Disease Status in this Season of Favorable Conditions for Infection

Posted: May 16, 2011

Some of you have contacted me wondering if we are still in the primary apple scab stage. There have been reports indicating that primary scab is over in neighboring states and it has also been suggested that ascospore maturation would progress much faster, and therefore terminate early in the kind of weather we are experiencing this spring. To clarify, today we counted ascospores in numbers that far exceed our monitoring threshold so we are still in the primary apple scab phase of the disease. Also, we observed the first fruits with apple scab symptoms on nontreated trees here at the Penn State Fruit Research and Extension Center this morning.
Apple scab

Apple scab


Dr. Henry Ngugi, Penn State Fruit Research and Extension Center Plant Pathologist

More on apple scab:  Be reminded that fruit are most susceptible to the scab fungus at this early stage and progressively become less susceptible as they mature. Given the prevailing weather conditions, this is the time to get the big guns into action, e.g., the new-generation DMI fungicides which have shown good scab control on fruit. 

Fire blight:  So far, we have had ideal weather for fire blight development and the risk will be higher in blocks that had, or are adjacent to those that had, the disease last year. Expect to start seeing symptoms anytime within a week from today so prioritize scouting.  

Powdery mildew:  Powdery mildew is another disease that is doing very well in this weather with a few dry days here and there that are characterized with high relative humidity. We have plenty of primary powdery mildew already on our nontreated trees. I assume that you all followed my earlier recommendations (remember the slide with arrows from the winter fruit meetings on how to manage scab in orchards with moderate levels of resistance to DMI fungicides and my Fruit Times article in March? - and used the DMI fungicides that we recognize as having best efficacy on mildew because at this time you should be switching to those we identified as having best control on apple scab especially on fruit. 

Bacterial spot on peach and nectarine:  Whereas I have not received a call from any of you, we are seeing high levels of infections on my peach and nectarines here at the Fruit Research and Extension Center. Colleagues in the Southeastern U.S. are also reporting very severe epidemics. By this time your second cover sprays should be on and in this weather you should tighten your oxytetracycline spray intervals. If this weather continues, you should plan to rotate oxytetracycline products with low rates of Kocide 3000 which has a label for cover sprays. Be careful about those low rates; you should apply them on days when you expect the spray solution to dry rapidly to minimize the risk of injury.