2026 Disease Update: Marssonina Blotch, Bitter Rot Risk

With warmer temperatures returning to the forecast this week and the threat of rain, conditions will favor several apple diseases. Growers need to be on alert, regardless if they have a crop. Apple scab, Marssonina blotch, and rust remain a threat to leaves. For those with fruit, the first major infection period for bitter rot will occur if it rains. Fire blight is starting to rear its ugly head in orchards, so growers will want to be vigilant and cut out branches as soon as they see them.

Marssonina Blotch, Apple scab, Rust, Cherry leaf spot

We are in the final weeks of the push for spores from overwintering leaves of apple scab, cedar apple rust, and quince rust. In addition, this coming week will be an important one for Marssonina blotch infection if the forecast of rain materializes. Temperatures will be warm and moderate throughout the week, with rain in the forecast. The Marssonina blotch infection window continues until about mid-June. Infection favors 68-77°F with as few as 4 leaf wetness hours. Tank mixing your rainfast mancozeb with FRAC Groups 3 and 7 products at this time. FRAC Groups 3 and 7 fungicides work well on scab, rust, and Marssonina blotch. Ideally, save your FRAC Group 11 fungicides for summer cover sprays to control rots. In addition, FRAC Group 11 fungicides are weak in controlling Marssonina. However, if you don't have fruit, you could also use FRAC Group 11 products, such as Luna Sensation or Merivon, as both contain a FRAC Group 7 fungicide, which will control Marssonina blotch.

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, particularly FRAC Groups 3 and 7. 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, tank-mixing mancozeb (3 lb/A) with captan (3 lb/A) is an option to consider. During the early season, this mixture suppresses disease better than either product used alone. However, the success of this combination works when the following are kept in mind:

• 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. 

Bitter rot

With spores available year-round, we have narrowed the timing to when fruits are 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. Those conditions may begin later this week.

Growers are encouraged to apply fungicides BEFORE the infection period (i.e. warm, rain events).  Tank mix captan (3 lb/A) or a rainfast mancozeb (3 lb/A), if you are not within 77 days of preharvest, with one of the following:

• Aprovia (FRAC Group 7; 30 day PHI)
• Omega 500F or Orbus 4F (FRAC Group 29; 28 day PHI)
• Flint Extra (FRAC Group 11; 14 day PHI)
• Cabrio (FRAC Group 11; 0 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; used 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.

Organic options are limited. We have observed control with sulfur; however, it 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.

Fire blight

Continue scouting your orchards for fire blight. The 2026 bloom experienced severe fire blight conditions, and growers are beginning to see infections. The plant growth regulator Prohexadione calcium (ProCa; Kudos or Apogee) is highly recommended to help slow infections. ProCa is a plant growth regulator that thickens plant cell walls and hardens off the shoots. It takes 10-14 days for the effect to occur in the tree, and repeated applications may be necessary to help control potential shoot blight that may arise from blossom infections, or leftover cankers in the trees (i.e. canker blight).

With recent reports of fire blight incidence in orchards, one option growers may want to consider is FungOUT (AEF Global Biopesticides), a 1.07% citric acid product that is considered a fungicide and bactericide. Anecdotal reports indicate growers have found success with this product when active fire blight infections are present. Consequently, at Penn State FREC, we decided to evaluate FungOUT this year in our fire blight research apple block. We hypothesize that this product should be effective against fire blight because Erwinia amylovora dislikes acidic conditions. According to the label, use FungOUT at 1.4% v/v every 2-3 days, and adding a spreader may improve quality. Growers are to use this product at their own discretion, as this is the first year we are testing it, and we do not know for certain whether it will be effective in stopping an active fire blight infection in its tracks.

If you encounter fire blight while scouting and choose to prune, we recommend a "triage" method when it comes to pruning decisions once fire blight has struck, going from highest to lowest priority:

• Young orchards 3 – 8 years old with just a few strikes. (highest priority)
• Young orchards 3 – 8 years old with many strikes.
• Older orchards with a few strikes.
• The "walk away" group: orchards with so many strikes that most of the tree would need to be removed; severe pruning can stimulate new growth that can become infected. (lowest priority)

Folks have been told all along to prune out fire blight when they see it; however, there can be too much of a good thing. It is very important to avoid excessive cutting when pruning out fire blight. Excessive cutting will encourage more shoot growth, worsening your fire blight problem. This is especially true for older orchards where old fire blight cankers may be lurking about in the tree. 

Items to keep in mind when pruning out fire blight infections 

• Do not cut out infections during wet weather since bacteria move via water.
• Cut out active infections early - before necrosis develops (limits the spread of bacteria).
• Pruning is most effective when incidence is low.
• Focus on salvaging tree structure and young high-density plantings when incidence is high.
• Bacteria can invade healthy tissue up to ~3 feet in advance of visible symptoms, which makes tool sterilization ineffective.
• Practice the ugly stub method: cut 6-12 inches below the margin of visible infection and remove later during winter pruning.
• Ideally, infected tissue should be removed from the orchard and burned. However, if pressed for time, prunings can be left in the row middles to dry out and subsequently chopped with a mower.

If you are pruning trees to remove infections, consider using the "Actigard Paint" method when cutting out fire blight. After cutting out the fire blight, apply a high-concentration spray of Actigard to the cut and 1 meter of the trunk below the limb with the fire blight. According to the label, mix 1 oz of Actigard in 1 quart of a 1% penetrant. The penetrant suggested on the label is Pentrabark (an organosilicone); however, a similar penetrant can be used. Use a one-gallon handheld sprayer for this method, and avoid spraying the leaves when spraying the cut and trunk. The leaves will be adversely affected by the high concentration of the solution. One quart will treat approximately 500 cuts. Do not apply within 60 days of harvest.

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.