Disease Update: Apple Scab Issues and Tree Decline

There have been reports about apple scab popping up, as well as many instances of tree decline. Here is a healthy review of why folks are seeing what they're seeing and what is in their control for the management of these pesky issues.

Apple Scab

Management failures do not mean fungicide resistance

Although the primary apple scab period is over, the issues with apple scab may continue for many people. This was a severe year for apple scab: many apple scab infection events occurred during the primary period (early April through early June). We detected the first symptoms of apple scab on May 7. A few weeks later, we observed the first instances of sheet scab on our untreated trees.

Needless to say, the apple scab pressure for the 2019 season has been very difficult. Keep this in mind if you come across apple scab in your orchard. Also, do not automatically assume you have fungicide resistance. As was emphasized during the winter meetings: seeing apple scab, despite fungicide applications, is most likely related to human failure and not related to fungicide resistance.

If you see apple scab, ask yourself the following questions:

• Did you stretch your spray intervals during the primary apple scab period too far: more than seven days for complete sprays; more than 3.5 days for alternate row middle sprays?
• Did you wait too long to reapply fungicides after a very wet period?
• Was your sprayer calibrated?
• Is the affected location in a low-lying area and not drying out completely?
• Have you tested your spray water: was the pH too high? (High pH can decrease the effectiveness of fungicides.)

For folks who are struggling with apple scab this season, the summer cover sprays will be important, and vigilance will be needed to limit disease on fruit.

Tree decline

Soilborne problems and unknown issues

There have been a lot of reports of declining apple trees throughout Pennsylvania. For some cases, the cause of decline is still a mystery. We know tree stress plays a role. Over the last year, our persistent soggy conditions might be the tree stress trigger for many of these trees.

For other cases, there have been soilborne disease issues. If the soggy conditions persist for the remainder of the season, growers may want to consider a proactive approach for dealing with any potential soilborne issue that could creep up later in the season. Regular applications of products containing active ingredients, such as potassium phosphite, phosphorous acid, fosetyl-Al, etc., will help against soilborne diseases caused by Phytophthora and Fusarium.

These products are often referred to as "phosphonates" and include the trade names, like Rampart, ProPhyt, Phostrol, Aliette, Phiticide. These products vary in their rates and application intervals; be sure to follow the label instructions for the product you are using. For more understanding about phosphonates, please refer to this article.

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 orchard. 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.