Peanut Disease and Insect Update for August

Dr. Bob Kemerait, UGA Peanut Pathologist

If the growing season was like a football game, many of our growers would now be early in the 3rd quarter. Our home team is the “Peanut Crop” and today we would be playing the “Pests”, a team primarily composed of weeds, insects, diseases, and nematodes. The “Pests” tend to put the diseases an nematodes on the field during the second half of the game. Winning the game requires both early detection of the diseases as they take the field and an effective play-calling to beat them At a UGA football game, it is a very good feeling is to be “way ahead of the opposing team” as one moves to the 3rd and 4th quarters of a game.

In terms of a peanut crop, being “way ahead” means that little or no disease of consequence can be found in a field and what little disease that is present is contained with an integrated disease management program. At this point, growers are well on their way to a harvest for which disease has little impact, other than the cost of the fungicide program. For growers who enter the month of August without disease issues in the field, I recommend the following.

1. Continue the current fungicide program, insuring that it includes appropriate fungicides for management of soilborne and leaf spot diseases. I use the word “appropriate” as from Peanut Rx, not all fields need to be treated in the same way. (See peanutrx.org for more information.)
2. Where conditions become more favorable for diseases, especially as we enter the “hurricane season”, anticipate the need to adjust dates of fungicide applications in order to maintain timeliness. Growers should take advantage of rain event to redistribute fungicides and to time fungicide sprays ahead of rainfall that could keep the tractors out of the fields for a period of time.
3. Scout your fields to ensure that you know what diseases are present not only for your management program this season, but also to anticipate fungicide programs in future seasons.
4. Evaluate your fungicide program not only for efficacy, but also to ensure that it is appropriate from a risk standpoint and from a fungicide resistance management standpoint. We want to make sure that fungicides are used (and not overused) to prolong their usefulness into the future.
5. Recognize that “perfect” control and disease-free fields are nearly impossible to achieve and are not necessary to maximize profits. For example, white mold “hits” confined to the size of dinner plates likely means the fungicide program is working; a jail-break of white mold running down the row means it is not working.
6. If no disease, or very little disease, is found in a field within three weeks of projected harvest, then growers may be able to further modify their fungicide program to reduce costs and maintain yield potential. For peanuts that are dug 135 days after planting, a “7-spray program” is likely enough. For peanuts that are dug 140-150 days after planting, an 8th spray may be beneficial.

Like in a football game,being “way behind” during the second half can be a very difficult deficit to overcome. Being “behind” also has significant impact on disease management decisions. “Getting behind” in a disease management program simply means that either one is late in timely fungicide applications or, more urgently, disease is now established in the field. “Established” does not necessarily mean “severe”. “Established” simply means that the grower, agent or scout can walk through the field and find, without too much trouble, signs and symptoms or white mold, leaf spot, Rhizoctonia limb rot, CBR, etc. Reasons why growers can get behind in their disease management program include the following.

1. The crop is at “high risk” because of short rotation.
2. The crop is at “high risk” because a susceptible variety has been planted.
3. The 20232 crop is particularly at risk to white mold because of hot humid days and warm humid nights. Sclerotium rolfsii thrives under such conditions.
4. The disease management program is complicated because there has been abundant rainfall that is favorable for development of disease and that keeps the grower from making timely fungicide applications.
5. Rainfall or irrigation has occurred to quickly after a fungicide application and the fungicide was washed from the leaves to quickly, increasing risk to leaf spot. Typically I hope that growers will have at least 8 hours before a rain or irrigation event following a fungicide application and not more than 24 hours if the material is to control white mold as well.
6. Rainfall or irrigation does not happen within 24 hours when a fungicide is applied for management of white mold and other soilborne diseases. Such can greatly reduce, but not eliminate, the efficacy of the product.
7. Less-effective products were applied when more effective products were needed OR lower rates were used when higher rates were needed.

