Apple IPM - Powdery Mildew

Overview

• Powdery mildew is caused by the fungus Podosphaera leucotricha, which infects the terminal leaves and developing buds of new shoots.
• Infection begins with overwintering fungus in apple terminal buds. In the spring, infected shoots with shriveled leaves emerge from these buds covered with white, powdery spores, which can cause new infections.
• Cultural control is best achieved by avoiding cultivars that are highly susceptible to powdery mildew, such as Cortland, Idared, Gingergold, and Jonathan. 
• Chemical control targeting powdery mildew should be used only in blocks with a history of the disease. Use fungicides that also control apple scab, making sprays at weekly intervals from tight cluster to terminal bud set.

Symptoms

Powdery mildew (PM) infections may occur on leaves, buds, shoots, blossoms, and fruit. Leaf infections appear as whitish, felt-like patches usually on the lower surface of the leaves. Lesions may also appear on the upper surface as yellow spots, or cover the entire leaf with powdery, white spores, and mycelium. Curling and crinkling of the leaves can occur as a result of infections along the leaf margin. Severe infections can lead to leaf drop.  Infected buds can become more susceptible to winter injury. Fruit infections on certain cultivars result in a netlike russeting.

Disease Cycle

The causal fungus, Podosphaera leucotricha, overwinters in dormant buds that had been infected in the preceding growing season. As the weather warms, the fungus produces spores that initiate the primary infections of the disease cycle in the spring. Optimum temps range between 60˚F and 80˚F with high relative humidity favoring infection.  The resulting infections of the young leaves and blossoms then provide inoculum for secondary cycles of the disease. Unlike the spores of the apple scab fungus, free water is not essential for germination of powdery mildew spores; however, high humidity is required. Fruit infection occurs between pink and bloom. The youngest leaves are most susceptible to infection, but become less so as tissues mature.

Economic damage occurs in the form of aborted blossoms, reduced fruit finish quality, reduced vigor, poor return bloom and yield of bearing trees, and stunted growth and poor form of nonbearing trees.

Management Strategies

Monitoring: Refer to orchard history to determine amount of likely overwintering inoculum of this disease. Begin to look for signs of infection when leaves emerge from the bud.

Control strategies

Cultural/Biological:

• Plant cultivars that are less susceptible to the disease whenever possible.
• Dormant-season pruning will open the canopy to light, air, and spray penetration.

Chemical:

• Refer to the New England Tree Fruit Management Guide for specific materials and rates recommended for managing Powdery Mildew in Apples.
• Apply recommended fungicides during the most susceptible period from tight cluster until terminal growth stops, particularly the period after petal fall when vegetative growth is rapid.
• Rotate fungicide materials from different FRAC groups to avoid promoting the development of resistant strains of this disease.
• Pears are also susceptible to P. leucotricha. Spores that infect pears are believed to come from infected apple trees within 200 meters (219 yards). Pear trees outside of this radius are not considered to be at risk.

Date: March 2020
Author(s): Angela Madeiras and Sonia Schloemann, UMass Extension

Additional information available on the MYIPM app: https://apps.bugwood.org/apps/myipmseries/

This work was supported in part by funding provided by USDA NIFA Extension Implementation Program, Award No. 2017-70006-27137