Bob Kemerait, Plant Pathologist, UGA
August is a month that is critically important for disease and nematode management for peanuts grown in Georgia. Heat, humidity, sporadic rainfall, days since planting, and growth of the peanut plants all put the crop at high risk for diseases, especially white mold and leaf spot. The heat, humidity, rainfall, and irrigation are near-perfect in August of 2022 for infection and development of fungal diseases. The dense canopy of foliage that has developed in many fields traps moisture and humidity, thus prolonging leaf wetness periods and increasing risk to diseases. The dense canopy of foliage also makes it more
difficult for fungicides applied to the leaves to reach the crown of the plant for protection against white mold.
By this time of the season, much of the crop is between 90 and 100 days after planting, which is sufficient time for leaf spot and white mold to become established in most, if not all, fields. Incidence of disease is should be lower in well-rotated fields and/or in fields where appropriate and timely fungicide programs have been deployed. Where crop rotation is short, or where there have been delays in fungicide applications, or where the choice of fungicide could have been better, peanut growers may find that August is the month to fight to find some way to re-gain control of disease in the field.
For disease management in August, I have five recommendations.
- Growers should continue to scout their fields, or to have their fields scouted for them, to ensure that there are no surprises as far as the development of white mold or leaf spot.
- Growers should recognize that even where there is good disease control in a field NOW, there is plenty of season left until harvest. NOW is not the time to relax on a disease management program.
- Growers should recognize that while near-perfect control of peanut leaf spot diseases is possible, though not necessary for top yields, it is nearly impossible to have “perfect” control of white mold. Initial infection for white mold is likely to occur from individual plants being infected by sclerotia in the soil close to the plant. From this initial infection, the disease can spread and burn along a row, resulting in significant yield loss. NO fungicide program will eliminate initial “dinner plate sized” hits of white mold in a field (though good crop rotation will do that). A GOOD fungicide program will stop the initial “hits” of white mold from burning down the row. If your program is not stopping the “burn”, then we need to figure out why. It could
be the fungicide is not reaching the intended target. It could be the fungicide is applied too late or at the wrong rate. Or, it could be that a better fungicide could be used. - There is significant interest in mixing SPECIFIC formulation of sulfur, remember that NOT ALL sulfur products are effective for management of leaf spot in peanut. It is my experience that sulfur in the right formulation mixed with the right product is similar to adding a pint of chlorothalonil. Adding sulfur to a fungicide for leaf spot control is not magic, but it can be a cost-effective way to improve leaf spot control.
- Better products and systemic products. I don’t look for any white mold products to be systemic, but some fungicides are better at fighting white mold than are others. Some leaf spot fungicides DO have limited systemic/curative activity against leaf spot diseases and these products can and should be used judiciously in the peanut fields.
Disease problems typically become most obvious during the month of August in Georgia’s peanut fields; however there are some problems that cannot be fixed now. Examples of these include damage from the Tomato spotted wilt virus and from nematodes. Opportunity to manage Tomato spotted wilt and peanut root knot nematodes largely ended when the furrow was closed at planting. Damage from these maladies is often quite evident in August as stress increases on the crop. Evidence of damage is a poignant reminder that grower should consider adjusting management strategies for the 2023 crop.