As in a football game, growers who are behind into the second half of the season likely need to change the game plan and be more aggressive in their management program. A “more aggressive” strategy could include tactics such as switching to more effective fungicides which could improve control (and also increase cost). More effective products could have increased systemic activity within the plant or could be more “active” against the pathogen/disease. Another tactic would be to shrink the spray interval from every 14 days to every 10-12 days, especially if weather conditions favor diseases.

In addition to being more aggressive in a field to improve disease management, there are two critically important considerations for the grower. The first is to recognize the need for FUNGICIDE RESISTANCE MANAGEMENT considerations, especially when disease is present in the field. When a grower feels behind in a disease program, he or she is often tempted to “throw the best fungicides at it” and to “worry about now; let tomorrow take care of itself”. Such could be a critical mistake. To protect the usefulness and longevity of a class of fungicides, farmers much limit use to appropriate number of applications and timings of applications. The second factor is to “know when to say when”. When disease is so bad in a field that there is little hope that control can be improved with an application of fungicide, then it may be time to adjust the program to avoid expenses that cannot be returned with increased yields. Most importantly, growers in this position should learn from problems this year ,
whether it is excessive tomato spotted wilt, nematode damage, leaf spot, or white mold, so as to avoid the same programs in the next season.

Dr. Mark Abney, UGA Peanut Entomologist

1. Spider mites: Spider mites are sporadic pests of peanut and rarely reach treatable populations in irrigated fields or in fields where rainfall is plentiful. Hot, dry conditions put fields at increased risk for mite infestation, and when a broad-spectrum insecticide like acephate or a pyrethroid is applied, the risk goes up even higher. Mites are currently abundant in cotton in GA, and this suggests that we could see mites in peanut if conditions become favorable. Mites can be moved from field to field on equipment, and growers should be cautious about moving from fields with known mite populations to peanut fields that are at high risk (dry and/or treated with a pyrethroid). Finding and treating infestations early are important for achieving good control. I recommend spray volumes of at least 20 gallons per acre when treatment is needed.
2. Caterpillars. Foliage feeding caterpillars will be present in Georgia peanut fields over the next two months. There will be a mix of species with velvet bean caterpillar (VBC) and soybean looper (SBL) likely being the most common. VBC is easy to kill, SBL is not. Proper pest identification and insecticide selection can save a grower a lot of money. VBC can defoliate a
   field FAST, but there is no reason for this to happen if we are scouting every week.
3. Three cornered alfalfa hopper: OK, this one is not a major concern of mine, but the insect can be VERY abundant in August and September, and growers can get understandably nervous about it. Can TCAH cause yield loss? Yes. Will it be noticeable? Probably not. Anything we can afford to spray on TCAH will increase the risk of spider mite infestation in a year when that risk is already elevated. My advice is to leave TCAH alone in all non-irrigated fields. If there are high numbers of nymphs in an irrigated field, adding a pyrethroid to a fungicide spray could pay for itself, but it is not a sure thing.
4. Lesser cornstalk borer: You are tired of reading about LCB, and I am tired of writing about LCB. There are still some lessers out there, and if a field is at threshold, it needs to be treated. Late planted, non-irrigated fields with sandy soil are the ones I would check first as they are most likely to be infested at an economically important level.
5. Rootworms: I don’t have a good sense for how rootworm pressure in 2023 compares to recent years. Depending on who I ask, the adult beetles are either as abundant, more abundant or less abundant than last year. Our trials in Plains (beside the highway and not close to corn) definitely have fewer larvae and less pod injury than we usually see this time of year. On the other hand, a trial that is at the back of the farm and near corn is very heavily infested. I received a report of heavy rootworm pressure in some fields in Berrien County; this is a county that has little or no history of rootworm problems in peanut. Options for management are few, and efficacy is inconsistent. Please let me know if growers contact you with questions, and we can try to work through what options are available.

If we can be of assistance to you at Worth County Extension, please let us know.