Spray trees now? Why would any gardener want to spend the waning days of autumn spraying his fruit trees? Well, it would not be necessary if it weren't for the potentially serious disease coryneum blight.
Coryneum blight is also known as shothole disease because of the interesting leaf symptom. It attacks all stone fruit trees but is more common on peaches and apricots. Nectarines are seriously affected some years, while plums and cherries have fewer problems.Control is necessary this time of year because the fungus infects trees through the bud scale scar. This scar remains when the leaf falls off the tree and leaves a small open area underneath each bud. This open area is invaded by the fungus and then infects the surrounding tissue. The fall infection produces symptoms during the following year.
The infection process is much more serious when rainy weather occurs during leaf drop. The infected buds are killed as growth begins in the spring. They have a shiny, varnished appearance and produce small amounts of gum surrounding the infection.
The fungus also infects the twigs and causes a dead tan spot one-eighth of an inch or more in diameter. This tan spot has a distinct purple margin. The spots grow larger and eventually girdle young twigs so the wood beyond where the infection occurs, dies. The disease is not fatal to the tree but greatly reduces the number of fruit and leaf buds by killing many of the small twigs.
The symptoms on the leaves are much more distinct and account for the common name, "shot-hole disease." Leaves appear to have been peppered with a shotgun blast because of the many small BB-size holes through the leaves. These holes start as small round spots with tan centers and purple margins. The center eventually dies and falls out leaving the shot-hole effect. If your trees show the shot-hole symptom, a preventative spray is in order.
As soon as the leaves have fallen from the tree, spray with either fixed copper or daconil. Fixed copper sprays include Lily Miller's Microcop, Acme Bordeaux mixture, as well as other supplies. Daconil is also available from many manufacturers.
Unfortunately, the disease organism survives from one year to the next, in the cankers on the infected twigs and buds. Newly developing fruit is often infected from these buds and develops fruit symptoms or many simply fail to develop. Each twig canker is capable of producing millions of spores that spread to other plant tissue by wind and moisture. Affected twigs should be removed during the spring pruning.
Fruit infections occurring in the spring are primarily a problem with apricots, though it will affect other fruits. The apricots develop small, raised purplish spots. They are evident when the green fruit is slightly larger than a pea. The spots expand as the fruit develops and may eventually cover the entire fruit surface. The fruit is safe to eat but has an unattractive appearance. Apricots are treated in the spring as the flower fall to the ground. Apricots should receive the same fall treatment as peaches, if the disease has been severe.
It's always a good idea to clean up leaves from around diseased trees. This is not only true of fruit trees affected by coryneum blight, but it's also true of aspens. Destroy the aspen leaves through the hot composting process described last week or use another means of disposal. This reduces the inoculum for aspen leaves spot disease which causes the black leaves on the aspen trees in our area. The inoculum source of powdery mildew on roses is also reduced by destroying fallen rose leaves.
Many gardeners want to prune trees this time of year. This is not recommended as it stimulates cell division around the pruning cuts. The newly divided cells are very susceptible to winter kill. Home gardeners should delay all major pruning of fruit trees until late winter or early spring. The only exception is dead or diseased wood. Those branches can be removed anytime they are pres-ent. Clean up and spray infected trees now, but delay your pruning. Your trees will be healthier and more fruitful next year.