2026 Disease Update: Bitter Rot and Summer Disease Season

We've experienced a drastic swing in temperatures over the last week, where there was a 30 - 40 degree temperature swing over a few days. Conditions have stabilized this week, and the forecast from May 28 to June 5 shows highs in the 70s and 80s, with little rain. However, we know how fickle Mother Nature can be, so growers need to continue vigilance. With these warm temperatures and threat of rain comes the increased risk of bitter rot infections. Being proactive is key when managing this troublesome disease in apple orchards. For those with fruit, you will want to double-check your spray records for May 20, May 25, and May 27. These dates suggest the conditions were met for bitter rot infection. Temperatures were in the 70s, and we had at least 12 hours of leaf wetness.

Additional diseases for the radar are Marssonina blotch, sooty blotch and flyspeck, cherry leaf spot, and bacterial spot of peach.

Marssonina Blotch, Sooty blotch and Flyspeck, Cherry leaf spot

We are in the final weeks of the push for spores from overwintering sources for Marssonina blotch. To address both bitter rot and Marssonina blotch at this time, tank-mix Aprovia (FRAC Group 7) or Omega/Orbus (fluazinam; FRAC Group 29) with captan (3 lb/A) to help control both diseases. However, if you don’t have fruit, you could also use FRAC 11 products Luna Sensation or Merivon, as both include FRAC Group 7, which will control Marssonina blotch. If you have fruit, separate control measures are not needed for sooty blotch and flyspeck; the recommended measures will also cover their management.

Although I always advocate using fungicide sprays preventively, some fungicides can be effective even when an infection is already active. Ideally, apply your fungicides within 24–48 hr after the rain event. The warmer the temperature, the shorter the post-infection window of susceptibility.

For those with a light to no apple crop and who want to be conservative with a spray program right now, switching to captan (3 lb/A) or micronized sulfur (10-12 lb/A) is your best option. However, when using only broad-spectrum fungicides during the season, a few words of wisdom need to be kept in mind when using:

• If you are using alternate row middle (ARM) sprays, please keep intervals short. The longer the interval is to cover the other side of the tree, the longer that side is without fungicide to protect it from disease.
• At the time of rainfall/infection event, keep in mind how recent the application is. The more recent the application, the more effective it is.
• Also be mindful of how much new growth has occurred since the last spray.

Even if you may not have a tart cherry crop this season, don't forget about managing for cherry leaf spot since cherry leaf spot can severely defoliate trees. Consequently, growers will want to protect trees to keep leaves on for as long as possible. Cherry leaf spot is similar to apple scab in terms of infection conditions. The optimal temperature range for the spread of this fungus is between 60 to 68°F. Many products labeled for apple scab control will work to keep cherry leaf spot in check. A good protection spray regime is necessary through postharvest. 

Beware of Bitter Rot

Bitter rot is one of the most important fruit rots affecting apple growers in the Eastern U.S. over the last several years, with Honeycrisp being an exceptionally vulnerable cultivar. The fungus causing the disease is one of the few fruit rot organisms that can penetrate the fruit's unbroken skin. When the spore penetrates the skin, the infection enters a dormant (quiescent) phase. During this time, the spore does not grow and is not susceptible to fungicides. Consequently, fungicides must be applied before the spores infect. In contrast to apple scab, you cannot rely on fungicides to control bitter rot fungus post-infection. Fruit maturity, temperature, humidity, and disease presence determine when the quiescent period ends and symptoms manifest. When you see symptoms, it is too late to control the disease. 

