Disease management late in the peanut season can either be very easy or it can be quite confusing. In fields where these is little disease, growers can generally “coast” to harvest with confidence that there is little (or nothing) to be done to finish the crop. Where disease is present, growers must decide what measures should be taken to protect their crop as best they can as they limp towards harvest. The decision for “best” practices is based upon 1) projected time until harvest, 2) how much disease is in the field, and 3) overall yield potential of the crop.
If there is not much time left until harvest (less than three weeks until digging), or if there is too much disease in the field, or if the yield potential is low because of other reasons, then there may not be any reason to spend more money on fungicides. However, in other situations growers can be justified in making a final fungicide application; the trick becomes what fungicide, or combination of fungicides, to apply. Perhaps the biggest change in disease management for peanuts over the course of my career has been that in the past peanuts were often dug 125-135 days after planting while we now wait 150 days (or more) before digging. This delay in harvest means that growers may need to consider an additional, final fungicide application to protect yield.
Below are some typical situations that peanut growers may find themselves in and suggestions for control.
1. Grower is four or more weeks away from harvest and currently has excellent disease control.
a. Suggestion: I recommend that the grower apply at least one more fungicide at least for leaf spot control. It wouldn’t hurt to use a mix of a protectant fungicide (chlorothalonil) + a curative with attention to the PHI (e.g. Alto has a 30 day PHI, Domark and Topsin have a 14 day PHI)
b. Suggestion: Given the low cost of tebuconazole, the grower may consider applying a tank-mix of tebuconazole (7.2 fl oz) + chlorothalonil (1.0 pt/A) for added insurance of white mold and leaf spot.
c. Notes on sulfur (specific formulations), Aproach Prima (picoxystrobin + cyproconazole), and Absolute MAXX (trifloxystrobin + tebuconazole)
i. SPECIFIC formulations of sulfur can be used with products like tebuconazole, azoxystrobin, Headline, and Aproach Prima to improve leaf spot control. I prefer that growers use such early-to-mid-season and not late in the season.
ii. If used late in the season, Absolute MAXX (14-day PHI) and Aproach Prima (30-day PHI) must be mixed with chlorothalonil. They are appropriate late-season where leaf spot is not well-established in the field.
2. Grower is four or more weeks away from harvest and has disease problems in the field.
a. If the problem is with leaf spot: Grower should insure that any fungicide applied has systemic/curative activity. If a grower wants to use chlorothalonil, then I strongly suggest that they mix a product like Provysol (mefentrifluconazole), thiophanate methyl (Topsin M), cyproconazole (Alto), tetraconazole (Domark) with the chlorothalonil. Alto has a 30-day PHI; the other fungicides have a 14-day PHI. Growers are NOT encouraged to use Priaxor or Miravis late in the season, especially where leaf spot is a problem.
b. Provost Silver is also performs well as compared to other fungicides for late season leaf spot management.
c. If the problem is white mold: Grower should continue with fungicide applications for management of white mold and leaf spot. If they have completed their regular white mold program and are within 40 days of anticipated harvest, then they should extend the program, perhaps with a tebuconazole/chlorothalonil mix, Provost Silver, or Fontelis. Each of these fungicides has a 14-day preharvest interval.
d. If the problem is underground white mold: Underground white mold is difficult to control. Applying a white mold fungicide ahead of irrigation or rain, or applying at night, can help to increase management of this disease.
3. Grower is three or less weeks away from projected harvest and does not currently have a disease issue. Good news! This grower should be good-to-go for the remainder of the season and no more fungicides are required.
4. Grower is three or less weeks away from harvest and has a problem with disease.
a. If leaf spot (or rust) is a problem and 2-3 weeks away from harvest, a last leaf spot fungicide application may be beneficial. If leaf spot is too severe (defoliation more than 20%), then a last application will not help. See above for suggested applications.
b. If white mold is a problem and harvest is 3 weeks away, then it is likely beneficial to apply a final white mold fungicide, as above. If harvest is 2 weeks or less away, then it is unlikely that a fungicide will be of any benefit.
c. NOTE: If harvest is likely to be delayed by threat from a hurricane or tropical storm, then the grower may reconsider recommendations for end-of-season fungicide applications.
Growers, in general, are encouraged to wait until appropriate harvest maturity to dig their peanut crop. However severe disease in a field may mean that growers should dig ahead of a projected digging date to minimize harvest losses.
There are no “hard and fast” recommendations for conditions when digging early is advised, however here are some suggestions for when the grower is within two weeks of harvest.
1. Tomato spotted wilt, even when severe, is generally not a reason to dig early.
2. Significant defoliation from leaf spot diseases, 50% and beyond is reasons to consider digging early to minimize harvest losses.
3. Active white mold in a field that affects greater than 40% of the crop could make digging early necessary.
Again, there are not hard and fast, but guidelines for timing “best” harvest.
If you have any questions about late season disease management in peanuts please contact your local county Extension agent.