclerotinia needs cool moist weather to really get going but can increase very quickly, especially when favorable weather develops late in the season when vine grow this lush. Scout carefully for Sclerotinia blight if you have a history of disease and apply fungicide at the first sign of an outbreak. Higher spray volumes (20 gal/A or more) will help to get the fungicide through the canopy to lower leaves and stems. However, coverage is important because all leaves and stems are vulnerable to infection and need to be protected. Be sure to allow for a post‐harvest interval (PHI) of 30
days for Omega applications.

Tropical storms and other diseases

September is our most active month for tropical storms. Leaf spots, particularly late leaf, Rhizoctonia limb rot, and Botrytis can really take off following heavy fall rains. Heavy rains and cool nights make a nightmare scenario for Sclerotinia blight. Maintaining a good disease control program through the entire season (as appropriate for the conditions to date) is critical for successfully weathering a tropical storm.If a storm is on the way and you have not applied a fungicide recently, protect your crop with a systemic fungicide that offers both leaf spot and soil borne disease protection. Apply a Sclerotinia fungicide if you have a history of disease (again, allow for a 30‐day PHI).

Looking ahead

Note problem areas just before harvest so that you can choose rotations, cultivars and disease control methods to reduce problems the next time peanuts are grown. Avoid moving equipment from heavily infested areas. It is tempting to harvest diseased areas first, but harvesting them last reduces the likelihood of spreading the problem to new areas. Generally speaking, early digging of diseased areas isa mistake unless disease exceeds 50%; that is, if one of every two plants is infected.

Clusters of stunted, yellow plants indicate potential nematode problems. In the case of the northern root‐knot nematode, roots will have numerous small galls and excess root branching. The peanut root knot nematode causes very large gall on roots, pegs and pods. Pods with small round holes could indicate problems with lesion nematodes. In addition to checking this year’s fields, take soil samples infields that will be planted to peanuts next year and submit them NCDA&CS for nematode assay.

We expect to see some new disease control products becoming available for the 2016 season. We are continuing to evaluate them this summer and will be reporting results this fall and winter. We also have been testing disease control programs on the new cultivars Sullivan and Wynne and will be reporting on those results this fall and winter.

Article first appeared as North Carolina Peanut Note (PNNC-2015-121)