Aug 19, 2019
Be sure to properly manage tree fruit rots in the 2019 season

Depending on your location, significant rainfall has occurred recently in Pennsylvania. Unfortunately, these rainy, humid conditions are quite favorable for rot, both in establishment and spread. Vigilance is needed in the orchard, especially if brown rot pops up. Some nuggets of wisdom to keep in mind:

Peaches and nectarines: A review of brown rot management strategies

Peaches and nectarines are ripening, which means ’tis the season for brown rot. The fungus causing brown rot is quite opportunistic: it can kill blossoms, and it can also ruin the fruit you’ve worked hard all season to grow. Brown rot disease is favored by warm, wet weather conditions. Under optimum temperature conditions, fruit infections can occur with only three hours of wetness when inoculum levels are high. Longer wet periods during infection result in shorter incubation times, so symptoms develop more rapidly. It’s not uncommon to have brown rot appear “overnight” on fruit.

Spores produced on early maturing cultivars can fuel a continuing outbreak on late-maturing cultivars – this is especially important for those who have battled rot infections already this season. To add another headache to the issue, insects can be important vectors of the fungal spores during fruit ripening: they can carry spores to injury sites produced by oriental fruit moth, Japanese beetle, green June beetle, and other insects that can injure fruit. Wounded fruit are much more susceptible to brown rot than unwounded fruit. It’s critical to be on top of insect management. Another concern to worry about is split pit. Unfortunately, these fruit are quite prone to rot problems. Keep in mind: under the right conditions, “healthy” fruit harvested can be contaminated and may decay later during storage.

Research at Rutgers has shown an effective brown management program near harvest includes: applying captan (no lower than 3.125 lb/A) for the last cover spray, followed by preharvest brown rot sprays at 18 days, nine days, and one day before harvest. This management regime provided greater than 95 percent control under heavy disease pressure. When following this regime, rotate chemistries used during the preharvest sprays by FRAC group for resistance management. Keeping in mind products that were used to control blossom blight, be sure to be in compliance by obtaining the current usage regulations and reading the product label. For example, one could spray the following (provided the maximum number of sprays has not been exceeded for that chemistry):

  • 18 days: Fontelis (FRAC Group 7; 0-day PHI)
  • 9 days: Indar (FRAC Group 3; 0-day PHI)
  • 1 day: Merivon (FRAC Group 7 +11; 0-day PHI).

Other options to rotate:

  • Luna Sensation (FRAC Group 7 + 11; 1-day PHI)
  • Luna Experience (FRAC Group 7 + 3; 0-day PHI)
  • Topsin M (FRAC Group 1; 1-day PHI)
  • Inspire Super (FRAC Groups 3 + 9; 2-day PHI)
  • Orius (FRAC Group 3; 0-day PHI)
  • Tilt (FRAC Group 3; 0-day PHI)
  • Quash (FRAC Group Code 3; 14-day PHI)
  • Flint Extra (FRAC Group 11; 1-day PHI)
  • Captan (FRAC Group M4; 0-day PHI)

As a result of the rain, postharvest diseases, such as Rhizopus rot, might create headaches. The spray closest to harvest will be important: the best options are Merivon or Luna Sensation. These products are labeled to control Rhizopus rot.

Additional options for peach/nectarine rot management

The key for growers who farm organically or prefer using alternative products is to spray as often as possible as disease conditions persist, manage insects, scout often, and prompt removal of infected fruit as soon as you see it. Spraying often ensures you have continuous protection; removing infected fruit from the trees ensures you are decreasing the number of spores available to cause disease and hopefully minimizing an epidemic. Knocking infected fruit to the ground will be enough to limit spread. Vigilance is important, and this may translate spraying every few days, especially if rain washes off products. According to studies at Rutgers, sulfur is not effective for controlling brown rot. Some organic options labeled for brown rot control are Cueva, Double Nickel, Serenade Opti (or Serenade ASO), and Regalia.

Apples: Protecting fruit from rot

We are nearing the home stretch of the apple season and folks will want to be considering sprays to keep their apples free of rot, especially while in storage. Not only a headache in the field, but the fungi causing fruit rots can be quite stealthy since spores will land on the fruit and cause symptoms only after the fruit have been in storage. This is especially significant if your apples are headed for a packinghouse or even fresh market.

Continue to protect apples from bitter rot.

Last year was a very challenging year for bitter rot. There have been several reports of growers already observing bitter rot on some of their immature fruit. The fungi that cause bitter rot initially infect fruit early in the season and then go dormant, i.e., the fungus stops growing. There are no obvious symptoms for these early infections. Usually, late July through early September, we begin to see these infections “awaken,” and fruit rot symptoms become evident. Honeycrisp and Empire are especially susceptible to bitter rot. Unlike other fungi causing rot, the bitter rot fungus does not require fruit wounding to establish an infection and can directly penetrate the fruit skin. Rot spots usually appear on the side of the apple directly exposed to the sun as small, circular brown lesions and change to sunken, dark brown lesions as they enlarge. During humid conditions, large numbers of creamy to salmon-colored spores are produced. Fruiting bodies visible to the naked eye appear after the lesion is one inch in diameter and are arranged in a concentric circle pattern in the center of the lesion (see photo). These spores are spread through the tree canopy via rain splashing.

For growers who struggled with bitter rot last year, I recommend at least one application of Omega (6.9 fl oz/A; 28-day PHI) in July to protect Honeycrisp and other vulnerable varieties. Be mindful of Omega’s PHI for earlier varieties. Rotate Omega with captan at 3 lb/A or 5 lb/A, depending on disease conditions: higher rate during very rainy periods; lower rate when not as rainy. I highly encourage growers to use Merivon (FRAC Groups 7 + 11; 0-day PHI) as their last one or two sprays prior to harvest since these products do show efficacy keeping rots, including bitter rot, in check while in storage. Based on our studies over the last year, Luna Sensation is not effective in controlling bitter rot. These recommendations are both for fresh market and juicing apples: the pack houses and processors will thank you!

For alternative methods, Serenade Opti or Serenade ASO (B. subtilus; 0 PHI) is an option. Our research has shown minimal fruit rot diseases in the field and storage, at least on Golden Delicious when we used this product during our last few sprays. These results may vary with other cultivars, depending on their susceptibility to certain rot diseases, as well as the severity of disease conditions. We are evaluating additional alternatives during our 2019 trial season.

Kari A. Peter, Penn State University

Photo at top: Warm, wet weather favor brown rot disease: keep peaches and nectarines protected. Photos: Kari A. Peter/Penn State University

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