From early July through the first half of September, our attention turns to peach harvests. And for disease control, a great deal of focus is on preventing brown rot development in the ripening fruit. Failure to effectively manage brown rot during the first half of the harvest season can set you up for significant yield loss in the second half of the season, especially if rains become frequent. Infected, sporulating fruit from early maturing cultivars provides fuel (inoculum) for infection of the mid- and late-maturing cultivars. Remember, the harvest season is one big epidemic for brown rot.
This article is a continuation of my earlier article titled “Improving your early season peach brown rot control program” (plant-pest-advisorty.rutgers.edu, April 17, 2019). This former publication discussed blossom blight control as well as quiescent / latent infections occurring in young, green fruit. If you haven’t read this article, or forgot much of it, now is a good time to read or review it. I assume you followed the advice of the former article and therefore do not have significant latent infection lurking in your fruit!
The Data Speaks … Again
Way back in 2010, a standard fungicide efficacy trial contained a single treatment of the fungicide captan (80WDG at 3.75 lb/A) applied at shuck-split and first cover through eighth cover. No sprays were applied during the subsequent 27 days prior to harvest, the period during which fruit ripen and become increasingly susceptible to brown rot. Regardless of the lack of preharvest fungicide applications, this cover spray only program significantly reduced brown rot at harvest, providing 60% control.
In subsequent years, from 2012 through 2018, similar cover spray only programs were tested using the protectant fungicides captan, sulfur, ziram, and thiram. Only the captan cover spray programs were observed to consistently reduce brown rot at harvest. In fact, the captan programs never failed to significantly reduce brown rot, providing 50 to 78% control, with an average 61% control. Furthermore, a bioassay was developed which showed that the control was due to the presence of fungicide residue on the fruit surface. Surprisingly, even heavy rains during the preharvest period failed to reduce the efficacy of the captan residue.
Cover Spray Program Refinements
Subsequent research during 2017 and 2018 refined the cover spray program in two ways. First, the rate of captan 80WDG could be reduced to 3.125 lb/A without any loss of control relative to the 3.75 lb/A rate. A lower rate of 2.5 lb/A would continue to significantly reduce brown rot, but the level of control would no longer be as effective as the high rate.
A second and perhaps more important refinement was that only the final two cover sprays need to be captan in order to obtain the control benefit. The bioassay once again demonstrated the presence of an effective captan residue on the fruit when just the two final cover sprays were applied.
An Improved Preharvest Program
Implementation of the improved program is easy. Simply apply captan in the final two cover sprays at 2.5 lb ai/A. For the 80WDG formulation, this is 3.125 lb/A. As with the earlier cover sprays, these applications should be applied at 10-14 day intervals. Use a 14-day interval for normal or relatively dry conditions and 10-day interval for wet conditions.
At 10-14 days after the final captan cover spray, begin the preharvest fungicide program. The recommended program for mid- to late-season cultivars consists of applications at 18, 9, and 1-day preharvest. The final spray can be applied between the first and second or third pickings, if necessary (assuming a 0 or 1 day PHI); it helps to protect the fruit during subsequent handling.
At least three different chemistries should be utilized in the preharvest sprays since these highly effective compounds are at-risk for resistance development. Some example programs are: Merivon / Indar / Merivon; Luna Sensation / Indar / Luna Sensation; and Flint Extra (at maximum rate) / Indar / Fontelis. Note other DMI fungicides, such as Orbit, PropiMax, Orius, and Rhyme could be substituted for Indar.
Improved Program Benefits
Several benefits are obtained when implementing the improved brown rot program. First, the two captan cover sprays provide a minimal protection level that has been shown to be quite resistant to wash off from heavy rains. So, if the at-risk materials cannot be applied at the above optimum timing due to unfavorable weather, rest assured that the captan will provide a “base level” of control. This should limit the occurrence of a major outbreak.
Second, the at-risk preharvest fungicides are all highly effective materials that are capable of providing excellent control without the captan. However, with the captan present, these materials no longer need to provide “all” the control. This combined effect should result in more durable and consistent control year-to-year.
Third, unlike the at-risk materials, the captan is a multi-site inhibitor and therefore not susceptible to resistance development. Thus, the combination of captan and use of three different at-risk chemistries provides an excellent strategy against resistance development in the preharvest fungicides.
Finally, the improved program does not require any additional sprays. Simply replace the final two cover spray materials (e.g., sulfur) with captan to obtain the benefits. It can’t be any simpler.
