Apr 10, 2015
Penn State: Spray for apple scab now

If not already completed, copper sprays are encouraged for fungal and bacterial disease control. Apple scab primary infection has kicked off this week.

Apple scab

As of April 6, we detected the initial release of mature spores from overwintering leaves in Biglerville. The primary infection period has commenced. We’re only seeing a few spores; however, with the rains we have been experiencing and the mild temperatures forecasted, I imagine those numbers will start to creep up. Remember: mature apple scab spores are not all released at once; the numbers start low (single digits), peak around bloom through petal fall (~20,000 spores), and then the numbers begin to fall to 0, which occurs around mid-June. Consequently, you need to be on your toes for controlling primary apple scab infection from now until about mid-June. If you already haven’t done it, it’s a good idea to apply copper to your trees to help knock back the population of any overwintering scab spores. Copper will also help knock back the bacteria causing fire blight, bacterial spot, as well as the spores from other fungal diseases. For apple cultivars at green tip or beyond, consider applying a broad spectrum (mancozeb, captan), or Syllit (dodine) plus mancozeb or captan right now.

Other diseases

Similar to last year (and the year before), those in the meteorological community are predicting spring is going to take a while to warm up. Consequently, the leaves may be slow to emerge on the trees. If conditions are similar to last year, I anticipate this will be another good year for peach leaf curl. Disease incidence is the greatest when rains wash overwintered spores into the bud and cool temperatures lengthen the time that the emerging leaves are exposed to the pathogen, before they are fully expanded and can resist penetration by the fungus. When leaf development is rapid, particularly during very warm days, peach leaf curl is not an issue because the spores do not have enough time to establish infection. For optimal control, treat trees with a fungicide (copper, chlorothalonil) prior to bud swell. Once the leaves are on the tree, the spores are protected and fungicide control will not be effective.

When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one’s own orchard. Before chemical products are applied, be sure to be in compliance by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS. Specific chemical recommendations are in: the Penn State Tree Fruit Production Guide and the 2015 Spray Bulletin for Commercial Fruit Growers for VA/WV/MD.

Kari Peter, Penn State Extension




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