As summer progresses, Wisconsin apple growers should be watching closely for two fungal diseases that tend to build pressure as the season goes on: bitter rot and Marssonina leaf blotch. Both diseases favor warm, wet conditions and can cause significant losses if fungicide programs lapse during the summer cover period. This article summarizes what growers need to know to manage both diseases through harvest.

Bitter Rot

Bitter rot is caused by multiple species within the Colletotrichum fungal complex. Symptoms begin as small, slightly sunken lesions that enlarge over time and eventually develop a characteristic bull’s-eye pattern, and cutting into infected fruit reveals internal rot in a V-shaped pattern. Infection can begin as early as bloom and continue through harvest, but symptoms may not become visible for several months after infection occurs, meaning fruit infected in the field can develop rot symptoms after it’s already in storage, shortening shelf life.

The pathogen overwinters in fallen fruit, mummified fruit, and in crevices in bark and dead wood, making sanitation an important piece of the management puzzle alongside fungicide applications.

Bitter rot is especially problematic during warm, wet weather and tends to be worse on highly susceptible cultivars such as Empire and Honeycrisp.

From bloom through first cover, the period when initial bitter rot infections tend to occur, single-site fungicides in FRAC groups 3, 7, and 11 provide strong protection against bitter rot. Growers should tank-mix and rotate chemistries during this window to avoid overreliance on a single mode of action, and should avoid complex tank mixtures containing captan during this period to reduce the risk of phytotoxicity.

Summer cover sprays and mancozeb (77-day PHI) can also be worked into the first or second cover spray under the label’s extended scab-control program, which provides collateral bitter rot control. As harvest approaches, combinations of FRAC 7 and 11 products, such as Luna Sensation, Merivon or Pristine, provide strong bitter rot control and have the added benefit of short pre-harvest intervals, which can also help limit post-harvest rot.
Additionally, the single-site fungicides such as benzovindiflupyr (Aprovia; 30-day PHI), and fluazinam (Omega; 28-day PHI), are considered effective options against bitter rot. Under moderate disease pressure, ziram provides good control. Notably, FRAC Group 3 and Group 9 fungicides provide little to no control of bitter rot as Colletotrichum species are not sensitive to those chemistries.

Fungicides alone won’t solve a bitter rot problem, particularly where cankers are present. Maintaining trees in a healthy condition is critical for controlling the canker phase of the disease, since cankers generally develop only on stressed or weakened trees. Growers should keep in mind that fungicides are not effective against the canker phase on already-weakened trees. Removing mummified fruit and infected wood throughout the season further reduces the inoculum available to infect this year’s crop and next year’s.

Marssonina Leaf Blotch

Reference: https://extension.psu.edu/apple-disease-marssonina-blotch

Marssonina leaf blotch, is not a new disease to Wisconsin, one of the first US reports of the disease was actually in Wisconsin back in 1903, but it has re-emerged as a significant concern in recent years, particularly in orchards that have received low or no fungicide applications.

Symptoms first appear on the upper surface of mature leaves as irregularly-shaped, grayish-brown spots with a purple border, roughly 1/8 to 3/8 inch in diameter, often containing tiny black fungal fruiting bodies within the lesions. As lesions expand, much of the leaf turns chlorotic and defoliation follows. Infection is favored by temperatures between 68-77°F combined with rainfall, and foliar symptoms typically appear on mature leaves in mid-to-late June, about eight days after infection occurs.

Growers in the upper Midwest should note that peak spore dispersal timing may differ here compared to regions like Pennsylvania, given Wisconsin’s distinct climate. A likely reason the disease has been showing up more in recent years is that fungicide applications tend to taper off as harvest operations ramp up, leaving trees unprotected during a period when the pathogen is still active.

Chemical control should begin around late bloom/early petal fall and continue through early-to-mid June. Ahead of rain events during this window, FRAC Group 3 and/or Group 7 fungicides tank-mixed with a rainfast mancozeb or captan product are the most effective option. Importantly, FRAC Group 11 fungicides should not be used during the early season for this disease, since that mode of action is ineffective against Marssonina leaf blotch; save Group 11 products for later cover sprays targeting fruit rots instead.

Looking at the Wisconsin-specific picture, FRAC groups 3, 7, and 11 provide good control of apple blotch disease overall, and captan in particular provides very good control. Because fungicides can wash off during rain events, reapplication may be necessary to maintain protection, and growers should consider extending their fungicide programs later into the season if warm, wet conditions persist, since it appears that lapses in coverage as harvest approaches are contributing to the disease’s re-emergence.

Organic growers have fewer options, but micronized sulfur, applied at 10-12 pounds per acre, is currently the most effective organic product identified for suppressing the disease, though research into additional organic options is ongoing.

As with bitter rot, sanitation plays a key supporting role. Removing overwintered leaves from the orchard floor and pruning to improve air circulation and speed foliar drying both help reduce disease pressure, and fall applications of urea to accelerate leaf litter decomposition are a commonly recommended practice as well.

For the most current, region-specific fungicide product and rate recommendations, consult the Midwest Fruit Pest Management Guide.