It has been an unseasonably damp and wet July so far for our region of southeastern Indiana. While the rains are good for maintaining the area’s water table, the extended duration of wet conditions contribute to summer diseases in the orchard. Humidity and hot temperatures are perfect “scab” weather. These weather conditions also add to the incidence of brown rot in peaches, powdery mildew, sooty apple blotch and fly speck.
Symptoms of apple scab on leaves. Image sourced from apsnet.org
So, what is scab? Scab is a fungal disease problem of apples and pears. Infestation of this fungal disease develops on the fruit as well as the leaves of the apple tree. The fungus overwinters on plant tissue and the ascospores are produced and released with rain events in the spring. The air and wind carry these spores and if they land on apple foliage or fruit they cause an infection when they stay wet for a few hours. After the initial infections, the secondary cycles begin in which spores are produced and reinfect the tree. Sporulation occurs during wet weather. Foggy, humid mornings when the leaves of the trees are dripping moisture or extended hours of rainfall with temperatures above 75 degrees are ideal for the spore infestation. Protective cover fungicide sprays are an absolute necessity to combat scab in our area – even with resistant varieties.
Brown rot affects the twigs and branches as well as the fruit of peach trees. Brown rot, being a fungus, is also caused by spores. These spores are often released from dried up fruit or “mummified” fruit left hanging on the trees or on the ground causing the fruit to have brown spots that actually cause it to rot. This disease can be controlled by fungicides applied to the trees and also by maintaining good clean-up methods in the orchard. Brown rot can start out as just a brown dot on the fruit, barely visible. However, it doesn’t take long for the spores to grow and completely deteriorate the fruit. Again, the warm, humid weather conditions aid the growth of the spores.
Apple scab fruit symptoms. Image sourced from apsnet.org
Powdery mildew is really self descriptive. The mildew fungus appear as tiny white patches and spread rapidly covering most of the leaf. This fungal disease is quite visible on all peaches, apples, and even rose bushes as well. Infected leaves become rolled up, stiff and brittle. Applying fungicides to the trees is a proven method of control.
Sooty blotch and fly speck are surface fungus infections that are common to apples and pears. Surfaces of the fruit have the appearance of sooty, dark looking blotches or small black dots in clusters. Fruit quality is not affected by the appearance of these fungal infections as both are cosmetic. Some of the visible symptoms of infection can be removed by washing the fruit.
While we welcome summer, the extended hours of daylight, warmer temperatures and all the activities that these conditions allow, the humidity and heat does create challenges for the orchardists. Even though we do our best to reduce disease through cultural practices (e.g. sanitizing orchards in the winter and spring, planting resistant varieties, planting trees at a wider spacing to reduce dry time, and in-season removal of limbs that are diseased), we still have to deal with the weather – especially when conditions are conducive for disease. This is just another reminder that we are just caretakers of the orchard, and we must do our best to try to manage the conditions that are presented to us.