Apple scab lesions on fruit Ohio State University Extension	Scab lesions on apple leaf Ohio State University Extension

Introduction

Apple scab (Venturia inaequalis) is a common fungal disease affecting apple and crabapple trees. No one will dispute the tedium of raking and disposing of the autumn leaves that litter our lawns. However, should an apple tree happen to grace your yard, this chore may be of extra importance to the health of your tree. Apple Scab lives within diseased leaves, and will survive even the most unforgiving winter only to spread again in the rainy days of spring. 

Symptoms

This most common of apple tree diseases can affect both the leaves and fruit of your tree, leaving it not only unsightly, but also devoid of fruit. As well, a reduction in the leaf-count of your tree may leave it vulnerable to winter injury or other diseases.

When it attacks the leaf it is characterized initially by soft, velvety, olive-green spots. These spots may, at first, resemble specks of mold such as those one might find on a loaf of bread. As the infection advances, the lesions enlarge and transform into thick, scabby growths.

Although, the fruit itself becomes less susceptible as it matures through the growing season, the tender growth of early summer is easily infected. As with the leaves, the early signs of Apple Scab manifest themselves in olive-green or brownish lesions. As the lesions scab over, deep cracks may penetrate the fruit, making it inedible.

Characteristics

The wet weather of spring and early summer is the ideal breading ground for the Apple Scab fungus since the disease favors such highly humid conditions. Carried on the wind or splashed amongst the leaves during rainfall, the spores then germinate in the residual water. Within a short period of time - as few as 9 days - symptoms may begin to appear. 

A tree infected with Apple Scab may loose both leaves and fruit if the fungus attacks early in the growing cycle. A seriously infected tree may shed many of its leaves, and the fruit will become deformed and drop before it is fully formed. While this is more serious for the individual who relies on the apple crop for their livelihood, it does deprive the homeowner of the pleasure one can expect from their garden.

Control

While preventing Apple Scab completely is a difficult task, there are several measures a homeowner can take to minimize the risk. Planting varieties with a higher resistance to infection is a viable option if one is in the beginning stages of a garden. Since most people 'inherit' their trees, other measures may be more practical.

Raking and burning - or disposing of - leaves in fall will help stave off infection the following spring. This is a good preventative measure, but it should be noted not an absolute solution. It only takes a few leaves to start contagion anew in the spring. 

If you own a tree prone to Apple Scab disease, the best course is to apply a fungicide in spring just prior to the first bloom. This will lessen the need for more frequent applications later in the season. In order to control the infection, it is important to spray the tree again in late spring and early summer. Since the germination period of the fungus is relatively short, a homeowner will have to approach the application of fungicide with a certain vigilance and a watchful eye. 

Recommend Products

Captan Fungicide for Fruit & Ornamental Trees, 8 oz.

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