Written by davethetreecenters • February 08 Apple Tree Diseases

Apples are one of the best fruit-tree choices for gardeners in cooler areas. The give a good yield of the most versatile and the most popular fruit there is, and they thrive in cooler climate zones than most other fruits. They will also grow in a range of soils and garden conditions, but they can have some problems that may need taking care of if you want to get the best possible results. There are some diseases that can affect your trees, so being able to recognize them, and deal with them, is basic knowledge for anyone who wants to grow this popular crop.

There are a lot of apple diseases listed, but most of them are rare or do not do serious damage. Which disease is a significant problem can depend on where you live, but there are just a few that are serious enough to regularly cause problems for many gardeners.

Apple Scab

This disease is both wide-spread and serious because it can affect both foliage and fruit. This fungus disease spends the winter in dead leaves from infected trees. In spring spores are released that travel to the trees, causing olive-colored spots to develop on the underside of the leaves and on the young fruits. It spreads most rapidly during warm, rainy weather – April showers can bring more than just flowers. As the disease develops leaves can become badly spotted and thick scabs develop on fruit. As the drier weather of summer arrives the spread of this disease is reduced, but during a rainy summer it may continue. Often with the warm, rainy weather of early fall there will be a new spread of the disease.

Apples that develop scabs early in the season will be deformed in shape, as the scabbed areas do not grow as rapidly as the healthy tissue. Scabs that develop later have little effect and since they are only on the skin they can be ignored, especially when cooking. They have no effect on the flesh or eating quality of the apple. Infected leaves will fall early and several seasons of bad infection can weaken the trees and reduce cropping.

There are three approaches to controlling this disease. First, look for varieties that are resistant to apple scab, especially if you live in an area where it is common. Secondly, remove all the fallen leaves beneath your tree in fall. If you shred and compost them, by spring they will be decomposed and cannot re-infect your trees. Thirdly, use a suitable fungicide early in the season, just after the petals fall from the blossoms. For organic growing, use sulfur as a protective coating on the leaves, applying it regularly if it is washed off by rain. Other chemicals provide surface protection, while some go inside the tree and prevent the disease spreading. If early control is used the disease cycle is broken and later infections will usually not occur – or at least only at the end of the season when they do little harm.


This is a bacterial disease that enters the tree during rainy weather in spring. It gets inside the tree through the blossoms and through the bark. Once in the tree if forms a damaged area on the bark, called a canker. This looks raised around the edges, but shrunken and dead in the center. During wet weather cankers ooze bacteria, which is then blown by the wind, or carried by insects, to other trees or to other parts of the same tree. The bacteria multiply beneath the bark, killing it and causing branches to then die. If you see the leaves on a branch suddenly wilt and turn brown, looking like it has been scorched, that is fireblight, and the appearance is why it has that name. This disease can also affect pears, quinces, hawthorns and firethorn bushes. Trees can die if this disease is not controlled.

If you have any sign of fireblight one year, before the next spring you should check your trees carefully and prune out any dead branches or those with cankers. Do this during cold weather, below 45 degrees, so that you don’t spread the disease around. Cut 8 to 12 inches below the canker, since the bacteria spread below the bark into healthy wood. If you have a lot of problems with this disease, agricultural streptomycin is available as a spray. Apply this during the blossoming season until the petals fall. Put the spray on at the end of the day so that it dries as slowly as possible – this makes it more effective. Watch for fireblight on ornamental shrubs in your garden and treat them in the same way, so that they do not act as a source of infection.

Bitter Rot

This disease is found mostly in southern states, since it needs warm weather to thrive. It causes soft patches to develop on the fruit, which spread and cause rotting and loss of the fruit. It develops during the hot weather of July and August and fruit quickly becomes soft and rotten, before drying out and turning into mummified apples that hang on the tree. There are no symptoms on the leaves. It spends the winter in the mummified apples and in cankers from other diseases or broken limbs. So to control this disease, remove all mummified fruit in winter and prune to remove dead wood, cankers and other damaged parts. Fungicides can also be applied if you live in an area where this disease is widespread – ask at your local garden center.

Other Apple Diseases

There are a number of other diseases that can occur from time to time on apple trees. They may cause leaf spots or mildew, or rotting of the roots and trunk. To reduce the likelihood of disease, make sure you plant in well-drained soil, with good air circulation around your trees and prune them correctly and at the right times of year. Although all these diseases can sound scary, do not be put off growing apple trees. The small amount of care and attention they require is more than repaid by the bountiful harvest of delicious fruit you will take from your trees.