Powdery mildew affects all fruit trees.
Description Powdery Mildew
- This fungal disease shows up on the leaves of trees as white, powdery mold. However at its early stage, these white areas show up as circular white spots on the tops of the leaves. Later in the growing season, the leaves and buds will become disfigured. Additionally younger leaves will turn yellow and dry out. Moreover, this disease can slow down the growth of the tree and impact the fruit yield and quality if it has spread extensively.
- Firstly, the wind can carry powdery mildew spores into the area. The fungi overwinter in the dormant buds. Then during the spring, they spread onto the flowers, leaves, and fruit.
- The optimal environment for powdery mildew to thrive is warm and dry (around 60-80°F / 15-27°C). In order for it to spread though, there needs to be high relative humidity. If the temperature is higher than 90°F (32°C), then it doesn’t spread as well. Similarly, if it is cool and rainy, the spread decreases.
- Powdery mildew tends to affect trees in shady areas more than those in sunny areas.The fungi do not overwinter in pear tree buds. However pear trees can get the fungi from nearby apple trees.
- Moreover, really cold winter temperatures can kill the infected buds since they are more susceptible to winter injury than healthy buds.
- The infection which will cause the fruit to russet can occur from about 3 weeks before bloom to 3 weeks after bloom.
Treatment and Management
- Firstly, plant and prune in a way so that the trees and branches aren’t overcrowded. This will increase air circulation. If you see areas of infection, prune those branches.
- Plant in areas with direct sunlight.
- If you know that powdery mildew occurs in your area, then do not plant susceptible apple varieties such as Idared, Monroe, Rome Beauty, Jonathan, Paulared, Gingergold, or Cortland. For pears, do not plant Anjou pears within 200 m of susceptible apple cultivars. However, Bartlett, Flemish Beauty and Winter Nelis are more resistant to powdery mildew.
- If you do see early signs of the disease or want to prevent the disease in the first place, then spray with organic fungicides such as sulfur, lime-sulfur, neem oil, and potassium bicarbonate.