December, 1997

Pruning fruit trees for pest management

Deborah I. Breth
Cornell Cooperative Extension

Fruit trees are pruned to contain tree height and spread, to open the tree canopy for access by workers, to allow sunlight penetration for fruit bud development, to renew fruiting wood, and remove injured or diseased wood.
While pruning can result in a reduction of some pest problems, it can also result in an increase in others. The positive effects of pruning relative to pest management include 1) sanitation, 2) improved drying conditions, and 3) improved spray coverage.
The negative effects result from the actual wounding of the tree that can be invaded by fungi that cause cankers and wood rot. Understanding how some of the diseases and insects survive in orchards from one season to the next help minimize the negative effects of pruning and maximize the positive.

Sanitation

Sanitation is the removal of infested or infected parts of the tree that serve as the source of pests.
Sanitation is a critical step in management of black rot, fire blight, Nectria twig blight, Nectria canker and black knot. The fungi and bacteria that cause these diseases survive in the diseased bark and produce spores or bacterial ooze that are washed to wounds or other infection sites. When we remove these sources of spores and bacteria, we are reducing the inoculum level and consequent disease pressure. In most cases, sanitation is only part of the control program required for canker type diseases, and complete removal is nearly impossible. But without any attempts at sanitation, it is an uphill and ineffective battle for the crop protectants.
Sanitation is not complete until the brush has been removed or destroyed. One option is to rake the brush out of the orchard and burn it where burning is not restricted.
A second alternative is to chop the brush in the orchard with a flail chopper. It is critical to chop the brush in very small pieces and strip the bark from the branches to speed up decomposition. This is especially important for canker diseases such as Cytospora canker, black rot, Nectria twig blight, Nectria canker, and silver leaf that can live on dead branches (prunings) as well as wounded weak branches in the tree.
Some diseases such as black knot on cherry and plum must be removed from the orchard and burned since the wood will not be chopped well enough to stop spore production for future infections. If brush is removed from the orchard, it is important to burn the brush since it serves as a source of black rot and Cytospora fungi, as well as food for shot hole and peach bark borers. Brush piles are excellent cover for rabbits that can do a lot of winter damage in nearby dwarf plantings. Do not allow brush piles to accumulate around the farm since they can be the source of a lot of pests in the orchard.

Improved drying conditions

After pruning, open canopies will dry more quickly to reduce wetting periods and relative humidity critical to fungal and bacterial infections. The length of wetting periods are critical factors for diseases like apple scab, sooty blotch/fly speck, black rot infections of apple, black knot on cherry and plum, brown rot blossom blight in stone fruit, and cherry leafspot in cherries. Since fire blight and other bacterial diseases require only brief wetting periods, there will be less impact on the control of these diseases from improved drying.

Improved spray coverage

Moderate pruning on an annual schedule instead of excessive pruning on a biennial schedule will result in improved spray coverage.
Annual pruning prevents the very vigorous flush of watersprouts from all the big cuts throughout the tree which are deterrents to adequate spray coverage. Research has shown that in medium to large trees, the hardest part of the tree canopy to cover is the top inside. Airblast sprayers must displace the air volume in the canopy with the air from the sprayer that carries the spray droplets. The thicker the tree canopy, the more difficult it is for the spray droplets to penetrate the canopy. If the air carrying the droplet is slowed down by the canopy density, the droplets may not have the necessary speed to impinge onto the surface of the leaves, resulting in unprotected growth.
The effectiveness of many of the crop protectants depends on good spray coverage throughout the tree canopy. Many of these materials must be absorbed into the leaf and are only locally systemic, not redistributed with rainfall like other older materials. Examples are Nova, Rubigan, streptomycin, Agri-Mek, Provado, and Vydate. If these materials do not cover the target where the pest occurs, the leaves will not have the dose required to control the pest.
Regardless of the material used, the nature of the pest can also require good spray coverage for efficacy of the pesticide. Apple scab, powdery mildew, fire blight, pear psylla, obliquebanded leafroller, scale and mites all require good spray coverage for adequate control since the pest pressure is throughout the tree canopy.

Timing

Timing of pruning must be controlled to result in a positive impact on pest management.
Dormant pruning in our climate should be delayed until the trees are winter hardy. If trees are pruned late in the fall or very early winter before a severe freeze, the pruning cuts can be winter injured and will serve as sites of infection for many canker diseases such as black rot and Nectria twig blight. It is not rare to see Empire pruned early December become infected with Nectria. Silver leaf, caused by a wood rotting fungus (Chondrostereum purpureum), infects through pruning wounds mainly in winter and early spring if the pruning wounds are less than one month old. Locate the sources of silver leaf spores on dead branches or tree trunks and remove them before pruning. In orchards where fire blight has been a problem, pruning while trees are dormant, is essential. If pruning past dormancy, there is a chance of spreading fire blight with subsequent cuts.
Stone fruit like peaches, nectarines, and sweet cherries should be pruned after they start to grow in the spring when the tree can out-grow infections by the Cytospora fungi which cause Cytospora or peach canker. Pruning during the dry weather will give cuts time to heal before spores are rainsplashed to the wounds.
Black knot infections on plums and cherries should be pruned out by March and burned by early May when the spores are released. Spores continue to be released by knots through mid-July, with a peak in shoot noted by John Northover in mid-June. There will be more knots that become apparent from 1997 infections in the spring of 1998 and they should also be removed. Knot removal may be necessary several times through June. Sanitation must be followed up with a good fungicide program.
Pruning is one of the most critical cultural pest management tools. The best results will be achieved with moderate, annual, well-timed pruning and removal of brush as necessary. Excessive pruning on alternate years, at poor timing, with inadequate brush removal will enhance conditions for growing diseases, not quality fruit.