Adapted from the article “Cytospora cankers can destroy an orchard” by Joe Connell, UC Farm Advisor Emeritus, Butte County and Franz Niederholzer, UC Farm Advisor, Colusa/Sutter/Yuba Counties in the October 2014 Sacramento Valley Prune News
Extreme heat, potassium deficiency, water stress, or heavy cropping can result in sunburn and borer attacks followed by cytospora cankers caused by the fungus Cytospora leucostoma. Cytospora canker is a weak pathogen that is spread by wind and rain to bark damaged by other stresses. Recent work suggests that cytospora can enter prune trees through pruning wounds (Figure 1). The fungus shows maximum growth in hot temperatures (around 90oF) and is particularly active in late summer to early fall.
These cankers first girdle branches causing branch dieback. If not pruned out properly, cankers continue to grow down into primary scaffolds killing more wood as they expand. Major scaffold limbs will die with trees compromised sufficiently to require tree removal if these cankers are not eliminated early enough through proper pruning.
To identify limbs killed or weakened by cytospora cankers, look for dark, sunken cankers on the bark of limbs showing dieback, or branches where dead leaves are still attached. Active cankers have distinct zonate margins (Figure 2). Small white spots called pycnidia found on dead wood confirm the presence of Cytospora (Figure 3).
If the pruning crew is not paying enough attention to their cuts to eradicate the infection they are wasting their labor and your money! Poor pruning (incomplete eradication) won’t control the disease. If limbs are not cut below the cankers the problem is not solved and trees will continue to decline. Pruners must cut into healthy wood several inches to one foot below any canker symptoms. Have them check the cut surface of pruned limbs to ensure that all disease has been removed (Figure 4).
Cutting out cytospora in vigorous orchards must be balanced with orchard performance and how much bearing surface is left after all damaged scaffolds and branches are cut out. All older orchards show some cytospora. Well-managed orchards – adequate water, nitrogen, potassium, etc. – will not be as impacted by cytospora as weaker blocks.
In order to maintain production in strong orchards, a grower may elect to leave some limbs with bark damage in the tree, even if sunburned or cytospora infected. If production is off and the orchard is filled with cytospora cankers, but strong water sprouts are found in the crotches and lower branches, remove larger older wood with cankers and improve irrigation and fertility. For a weak orchard riddled with cytospora and wood rot, the best approach might be replanting. Bottom line: all growers should spend a day with a saw and this pocket guide to evaluate each orchard.
There is no chemical control for cytospora. Cankered wood must be removed from the orchard and burned. After cankers are pruned out, paint exposed trunks and scaffold crotches with white interior latex paint to protect them from sunburn. Maintain adequate orchard water status, especially after harvest, and avoid potassium deficiency, spider mite infestation, or prune rust defoliation that can increase sunburn and disease potential.
For more detailed information and photos visit the IPM web page.