2022 Disease Update: Scout for Fire Blight and Apple Scab Infections

It is important growers scout their orchards for potential infections. The earlier an infection is caught, the easier it will be to manage. During dry days, assess disease in your orchards and take remediation steps, if necessary.

Fire Blight Management

If infection occurred in your orchard, symptoms would start to be visible beginning May 27. Consequently, it is imperative growers begin scouting their orchards this week and next week. The earlier fire blight infections are caught, the easier it is to manage the disease.

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 observe fire blight, consider using the “Actigard Paint" method:

• 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 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. Do not apply as a spray since 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.
• If an infection is evident, consider an application of the highest rate of ProCa (12 oz/A) to shut down the infection spread. Another option would be three applications of ProCa (6 oz/A) + Actigard 2 oz/A.

If it is a bad fire blight year, 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 severe 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 during the season 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 and make your fire blight problem even worse. This is especially true for older orchards where cankers may be lurking about in the tree.

Apple Scab Management

We are at the end of the primary phase of apple scab. During this time, we experienced several scab infection events when the spore counts were high (April 25 – May 16). It is important growers scout their orchards for scab infections to determine if there are any issues that need to be addressed.

To scout: For each orchard block, follow a "W shape" pattern within the block when scouting. Evaluate 10 trees by examining 20 leaves on each of 5 limbs per tree and record the number of leaves showing any scab lesions.

1. Begin with the flower bud (spur) leaves where early infections are most likely to be noticed.
2. Start with observing the undersurface of leaves since the undersurface of leaves may become spotted before the top surface. Take notice of early lesions which may be small light brown-black spots.
3. As scouting continues during early spring, be sure to observe both the topside and underside of the leaf. Apple scab infection appears as brown velvety lesions, which will become darker as they age.
4. After fruit has set, in addition to leaf observations, also examine 20 fruit on each tree and record the number showing any scab lesions. Use this information to better manage scab in the future.

Refer to Orchard IPM – Scouting for Apple Scab for more information.

If apple scab is observed, applications of fungicides contain FRAC Group 3 + a broad spectrum fungicide should help slow down the infection. Sulfur (8 – 10 lb/A) is also recommended; however, sulfur can be phytotoxic on fruit when temperatures any time during the day are 80°F or higher. It is important that any trees with scab infection be monitored regularly since scab can cause repeated infections throughout the tree once established.

Figure 2. (left) Don't let fungicide protection lapse since apple blotch spores are active. (right) Apple blotch defoliates trees very quickly. Be sure to reapply fungicides after rain events during the summer. Photos: K. Peter, Penn State

Beware of Apple Blotch Disease

When the threat of one disease ends, another pops up. While we have been monitoring apple scab spores, we have been observing a lot of Diplocarpon coronariae spores in our samples. This fungus causes apple blotch (formerly known as Marssonina blotch), which can defoliate trees very quickly during a heavy infestation. It is critical growers do not let their fungicide applications lapse during the summer, especially after heavy rain events. More will be discussed in future disease updates.

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 assist with management decisions (especially infection events), growers can use the NEWA website, which has weather stations all over Pennsylvania. Before chemical products are applied, be sure to comply by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS.