What Causes Black Spots on Young Apples?

Causal Agent

• Apple scab is caused by the fungus Venturia inaequalis. The fungus survives in the form of reproductive spores called ascospores and conidia, in the infected debris under the tree from the previous year. These reproductive spores start to infect new foliage, flowers and fruit as soon as spring weather supports the growth of fungal spores.

Symptoms

• Disease starts with the appearance of light green, scabby legions on foliage. The spots gradually spread to blooms and fruit as darker colored spots. The infection can occur on fruit at any stage of growth. When infection occurs in an early fruit growth stage, the tree suffers extensive fruit drop of young fruit. Infection near the petiole causes leaf drop. Very small black spots occurring late on the fruit are referred to as "pinpoint scab." Pinpoint scab frequently develops on fruit kept in storage.

Cultural Management

• Cultural management in the form of sanitation around the apple trees is among the best ways to reduce chances of infection. Remove all fallen leaves from under the trees during late fall and destroy by chopping. Apply 5-pecent urea solution to the tree's leaves in the fall to promote the decomposition of leaves before the fruiting bodies get a chance to develop. Apply the urea just before leaf drop. Applying too early leads to succulent growth that becomes susceptible to winter injury.

Chemical Management

• A number of protective and curative fungicides are used on apple trees to minimize damage from infection. Apply protective fungicides before the usual time of infection every seven to 10 days. Start protective fungicide applications just before bloom time and continue until petal fall and dry weather. Start using curative fungicides at the first signs of infection. Recommended chemicals include dodine, captan, mancozeb, thiram or metiram.