Have you ever felt that you did everything right at controlling brown rot during the preharvest period, but still got a significant amount of brown rot at harvest? Your preharvest fungicide applications were at tight intervals during the roughly 21-day fruit ripening period. Rains occurred but you were protected. Even when you looked back at the bloom period you saw no obvious gaps in coverage. Yet, the brown rot appeared at harvest. If this is you, read on.
Traditional Coverage
There are two “traditional” phenological periods during which we eastern peach growers must control brown rot. The first period, of course, is the bloom period, during which fungicide sprays are applied at pink, full bloom, and petal fall. We need to prevent flower infection because these infections results in blossom blight cankers, which then produce inoculum (spores) that can infect fruit and cause rot. It’s worth noting that blossom blight is almost never so great that it reduces yield; peaches are so prolific. It’s all about the cankers and the inoculum.
The second traditional phenological control period is during fruit ripening. During mid-to-late summer, the green fruit are not susceptible to brown rot. However, beginning at approximately three-weeks prior to harvest, the fruit will begin to ripen. The background fruit color will begin to change to yellow. As the fruit ripens, they become increasingly susceptible to infection by Monilinia fructicola, the brown rot pathogen. Hence, we apply two (early season cultivars) to three (mid-late season cultivars) fungicide applications during the ripening period to control the fruit rot phase. This should all sound quite familiar.
Hidden Fruit Infection
There is a third phenological period during which brown rot control can be exerted. Early research in western U.S. discovered that infection of young fruit, soon after flowering, can result in the formation of quiescent infections. These infections can remain invisible (latent) or appear as small, inactive lesions (quiescent). Later in the season, as the fruit ripen, the pathogen becomes active and begins to form the normal, large brown rot lesions. It’s too late for your preharvest fungicides; the infection took place earlier in the season.
Quiescent or latent infections are not common in eastern U.S orchards. At most, only a few percent of young fruit have been observed to become infected. Consequently, little attention was given to control of brown rot at this time. After all, most fungicides applied during this period for peach scab control would also provide control of quiescent brown rot infections (except sulfur!).
The Data Speaks
During 2017 and 2018, a study was conducted to determine the contribution of summer cover sprays to management of brown rot. One particular treatment in this study consisted of captan sprays at shuck-split, first cover, and second cover. No sprays were applied before shuck-split or after second cover.
Surprisingly, when brown rot was assessed at harvest, the captan sprays provided 40% control of brown rot in both years. This result indicated that considerable latent or quiescent infection must have been occurring on the young green fruit. A direct assessment of infected fruit needs to be conducted, but the disease control data provided strong evidence that significant numbers of young fruit were infected during this period prior to pit-hardening (~ second cover).
A Third Control Period
Given the above finding, it is recommended that captan be applied at shuck-split, first cover (10-days later), and second cover (12-days later). The recommended rate is 3.125 lb/A of captan 80WDG. This rate provided control equivalent to 3.75 lb/A, but significantly better than 2.5 lb/A. This program will provide control for both quiescent / latent brown rot infections and normal levels of peach scab. Do not use sulfur during this period, as it will only provide effective control of scab, not brown rot.
High Levels of Scab?
If at least 5-10% of fruit from a particular block had scab lesions during the previous season, then a more effective, but temporary, scab program should be added to the quiescent brown rot control program. At early shuck split and first cover, add either Flint Extra (3.8 fl oz/A, formerly Gem) or Abound (15.5 fl oz/A; note toxicity to apples). These materials will help reduce sporulation of the scab twig lesions (source of inoculum), as well as provide additional protection for both scab and brown rot fruit infections.
Note that the above scab program is only for rehabilitation of blocks that have significant amounts of scab inoculum. Once employed, the fruit later that season should have significantly less scab (< 5%). In this case, the block can go back to the standard captan program in the following year. Using the rehabilitation program every year unnecessarily adds to the program cost and increases the likelihood of resistance development to the at-risk fungicides.
Next Step
After second cover (~ pit hardening), the green fruit are no longer susceptible to brown rot, so the remaining cover sprays can be either captan or sulfur. Both materials will control scab. However, if anthracnose has been problematic (not common), then captan should be used. These covers are typically applied at 14-day intervals, although a shorter 10-day interval should be considered if frequent rains occur.
Stay tuned for details on improving your late-season brown rot control program …
