Powdery mildew is a very common fungal disease on several types of plants. Different species of the mildew have different host plants. Apples and pears are attacked by Podosphaera leucotricha, while stone fruits are attacked by Podosphaera clandestina and Sphaerotheca pannosa. Grapes are another frequent victim attacked by Erysiphe necator. Raspberries, strawberries, and many ornamental plants are also sometimes affected by mildew species.
Powdery Mildew on apple and grape leaves.
Mildew can affect both the fruit and the leaves of infected plants. Leaves on all infected plants appear to be covered with white powder; new growth is often stunted or distorted as well. In severe cases of infection, the resulting loss of photosynthesis can reduce plant growth and fruit set. Infected grapes can crack, split or fail to ripen. Apples and occasionally stone fruits can be covered with a raised webbing called ‘russeting’.
Although powdery mildew does require moisture to infect, unlike most fungi it will continue to spread with or without humidity. Most powdery mildew overwinters in terminal buds, and it tends to be more severe in seasons that follow mild winters. Severe winter temperatures can reduce mildew occurrence by killing infected buds. Therefore, instance of powdery mildew may be lower this year because of the harsh winter we had.
CONTROL OF POWDERY MILDEW
There are several options to control this fungus. Resistant varieties are available for most fruits affected by powdery mildew. Cultural controls include avoiding the overcrowding of trees and branches and pruning out twigs with the powdery mildew on the surface. Pruning to open up the tree allows for a bit more airflow, which in turn helps to reduce wet surfaces that promote fungal growth. There are also several organic sprays available that adequately control powdery mildew. Spraying sulfur, copper, neem oil, or a baking soda (sodium bicarbonate) solution beginning in the early spring when buds start pushing. Continuing sprays every 2-3 weeks throughout the growing season greatly reduces the instance of powdery mildew and other problematic fungi. Copper and sulfur should only be used in severe cases of infection; although organic, they can also have severe negative effects on beneficial fungi and micro-organisms.
This edition of POP TIPS prepared with assistance from POP Intern Nettie Laugher.
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