Cankers and limb dieback of cherry trees can be caused by bacteria or fungi. In California, the most common bacterial cause is Pseudomonas syringae, the causal agent of bacterial canker and blast, which affect all stone fruit crops. Cherries are especially susceptible to bacterial canker; severe losses in recent years in young ‘Coral’ plantings have demonstrated that this variety is particularly vulnerable.

Plant pathologists throughout the world have worked for decades on the biology and control of P. syringae. The fact that it is still a major cause of tree death and decline speaks to the complexity of this disease and difficulty of controlling it.

In cherries, we are left to rely mainly on three tactics: avoiding planting orchards in areas with 1) warm and wet spring weather (for example, in northern and coastal California) or 2) on soils infested with high populations of ring nematode, and 3) managing orchards in ways that minimize “stress” from improper irrigation, nutrition, salts, and other diseases. These measures are especially important in young orchards.

More detailed information on bacterial canker can be found at http://ucipm.ucdavis.edu/PMG/selectnewpest.cherries.html.

There are several groups of pathogenic fungi that cause cankers and limb dieback in cherries. Long a serious problem of older cherry blocks, in recent years we have seen increasing problems in young orchards. Since 2006, a team led by Dr. Doug Gubler of UC Davis has been investigating fungal causes of branch and limb dieback in cherry trees. Their work is beginning to shed new light on fungi that cause dieback and steps growers can take to control them.

For a number of years since it was first identified in a Linden area cherry orchard, we have known that Eutypa lata, a major fungal pathogen in grapes, can infect cherry trees, cause cankers, and kill large limbs. Dr. Gubler’s group has now conclusively shown that two other fungi – a species of Phaeoacremonium and Calosphaeria pulchella – are also commonly associated with cherry cankers and may cause limb dieback. Work is underway to better understand when and how these fungi infect cherry trees and how they can be controlled.

While we await the results of these investigations, there is much about these fungi we already know that can be used to manage orchards and minimize risks of infection. These fungi enter into trees mainly through pruning wounds, they release airborne or water-splashed spores mainly during wet or rainy periods, and these spores germinate when they are wetted. The optimal pruning time for disease prevention is therefore during dry weather, spring through early fall.

When a pruning wound is made, natural defense mechanisms are triggered that help protect the wound against subsequent infection. This process takes two or more weeks depending on the severity of the cut and how and when it was made. Larger cuts, “flush” cuts made very close to larger limbs or “stub” cuts which leave a portion of the removed branch on the tree take longer to heal and are susceptible longer to fungal infection than small cuts and those which leave intact the “collar” of tissue where the pruned branch joins the one from which it was removed. Pruning cuts heal much faster during the growing season when trees are active than during the winter.

While these disease considerations need to be coupled with horticultural goals in deciding when to prune, a strong case can be made that pruning during the winter is a generally bad idea. Fall pruning done too early can stimulate unwanted regrowth — especially in young orchards — and spring pruning done too late can reduce tree vigor. Pruning time needs to be determined on a case-by-case basis.

Research has shown that treating wounds with fungicides can help prevent infection, but treatments must be applied directly to the wound and very soon after the wound is made to be effective.

To my knowledge, however there are no effective fungicides currently registered for this kind of use on cherries in California. Dr. Gubler’s group is working to develop a post pruning, tractor applied fungicide program with Dow Agrosciences that could be ready in 2009. Experiments with protectants like paint, wax, and other sealants have generally shown that these treatments provide little or no benefit, especially when compared to pruning at the proper time with good techniques.

Understanding wound-related disease problems will become increasingly important as we prune heavier and more often to improve fruit size and quality.

Dr. Gubler and other cherry researchers will present results of their work at the California Cherry Research Review on Jan. 27 in Stockton.