As primary apple scab season winds down across Wisconsin, many growers are beginning to ask an important question: Can I reduce my focus on scab and start thinking about summer diseases? The answer depends largely on whether active scab lesions are present in the orchard.

Secondary Infections Driven by Existing Lesions

While primary apple scab infections originate from ascospores released from overwintered leaves, secondary scab is driven by conidia produced on existing lesions. If lesions are present, they can continue producing inoculum throughout the summer and initiate new infection cycles whenever rainfall and leaf wetness conditions occur.

At the same time, growers should also be thinking about Marssonina leaf blotch, a disease that has become an increasing concern in Wisconsin orchards. Recent periods of frequent rainfall and prolonged leaf wetness have created favorable conditions for both diseases.

Secondary Apple Scab in Wisconsin

Once primary scab lesions become established, they serve as a source of inoculum for the remainder of the season. Unlike primary scab, which typically ends when ascospore release is complete, secondary scab can continue as long as active lesions remain in the orchard and environmental conditions favor infection.

Secondary scab can result in:

• Additional leaf infections
• Fruit infections and russeting
• Increased inoculum production throughout the season
• Premature defoliation in heavily affected blocks

The most important management decision at this time of year is determining whether active scab lesions are present. Orchards that remain free of lesions after primary scab season have substantially lower risk than orchards where infections became established earlier in the year.

Scouting for Secondary Apple Scab

Take time to carefully scout representative areas of each block, especially:

• Lower canopy leaves
• Interior canopy areas with reduced air movement
• High-risk cultivars
• Areas where spray coverage may have been compromised during rainy periods

Young scab lesions often appear as olive-green, velvety spots on leaves before becoming darker and more distinct.

Marssonina Leaf Blotch

Even if scab pressure appears low, growers should remain vigilant for Marssonina leaf blotch. Unlike apple scab, which often begins to decline once primary inoculum is exhausted, Marssonina can become increasingly important during the summer months.

Marssonina leaf blotch symptoms typically begin as small dark green to black lesions surrounded by yellowing tissue. As disease progresses, leaves may develop extensive yellowing and drop prematurely. Severe defoliation can weaken trees, reduce fruit quality, negatively affect return bloom, and increase winter injury risk.

The disease is favored by:

• Frequent rainfall
• Extended periods of leaf wetness
• Dense canopies with poor air circulation
• Warm summer temperatures

Many growers first notice the disease after extended rainy periods in July and August, but infections often occur earlier than symptoms become visible. I have already started to observe symptoms in some Wisconsin orchards.

Fungicides for Apple Scab and Marssonina Leaf Blotch

The presence or absence of scab lesions should help guide fungicide decisions over the next several weeks.

If No Scab Lesions Are Present

Growers may be able to gradually transition away from an intensive scab-focused program while maintaining protection against summer diseases such as Marssonina leaf blotch, bitter rot, black rot, sooty blotch and flyspeck.

If Scab Lesions Are Present

Continued protection is warranted to suppress secondary scab spread while simultaneously maintaining activity against Marssonina and other summer diseases.

When selecting fungicides:

• Rotate FRAC groups to reduce resistance risk.
• Avoid repeated applications from the same mode of action.
• Maintain coverage during periods of frequent rainfall.
• Consider the activity spectrum of products against both scab and Marssonina.

Several fungicide classes commonly used during summer cover sprays provide activity against both diseases, making product selection an important component of integrated disease management. Products effective against Marssonina leaf blotch include fungicides in FRAC 3 (ex. Cevya, Indar, Rally) and FRAC 7 (ex. Aprovia, Excalia, Sercadis). For organic systems, micronized sulfur is recommended.

Both secondary apple scab and Marssonina leaf blotch require moisture for infection. Given the frequent rainfall experienced in many Wisconsin orchards this season, maintaining awareness of infection periods remains important. 

Additional Reading

• Marssonina Leaf Blotch (Penn State Extension)
• Apple Blotch Disease Update for Wisconsin Growers
• Scab Frost: Now What?Ω