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The cooler start of the season has peanuts a bit smaller than typical. Fields planted May 1 are 41 days old today, with those planted April 20 at 54 days old. Herbicides are continuing to be applied, and fungicides are beginning to be applied.
Leaf Spot Fungicide on Small Peanuts?
Traditionally, we recommend starting leaf spot fungicides no later than 45 DAP, and in fields with histories of increased leaf spot pressure and/or volunteers nearby, starting closer to 30 days. This year, even though the peanuts may have been growing a little slower so far, I would still plan to have a leaf spot material out by around the 45-day mark. If field histories and rotations have been marginal to poor, I would even consider maintaining an earlier start. One of the reasons for this is how leaf spot has a lower temperature requirement than peanut growth. Optimal leaf spot spore production and infection occur at ~68F, whereas optimal peanut growth is ~86F. Later in the season when temperatures are generally warmer and when we normally associate with increased amounts of leaf spot, nighttime temperatures and available moisture can help to further leaf spot development at times when daytime highs would result in lower overall infection amounts. In both cases (earlier versus later in the year), the compound-interest reproduction cycle of the fungus increases with each generation, making timely early management prudent.
While volunteers are often the first to show lesions and should still be attentively managed, before a peanut plant is infected the spores are first produced and dispersed from the soil where they can survive on old peanut residue. Volunteers are more visible like a flag, while the spores from the ground are not so obvious.
While synergistic fungicides can inhibit spore/hyphal development in the plant during the first couple days of infection, most all our fungicides are still preventative in nature. This is a main reason we consider diseases as easier to prevent than to try to manage after they become established. Smaller canopies allow better spray penetration, helping protectant materials (including bravo) get good coverage. More expensive products can be saved for a later application (60/75+ days) when there is more peanut canopy to protect.
For white mold management, our traditional critical period for management starts around 60 DAP. This is still a good target to maintain this year. Like leaf spot, white mold is easier to prevent. The reasoning for our critical period starting around 60 DAP is related to the growth of the peanut, since this is generally when pod fill starts, in addition to standard schedules being based on 15-day intervals. If peanuts are delayed a week or two, pegs on their way to becoming pods would likewise benefit from protection. In previous years, such as 2019 which was readily hotter earlier in the year compared to this year, we saw benefits of earlier application (45 DAP) of effective white mold fungicides. In that year, applications at 60 days still provided good benefit, the earlier applications just provided additional earlier protection when the disease was also active. While 2023 may not see as much benefit from an early starting of white mold fungicide, including white mold fungicide protection at 60 days will allow us to take advantage of increased ground deposition through smaller canopies.