How to Manage Pests
UC Pest Management Guidelines
Pathogen: Cytospora leucostoma
(Reviewed 11/09, updated 11/09)
In this Guideline:
Symptoms and Signs
The first noticeable symptom of a Cytospora infection is usually wilting or flagging of a branch. A close look at the branch reveals a dark-colored bark canker with a depressed center. Amber-colored gumming can occur at the edges of the canker. The canker eventually girdles the branch, causing it to die; dead limbs become evident in mid- to late summer.
Cytospora canker can be distinguished from other cankers by the presence of pycnidia, which are pimplelike structures that form on the outer bark of the canker. Pycnidia are initially black, but turn white. In humid, but not excessively wet conditions, amber tendrils containing spores exude from the pycnidia.
If pycnidia are not present, Cytospora canker can be differentiated from bacterial canker by the difference in the canker margins. Removing the bark from a Cytospora canker reveals abrupt margins that frequently exhibit a zonate (bathtub-ring) pattern whereas bacterial cankers have irregular margins. The zonate pattern is created when established Cytospora cankers overwinter and resume growth the following year.
Comments on the Disease
Cytospora leucostoma is a relatively weak pathogen. Its spores are spread from cankered branches by rain and wind and can infect any type of bark wound, such as those caused by sunburn, old bacterial cankers, boring insects or other damage; it cannot infect healthy, undamaged bark. Damage is usually limited to bark tissue. Shothole borers may also vector this disease.
Cytospora canker tends to be most serious in weak orchards where infections often develop into cankers and continue to grow for several years, killing major limbs and causing significant economic loss. Generally this disease is of little consequence in vigorous orchard blocks.
Water stress, potassium deficiency, overcropping, and high ring nematode pressure increase tree susceptibility to the spread of infection (canker development). Trees planted on shallow or heavy textured (clay) soils are generally more likely to suffer economic damage from Cytospora, because water and potassium management on these soils can be challenging.
Cytospora canker is a warm-season (summer) disease with peak fungal growth occurring just above 90oF. Canker growth potential is highest when temperatures are high and prune tree growth activity is low (July-September).
There is no known chemical control for Cytospora. Manage infection and spread of the disease by avoiding tree stress, and by removing and destroying cankered wood from the orchard.
Avoid stress factors such as potassium deficiency, sunburn, high ring-nematode (Criconemella xenoplax) populations, trunk borers, and soil moisture stress that specifically predispose prune trees to the spread of the disease. Prune trees to minimize sunburn potential, and paint exposed trunks and scaffold crotches with white interior latex paint to further protect them from sunburn. Avoid, where possible, planting on marginal soils, especially shallow or clay soils. Maintain adequate orchard water status, especially after harvest. Avoid defoliation (sunburn potential) caused by prune rust infections. Pruning cuts and leaf scars are not important infection sites. Thin heavily cropped trees.
During the growing season, remove (cut out) cankers and destroy dead or damaged wood. Pruning during the growing season allows you to better identify branches with cankers. To ensure that all the disease is removed, cut several inches to one foot below any canker symptoms into healthy wood. Check the cut surface of damaged limbs to ensure that all the disease has been removed. Incomplete canker removal wastes time and money with little to no benefit in disease management.
In stressed orchards where infected wood (cankers) are allowed to remain on the tree, continued canker growth and scaffold death can occur. Growers must decide, on a block-by-block basis, whether cutting out cankers or replacing the orchard or portion of the orchard is the best economic decision. Trees or limbs killed by Cytospora canker and left in the orchard or adjacent to living trees provide inoculum for further infection and should be removed or destroyed.
UC IPM Pest Management Guidelines:
J. E. Adaskaveg, Plant Pathology, UC Riverside
Acknowledgment for contributions to Diseases:W. D. Gubler, Plant Pathology, UC Davis
B. L. Teviotdale, Kearney Agricultural Center, Parlier