Observations and Predictions for Mid-Season Disease Control

— Written By

If only I had a crystal ball . . .

Barbara Shew

As I write at the end of July it’s almost impossible for me to figure out what disease problems we’ll be seeing by the time this article is published. It’s been an unusual season to date, with heat and drought predominating, but with rain amounts and timing varying widely across the state. Be sure to have a look at David Jordan’s article about digging dates. Variable weather also means variability in maturity, which in turn affects disease control decisions. Below I break down some thoughts on late-season disease control for some different scenarios.

Irrigated fields

Even irrigated fields tend to have low-moderate leaf spot pressure in hot, dry years. Humidity periods are shorter with irrigation than with wet weather. Nevertheless, irrigated fields have the highest risk of leaf spot, stem rot, Rhizoctonia limb rot and Sclerotinia blight due to greater moisture and heavier vine growth. Maintain regular fungicide applications until 2-3 weeks before harvest and be sure your program includes fungicides active against soilborne diseases. Scout carefully for Sclerotinia blight if you have a history of disease and apply fungicide at the first sign of an outbreak. However, be sure to allow for a post-harvest interval (PHI) of 30 days for Omega applications.

“Just enough” rain

In years when we have “just enough” rain to get by, disease pressure tends to be on the low side. Pathogens newer really get a chance to take hold and build up numbers sufficient to cause a lot of disease losses. This can be particularly true with leaf spots. As of August 1, many locations have had no or only one spray advised following the initial fungicide application in early July. Of course, things can change for the worse very quickly – all it takes is a little rain and humidity in August to get a leaf spot epidemic started. Likewise, rain after a dry spell is thought to trigger stem rot outbreaks. If you are in a “just enough” situation, keep an eye on the weather (or better yet, leaf spot advisories) and be sure to maintain a good disease control program through early September in anticipation of digging later in the month.

Drought stress and delayed maturity

If your fields are under drought stress, it is very important to avoid excess fungicide applications due to the risk of spider mite outbreaks. Hot, dry weather does not favor leaf spot development so fungicide application is not necessary, if not actually detrimental. If you plan to delay harvest in hopes of getting some late-season rain, spray decisions can become difficult. Weather in September can be all over the place. In typical seasons (something we haven’t seen in years, it seems), September has low rainfall, less humidity, and cooler nighttime temperatures, making conditions less favorable for leaf spot by the mid-month. However, in the past few years, warm and humid weather has extended to the end of the month. These conditions are much more favorable for disease than in typical years. Under these conditions, fungicide programs need to continue until low humidity and nights in the 50’s finally begin to take hold. Because September weather can be so unpredictable, the best approach is to follow disease advisories to make fungicide application decisions on late harvested crops.

Tropical storms

September is also 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 soilborne disease protection. Apply a Sclerotinia fungicide if you have a history of disease (again, allow for a 30-day PHI).

Wrapping up

Fumigant labels will have additional restrictions in 2012, including the institution of buffer zone requirements. Unfortunately, the exact wording of 2012 labels will not be known until late this year. County agents, NCDA&CS personnel, and fumigant suppliers will help to provide information and guidance about new labels when they become available.
With the loss of aldicarb and the new restrictions on fumigation, it is more critical than ever to keep records about the disease situation in each field. At harvest time, note areas where CBR is present. We have seen spotted wilt in some areas this year, so check to see if dead, wilted and yellowed plants have CBR or spotted wilt. Brick-red fungal structures on the crowns, lower stems, and pods are a sure sign of CBR, but are not always present. Roots with CBR are black, brittle and rotten. Scattered yellow, wilted or
stunted plants without root or pod rot probably have spotted wilt. 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. Pods with small round holes could indicate problems with lesion nematodes. In addition to checking this year’s fields, take soil samples in fields that will be planted to peanuts next year and submit them NCDA&CS for nematode assay.

Article first appeared as North Carolina Peanut Note (PNNC-2011-033)