Marin IJ Articles
August 27, 2016
To promote bountiful fruit production and minimize the chance of disease, apricot (Prunus armeniaca) and cherry trees (Prunus spp.) should not be pruned in the fall or winter when the trees are dormant, but in summer after harvesting the fruit.
If pruned in winter as most fruit trees are, apricots and cherry trees are prone to a fungal infection called eutypa dieback (eutypa lata), also known as cytosporina, limb dieback or gummosis, a disease characterized by sudden limb dieback and oozing cankers. Eutypa is spread when its spores are carried through the air during rainstorms and settle into fresh pruning wounds. Those spores germinate and grow in the wood tissues, causing darkened, malformed branch and trunk cankers to develop around the old pruning wounds one to two years later. The disease progresses for many years, often killing the tree.
Ideally, pruning should be completed at least six weeks before the first fall rains. In inland areas restrict pruning to July and to no later than mid-September in coastal areas. If trees are not pruned at the recommended time, Eutypa dieback can cause limbs to suddenly wilt and collapse in mid- to late summer with wilted leaves still attached to the tree. The bark develops a dark discoloration with amber-colored gumming. While infections can occur at any time of year during rainy periods, the greatest incidence is in fall and winter. Unpruned apricots and cherries are not affected.
In fruit trees of all types and ages, proper light and air circulation reduce the risk of fungal disease. Summer pruning of apricot and cherry trees is done anytime between mid-summer up until Sept. 15 only if rain isn’t in the forecast. The goals of summer pruning are to remove dead, damaged and diseased limbs, and to remove crossing or rubbing limbs and limbs that grow inward toward the tree’s center. Opening up the canopy allows more light onto the branches which improves the development of fruiting wood and the quality and size of the fruit. Pruning creates a sturdier structure that better supports fruit.
Both apricot and cherry trees are trained when young to an open center or modified central leader form. Using a combination of heading and thinning cuts, strive for new shoot growth of 12 to 18 inches so light bathes all branches. Heading cuts are used to redirect growth by cutting back a portion of a branch to just above a healthy bud or side branch; thinning cuts reduce crowding by removing an entire limb at its base. More severe pruning will reduce your crop, but the fruits will be larger. Fruit thinning is unnecessary unless the crop is heavy, in which case thin the fruits so they are about two inches apart. Avoid pruning if an extended period of hot weather is expected; use precautions to prevent trees from sunburn.
If you prune at any other time than July through mid-September treat the cut areas with a fungicide applied according to manufacturer’s specifications. Studies have shown that hand-painting the fungicide onto large wounds afforded protection for six weeks or more, by which time wounds were largely immune to infection. Existing infections can be pruned out at least 8 inches to 1 foot below the canker and destroyed. Do not apply emulsions, paint or other materials to pruning cuts. Leave the cuts open to the air so they can dry out and heal naturally. Remove abandoned grapevines in the vicinity as these can be sources of infection.
Both apricot and cherry trees flower early in the spring and grow best when planted in well-drained soil in a sunny site protected from strong winds. Apricots require 600 to 900 hours of winter temperatures below 45 degrees to set fruit; sweet cherries need 700 to 1,000 hours while tart cherries need 1,000 hours. Fruiting occurs on both trees on two year or older spurs.