Several recent overnight cold periods have damaged newly fertilized flowers and/or very young fruit in New Jersey peach orchards. The percent of injured fruit in any particular orchard block varies with cultivar and location. Some blocks have enough un-injured fruit to be salvaged for the growing season. Their economic return will be sufficient to warrant a continuation of standard practices.
In those orchards that lack sufficient fruit, the tendency may be to abandon the spray program. After all, these blocks will produce no revenue, so any activity will result in a net economic loss. However, this thinking is short-sited. Without any form of control, the pathogens will use this summer as an opportunity to increase their populations and, therefore, the amount of overwintering inoculum for the 2021 season. Next year will be a struggle to control disease, especially if the weather is favorable for disease development.
In cold-injured, non-bearing orchards, the three diseases of concern are scab, bacterial spot, and brown rot. The goal will be to provide enough control for disease suppression while keeping costs to a minimum. Details for their control are presented below.
Peach Scab. Fusicladium carpophilum, the causal agent of peach scab, overwinters in twig lesions on current season fruiting wood. In southern New Jersey, spores are produced on these lesions from bloom through early July (~ mid-July in north Jersey). During rain periods, these spores are disseminated to both fruit and the current season’s vegetative shoots. So, even though fruit may not be present (or in low numbers), control is necessary to prevent build-up of shoot infections, which will provide inoculum for the 2021 season. Note these lesions will not be visible until next spring.
If control of fruit scab during the 2019 season was excellent (≤ 5% fruit infected), then this is an indication that the current number of overwintering lesions on shoots may also be low. In this case, the application of Captan 80WDG (2.5 lb/A) at shuck split (SS), first cover (1C), and second cover (2C) followed by bi-weekly sulfur (8-10 lb actual/A) sprays until early July should be sufficient for scab control this season. The captan applied from SS-2C will also control any latent brown rot infection on young green fruit (susceptible until pit-hardening at ~ 2C), assuming they are present.
If >5% of fruit were infected with scab at harvest in 2019, then the current number of overwintering lesions on shoots may also be high. In this case, applications of Flint Extra 3.8 fl oz/A (formerly Gem) at SS and 1C and Quadris Top 12 fl oz/A at 2C followed by bi-weekly sulfur sprays will provide a very strong program for scab control; this program will also manage latent brown rot infection on any green fruit present. If Quadris Top cannot be used because the sprayer is also used on apples (phytotoxicity! see label), then substitute Captan for the Quadris Top.
An intermediate scab program can also be considered. In this case, Flint Extra is applied at SS, Captan at 1C and 2C, followed by the sulfur cover sprays every two weeks until early July. In all of these programs, the Flint Extra acts as an anti-sporulant on the overwintering scab lesions, reducing spore production by 70% for the entire season.
Bacterial Spot. During the growing season from about petal fall onward, the bacterial spot pathogen Xanthomonas arboricola pv. pruni infects fruit, leaves, and current season vegetative shoots (summer cankers) on susceptible cultivars. Thus, the lack of fruit doesn’t prevent the occurrence of an epidemic on the foliage and shoots. If no control is applied, then a larger than usual number of infected leaves, summer cankers, and epiphytic inoculum may be present going into the post-harvest season. This higher level of inoculum may result in a greater number of spring cankers formed for the 2021 season.
Given the above scenario, some form of disease control or suppression is warranted. Bi-weekly sprays of a copper bactericide at 0.5 oz actual (metallic) copper per acre should be sufficient. The goal is to keep inoculum levels low advancing into the post-harvest season. Oxytetracycline, applied prior to critical warm, rainy periods, could also be used. However, the expense of this antibiotic is probably not warranted given the lack of economic return from a non-bearing block.
Brown Rot. If no fruit are present, then control is not needed. In low-yielding peach blocks not destined for harvest, the main goal for brown rot control is to prevent the formation of overwintering mummies. If the brown rot pathogen Monilinia fructicola is allowed to infect and colonize fruit very quickly, then the abscission layer in the petiole may be killed. This action will thereby prevent fruit from falling naturally; the fruit remains attached, becoming a mummy.
Fruit should be marginally protected so that they mature, ripen, and fall from the tree before a significant amount of brown rot occurs. A single spray of any fungicide rated with good to excellent control (e.g., Captan, Indar, Bumper, Fontelis) applied between 9 and 18 days prior to harvest should be sufficient.
An alternative to fungicide control at preharvest would be to simply remove the mummies during the winter or 2021 pruning operation. Or the fruit could be removed earlier in the 2020 season. Either way, fruit or mummy removal eliminates the need for a preharvest brown rot spray. Early 2020 fruit removal also eliminates the possibility of scab or bacterial spot infection on any fruit that survived the cold period.
