Kelly Jackson
Christian County Extension Office
Apples Vary in Resistance to Powdery Mildew
In Kentucky, apple trees often show symptoms of powdery mildew, particularly following mild winters (where temperatures do not drop below +10F). Powdery mildew is caused by the fungus Podosphaera leucotricha. This time of year, the fungus is overwintering in dormant buds. In early spring, spores produced in the overwintering buds infect nearby young leaves, blossoms, and fruit. These primary infections provide more spores for secondary infection of young leaves, shoots, and fruit.
On apple leaves, powdery mildew appears as whitish patches of fungal growth which may spread to the entire leaf. Powdery mildew developing on leaves that are still expanding may cause the leaves to be deformed. Infected shoots are often stunted with curled and deformed emerging foliage covered with a silver-gray mat of mycelium. Later in summer, the grayish-white fungal growth turns brown. When apple fruits are infected their growth is stunted and the fruit surface is russeted (brown corky cells) in a net-like pattern. Chronic foliar and fruit infection can result in reduced vigor and yield.
Selecting apple cultivars with disease resistance is the best method of reducing occurrence of powdery mildew. Data provided from a ten year study of apple cultivars in the northeast provides information on the best and least resistant apples available.
In general, the cultivars Crimson Crisp, Delblush, Ginger Gold, GoldRush, and Suncrisp were the most susceptible, with incidence of powdery mildew reaching 40-50% on the most susceptible cultivars. Enterprise, Gala Supreme, Hampshire, Pristine, September Wonder, and Zestar had the lowest incidences of mildew. It is noteworthy that the mildew-resistant cultivars Pristine and Enterprise are also among the most resistant to scab, a fungal disease effecting leaves and fruit.
By contrast, the scab-resistant cultivars Crimson Crisp, GoldRush, Princess, Scarlet O’Hara, and Sundance are highly susceptible to powdery mildew and would require early season fungicide applications for mildew and rust management in high inoculum areas in spite of their resistance to scab.
Many of the traditional cultivars grown in Kentucky, i.e., Ginger Gold, Golden Delicious, Granny Smith, Idared, Jonagold, Jonathan, Paulared, Rome Beauty, Stayman Winesap, and Winesap are known to be moderately or highly susceptible to powdery mildew. In areas where powdery mildew is a threat, growers attempting to grow apples as organic or with reduced pesticides will want to select scab-resistant cultivars that are also resistant to cedar rusts and powdery mildew.
