Cold injury to peach flowers and primordial fruit occurred during the evening of April 5-6 as temperatures dropped below freezing. At the Rutgers Agricultural Research and Extension Center, air temperatures first dropped below 32ºF at 10 pm Tuesday and remained below freezing until 9 am Wednesday morning. The lowest temperatures of 22 to 23 ºF occurred for four hours between 4 and 7 am. The average low temperature during the entire 12 hour cold period was 25.5 ºF.
The extent of injury to flowers and fruit varied by cultivar and stage of flower development. In our research blocks, the cultivars Bounty, O’Henry, Suncrest, Encore, and Autumnglo were assessed to have 5, 36, 59, 37, and 64% viable fruit, respectively. GaLa peach trees were not assessed, but appeared to have fruit loss similar to the Bounty.
Peach orchards that have little or no fruit still require disease control to avoid build-up of inoculum for the 2017 growing season. Below are recommendations for these orchards.
Scab. Although fruit infection is not a concern, spores from overwintering scab lesions will continue to infect the new current season shoots. These infections will result in overwintering lesions that will provide inoculum for the 2017 season. Thus, fungicide sprays need to be applied to protect these shoots.
Fungicide sprays should be applied until approximately 100 days after the pink stage, which will be early to mid-July for most cultivars in NJ. At this time, the current overwintering lesions cease to produce inoculum. A basic program would consist of captan or sulfur sprays beginning at early shuck-split (estimate this timing). If many scab lesions are present, an application of Gem or Abound at petal fall, shuck-split, or first cover would help to reduced inoculum levels by acting as an anti-sporulant.
Brown rot. If a small number of fruit are still present in the orchard, some of these fruit may become mummies, thereby providing primary inoculum for the 2017 growing season. This mummification is even more likely in the absence of preharvest fungicide sprays. However, no additional work need be performed as these mummies can be handled in the normal fashion.
Ideally, mummy removal is best during the fall by simply knocking them to the orchard floor where they will have time to decay before next spring. Removal of the mummies during spring pruning is another, perhaps more cost effective, option. However, unless removed from the orchard, these mummies will still sporulate on the orchard floor and provide inoculum for blossom blight.
Preferably, the fruit peduncle (fruit stem) should also be removed when detaching the mummy since it also provides inoculum. Knocking off the fruit often leaves the peduncle attached to the tree, so a knife cut along the stem would be required to remove both mummified fruit and stem. This operation may well be too laborious and costly. Thus, growers should be aware that they may have more infected peduncles next year in the canopy and make sure to apply a robust blossom blight program.
Bacterial spot. In a normal year with a crop, low doses of copper bactericides are typically applied during the post-bloom cover spray to protect fruit from infection. This program reduces the incidence and, in particular, the severity of disease development on fruit; there is much data to support this claim. However, these sprays rarely improve foliar health. While the copper may prevent some leaf infections, it also causes its own injury, and along with the bacterial spot not controlled, it contributes to defoliation. Furthermore, it has not been demonstrated that copper can prevent the formation of summer cankers on the current season’s new growth. Thus, copper is not recommended for orchards without fruit.
If a particular peach block consists of a highly susceptible cultivar, then it is possible that severe defoliation may occur in a disease-favorable growing season. Thus, these blocks should be closely monitored for the degree of disease development. If foliar infection appears to be increasing in severity (number of lesions; defoliation), then one or more applications of the antibiotic oxytetracycline (Mycoshield, FireLine) may be needed. For best results, these applications should be applied before warm (75-85ºF), wet, and windy conditions.
Constriction canker. The lack of fruit has no bearing on the development of constriction canker, which occurs exclusively on shoots. Thus, normal control practices should be followed. Canker removal during the summer, prior to leaf drop, and fungicide applications during fall and spring bud-break periods are recommended for susceptible cultivars currently harboring cankers. Further details can be found in the “Tree Fruit Pests and Controls” section of the 2016 New Jersey Commercial Tree Fruit Production Guide.
