Some disease and insect problems can only be found at digging.
Editor’s Note: Pod rot and burrower bug symptoms can really only be found after peanuts are dug. Taking an assessment of any disease or insect symptomology will help with subsequent crops. In the case of burrower bug, knowing it was found in a field will help entomologists assess the scope of the problem with this bug and track its movement. See the sidebar from UGA peanut entomologist Mark Abney for more on this. Below is information on Sclerotinia blight and assessing for pod rot from Jason Woodward, peanut specialist, Texas A&M Agri – Life Extension, Texas Tech University.
Weather conditions that follow a pattern of moderate temperatures with rainfall and high humidity are ideal for the development of Sclerotinia blight. Two closely related fungi are capable of inciting this disease: Sclerotinia minor, which is more prevalent and aggressive, and Sclerotinia sclerotiorum.
Samples submitted for diagnosis have recovered S. sclerotiorum. Although this fungus has been associated with the disease, the overall response of the fungus to fungicides is poorly understood. If you are dealing with S. sclerotiorum, the same fungicides labeled for control of fields with a history of pod rot have been used to treat previous to this season.
When considering a fungicide application, weigh the opportunity for damage, the length of season and pre-harvest intervals. For example, if moderate amounts of disease are being observed late in the season, the potential to increase yields above and beyond the cost of the fungicide are limited. Furthermore, many fungicides labeled for Sclerotinia blight have a 14- to 30-day PHI.
The best time to assess pod rot severity within a field is after digging. This disease complex is comprised of a number of different fungi including Pythium spp. , Rhizoctonia solani and Thielaviopsis basicola . However, the overall effect on peanut pods is similar causing a dark brown to black discoloration. Differences in Pythium and Rhizoctonia pod rot can sometimes be observed based on the appearance of the pods.
Pythium is characterized by greasy, water- soaked necrotic lesions on the pod, whereas, Rhizoctonia has a more pronounced dry-rot appearance. Symptoms caused by T. basicola , or black hull as the disease is commonly referred to, are more superficial and generally do not affect the kernels.
Assessing Burrower Bug Situation
Burrower bug continues to be a significant threat to Georgia peanuts, and after two years of little to no pressure, the insect caused severe damage to portions of the crop in 2014.
The burrower bug joins lesser cornstalk borer and two spotted spider mite as pests that thrive in hot, dry conditions. While sometimes a problem in irrigated fields, all three of these pests are most prevalent and damaging in the state’s non-irrigated acreage.
The factors that are known to increase the risk of burrower bug damage are conservation tillage and hot, dry soil conditions. Conversely, deep turning and irrigation reduce the likelihood of infestations.
Many growers have asked the question, “Will abandoning conservation tillage in favor of a turning plow eliminate the burrower bug problem?” Previous research suggests that the risk of damage will decline, but there are no certainties. Burrower bug damage has been reported in Georgia from fields that were turned prior to planting. Work is currently underway to quantify the risk associated with different production practices, location, soil type, etc. Granular chloropyrifos, Lorsban 15G, is the only insecticide that has been shown to have any efficacy against burrower bug, but it does not provide complete control.
In the near term, management options available to growers who have experienced significant losses due to burrower bug are limited to deep turning and application of granular chloropyrifos.
To know more about the extent of burrower bug problems, if you find burrower bug in your field after digging, contact your county Extension agent or, in Georgia, contact your Extension agent or Dr. Mark Abney, UGA peanut entomologist at 229-386-3374.
Know Problem Areas
As anyone who scouts peanuts during the season will attest, the distribution of pod rot within a field can be quite sporadic. While the disease may be clumped in some areas, the random nature of where symptoms occur can limit one’s ability to quantify severity of the disease. Arbitrarily or randomly choosing several areas within a field and estimating the percentage of infected pods can provide insight into the distribution of pod rot within a field. Likewise, assessments made during harvest operations, both digging or combining, may also shed some light on disease severity and/or distribution.
Subsampling infested areas and scoring disease based on the severity of symptoms will provide additional information about the disease within a field. Although identifying infested areas within a field may be difficult, having a better understanding of problem areas within a field may be useful when scouting or treating subsequent peanut crops.