Apr 12, 2019
Disease conditions favorable for fruit trees in Pennsylvania

Cedar apple rust galls are maturing and will be active during wetting events. Conditions will also favor brown rot infections in open stone fruit blossoms. Fungicide applications are encouraged prior to the wetting event.

Well, there is no denying it: the season is here in full force, regardless if one is ready or not. The summer days we have been experiencing this week have pushed the fruit trees in Pennsylvania. The bud development on all fruit trees is moving along. With rain in the forecast for all of Pennsylvania April 12–14, the fungal spores for many pathogens are getting ready for their first big party of the season. Since green tissue is present on apple trees and stone fruit blossoms are open (or ready to open), vigilance is needed. As always, it’s best to apply your fungicide sprays prior to the infection event.

Apple Scab, Powdery Mildew, and Rust

Optimizing management for apple scab, powdery mildew, and rust from green tip through tight cluster

Coinciding with green tip, the first mature apple scab ascospores have started to disperse. We detected a few mature ascospores from overwintering leaves on April 9. The numbers are still very low, but they are on the rise. The ascospores do not all mature and release at once. The ascospores will gradually disperse, peaking from late pink until petal fall (see figure below). Consequently, save your best apple scab control products (FRAC group 7) for your sprays from pink until petal fall when the disease pressure can be at its greatest.

The forecast is favoring an excellent scab infection event: three days with average temperatures around 60°F and more than 6 hours of leaf wetness hours each day. The greatest threat for infection occurs when the average temperature ranges from 61–75°F combined with a minimum of 6 hours of leaf wetness. Although the amount of available mature ascospores is still low, fungicide protection is a must for trees with any green tissue present. Cedar apple rust will also be of concern since rust galls are mature and ready to go once a significant wetting period begins. Using mancozeb will help control rust, as will any FRAC group 3 products. Unfortunately, effective alternative options remain elusive for managing rust. For growers using organic management to control for pests and diseases on apples, your best bet is to limit cedar trees in or around your orchard.

If your trees are at ½ inch green tip, consider using the following:

  • Mancozeb only
  • Syllit 1.5 pt/A plus mancozeb 3 lb/A
  • Alternative options: Potassium bicarbonate 3 lb/A (Kaligreen, Milstop); sulfur

If your trees are at tight cluster, powdery mildew control will begin. The dry periods in between wetting events will favor powdery mildew. Consider using the following to control scab, powdery mildew, and rust:

  • Mancozeb 3 lb/A plus FRAC group 3 (Rally, Indar, Procure/Trionic, Rhyme, Inspire Super)
  • Mancozeb 3 lb/A plus sulfur (refer to the product label)
  • Alternative options: Potassium biocarbonate 3 lb/A; sulfur

To review apple scab, especially the relationship between average temperature and required leaf wetness hours, be sure to check out our Orchard IPM – Scouting for Apple Scab fact sheet.

Brown rot

Have brown rot on your radar for stone fruit during bloom

Many apricots are in full bloom; peaches and nectarines aren’t far behind. Blossom infections from the brown rot fungus can occur whenever pistils are exposed, and a favorable climate exists. Infections can occur during any wetting period when temperatures are between 41 and 86°F. However, optimum conditions for infection occur with wetting and temperatures in the mid-70s. During long wetting periods (several days or more) blossoms can be infected regardless of temperature.

Generally, infections that occur when conditions are sub-optimal are less severe. Blossoms and fruitlets will remain susceptible until the pistil desiccates (sometime between petal fall and shuck split). Keep blossoms protected with fungicides for blossom blight using products with the active ingredient iprodione (FRAC group 2; Rovral, Meteor) mixed with captan. This will give you adequate protection. Save the other FRAC groups (1, 3, 7, 9, 11) for your preharvest sprays later in the season. Alternative management options to manage blossom blight include sulfur.

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.

Kari Peter, Penn State University

Photo at top: Cedar apple rust galls are ready to pop! Be sure to have apple trees protected. Photo: Kari Peter/Penn State University


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