See:
Cherry (Prunus spp.) - Fungal Cankers
Cause Cytospora canker, also known as Leucostoma canker or Valsa canker, has been the most prevalent canker disease on cherry after bacterial canker. Cytospora spp. have a wide host range and can infect virtually all stone fruit and pome fruit trees. This fungus is a vigorous wound invader (including wounds from sunscald and winter injury as well as fruit picking wounds) that ramify throughout the bark and cambium and to a lesser extent the tree's structural wood. Leaf and bud scars can also be entry points for the fungus but less so than wounds. Cytospora populicola and C. sorbicola have been found in British Columbia and are likely present in the US as well. These fungi have a wide host range and can infect virtually all stone fruit and pome fruit trees. Overwintering is in infected stems and stem debris on the ground. Conidia are produced in cherry and peach orchards in eastern Washington throughout the year and the inoculum concentration is highest during the spring and summer. Over-the-canopy and under-tree sprinkler irrigation can promote sporulation and disperse conidia.
Symptoms This disease is noticed either in the spring when buds fail to sprout and foliage appears stunted and/or in summer when dead limbs become apparent following heat or water stress. Symptoms include branch and main scaffold dieback as well as cankers and vascular necrosis. Leaves above stem infections droop and discolor through shades of green to various shades of brown; they remain attached, sometimes through the winter. These flag shoots are caused by stem invasions that girdle or nearly girdle the stem immediately below the flag.
Cankers are small at first but slowly enlarge elliptically; sometimes they streak rapidly up and down the stem without immediate girdling. Cankers are sunken due to dead tissue in the middle and living tissue at distinct margins that continue to grow. Cankers may or may not have gummosis. For Cytospora canker, a callus forms at the canker margins and it stops growing but fungal activity resumes again next spring. If not removed, continued expansion of these cankers each year gives it an elliptical look with concentric rings, which is why it has also been called perennial canker.
Cytospora canker surfaces develop scattered raised pinhead-size pimples (pycnidia) that push through the bark, but these may not be readily visible. Amber to orange, hair-like spore tendrils extrude from the pycnidia in humid weather. These break off easily and can be blown about, tracked around by insects, and splashed by rain. In very wet weather, an amber spore droplet instead of the spore hair may form on the pycnidia.
Cultural control
- Remove dead branches, cutting well below (at least 1 ft) any visible discoloration in the bark. Infected material should be removed from the orchard and destroyed if possible.
- Do not establish new orchards close to or downwind from badly diseased orchards.
- Treetop or other hedgerow pruning and overhead irrigation are not advised where the disease is a problem.
- Take special care to avoid wounding trees during the growing season. Tractors and sprayers commonly inflict such wounds, as do shakers, ladders, and pickers' feet.
- Control all insect and disease problems, such as brown rot of stone fruits, even in the first few nonbearing years.
- Train trees properly.
- Avoid rodent injury.
- Avoid excessive nitrogen fertilization, nitrogen applications late in the growing season, or late-season cultivation or irrigation.
- Paint the southwest side of tree trunks with white latex paint to avoid winter injury.
- Prune as late as possible in the dormant season. Prune in dry weather.
- Summer pruning can be helpful as long as wounds can be kept dry for 2 to 3 weeks.
- Surgically remove cankers from trunks and scaffold limbs in summer.
- Pruning wound treatments with sealants or paints have been effective when used within 24 hours of the pruning cut. These products include B-Lock, Spur Shield, or Vitiseal. See labels for details.
Chemical control Use of latex paint, Captan and/or Topsin was shown to be effective wound protectants prior to Cytospora infection in Colorado. Latex paint, however, did not result in long term effective control while lime sulfur gave variable results. Use of these materials after pruning or winter injury may be helpful. Use of copper-based fungicides was ineffective.
- Topsin 4.5 FL at 30 fl oz/A as a spray directed onto cuts within 24 hr of pruning. A second spray 2 weeks later is recommended. Topsin 4.5 FL may also be used at 4 fl oz/1 gal water and applied as a paint to cut or pruned surfaces. Use when rain is not expected after application. Group 1 fungicide. 2-day reentry.
References Grove, G. G., and Biggs, A. R. 2006. Production and dispersal of conidia of Leucostroma cinctum in peach and cherry orchards under irrigation in Eastern Washington. Plant Disease 90:587-591.
Úrbez-Torres, J. R., Boulé, J., Walker, M., Hrycan, J. and O'Gorman, D. T. 2024. Identification of fungal pathogens causing fruit tree die-back in British Columbia. Canadian Journal of Plant Pathology, 46:89-105.