Effective bitter rot management during the season

Over the last several years, we have been studying how best to manage bitter rot. Our research has shown the following:

• Bitter rot spores are available all season long, most likely residing throughout the tree in buds and mummified fruit left in the tree.
• The spores are dispersed by rainwater, and high disease pressure is favored by warm temperatures (>75°F) and prolonged periods of moisture. Based on research in other systems, the fungus causing bitter rot requires at least 12 hours of wetness to infect fruit.
• With spores available all season long, we have narrowed the timing for when fruit is most susceptible. Fruits are most susceptible to infection when these conditions (wetness hours lasting at least 12 hours and temperatures >75°F) are most favorable. For the last few years, these conditions have been met as early as mid-May and lasting through harvest. Growers should be vigilant for bitter rot infection conditions starting in May.
• Growers are encouraged to apply fungicides Before the infection period (i.e., warm, rainy events).
• Tank mix captan (3 lb/A) with one of the following:
  • Aprovia (FRAC Group 7; 30 day PHI)
  • Omega 500F or Orbus 4F (FRAC Group 29; 28 day PHI)
  • Cabrio (FRAC Group 11; 0 day PHI)
  • Flint Extra (FRAC Group 11; 14 day PHI)
  • Luna Sensation (FRAC Groups 7 + 11; 14 day PHI)
  • Merivon (FRAC Groups 7 + 11; 0 day PHI)

Topsin (FRAC Group 1) and Fontelis (FRAC Group 7; applied at 20 fl oz/A) are additional options; however, they are less effective at controlling the dominant fungal species causing bitter rot in Pennsylvania. If used, captan must be tank mixed.

Be sure to rotate FRAC groups to manage fungicide resistance. Growers need to remember what was applied in the early season to manage apple scab, since FRAC Group 7 and FRAC Group 11 fungicides, whether in a premix or as a stand-alone product, are limited to four complete spray applications per year per the label. If there are multiple rain events in one week, be mindful of the amount of rain that has fallen and when the fungicide was last applied to determine whether reapplication is necessary and whether alternate row middle (ARM) is being used. For growers sending fruit to packers or holding fruit in storage, it is strongly recommended to save a Merivon application as a last spray near harvest, as this will help address postharvest rot issues that may occur during storage.

Additional products can be added that have been observed to help with bitter rot management. Research in the Southeastern U.S. has shown that adding potassium phosphite products (Rampart, Reveille, etc.) to fungicide sprays helps control bitter rot.  During years of moderate disease pressure in Pennsylvania, the addition of these products has reduced the incidence of bitter rot during storage. However, these products contain potassium, so cultivars where the potassium–calcium balance needs to be monitored due to susceptibility to bitter pit should be used with caution. We have also observed that Regalia (1-2 qt/A) tank mixed with a conventional spray application provides a modest boost in protection, which would be most beneficial during seasons with frequent rain events.

Organic options are limited. We have observed control with micronized sulfur; however, sulfur can cause fruit russeting at 80°F or higher. Serenade (ASO or Opti), Double Nickel LC, and OSO 5%SC have provided limited rot protection and must be reapplied since these products can be washed off. If using Regalia for organic control, it's best used when added with another product (sulfur, Serenade, etc.) and not alone. We are currently researching additional alternatives to increase the tools in the grower toolbox for both organic and conventional growers.

Bacterial spot of Peach (and Nectarine, Apricot, Plum)

Conditions are favoring bacterial spot on stone fruit. Management programs for susceptible stone fruit varieties (peach, nectarine, plum, apricot) should be underway and continue until about 3 weeks before harvest, when preharvest brown rot management program occurs. Use 0.5–1 oz of metallic copper per acre. For more information on this copper use, please refer to Dr. Norm Lalancette's article on copper bactericides. Rotate copper with the following:

• Serenade ASO 4 qt/A
• Serenade Opti 14 oz/A
• Double Nickel 2 qt/A
• Regalia 2 qt/A
• Mycoshield/FireLine 12 oz/A

For additional resources

Dr. Tree Fruit and Don Podcast 

Penn State 2026 - 2027 Tree Fruit Production Guide

For commercial fruit growers, please note: When controlling for disease, weather, and tree growth conditions need to be monitored at a local level within one's own orchard. To support management decisions (especially during infection events), growers can use the NEWA website, which has weather stations across Pennsylvania.  Before applying chemical products, be sure to comply with current use regulations and read the product label. Product information can easily be obtained from CDMS, Agrian, or Greenbook.