Missoula County Weed District & Extension

Cytospora Canker




Plant Diagnostic Database

Cytospora Canker  (Valsa spp., Leucostoma spp.)












Apple, Ash, Aspen, birch, boxelder, cottonwood, elm, linden, honey locust, maple, mountain ash, oak,poplar, sumac, willow, spruce, and stone fruits.


Symptoms vary depending on host and species of Cytospora. Cankers are usually irregular in shape and elongate when they appear on limbs and trunks. Discoloration of the outer bark may be yellow, brown, red-brown to gray or black depending on the host plant affected. Pimple-like fruiting structures (pycnidia) often develop in the canker areas. Under moist conditions, pycnidia ooze orange, thread-like spore tendrils. On aspen trees, the substance that oozes from the canker is in a liquid form; on cherry and plum trees, it is gummy.

On spruce, dying or dead branches can indicate canker development. Older branches are more susceptible than younger ones. Lesions appear as sunken areas surrounded by swollen callus tissue. Small black fruiting structures may be evident on the canker; however, large amounts of clear amber resin can flow from the infected areas and may obscure the canker location.

Life Cycle:
The fungus overwinters in cankered bark. Spores are dispersed by rain, wind, insects, or birds. Infection occurs only through bark wounds, dead tips of twigs, or branch stubs when temperatures are 70 - 80° F. and weather is humid. The fungus grows in the bark until limited by the defense processes of the tree. During periods of active tree growth fungal growth is temporarily inhibited. Fruiting bodies form in the infected bark to complete the life cycle. Drought-stressed trees are more susceptible.


  • Prune in late spring after trees have leafed out; pruning wounds heal more rapidly at this time.
  • Prune at least one foot below any discoloration. Disinfect your pruners between cuts.
  • Prune on a dry day.  Avoid overhead irrigation.
  • Avoid excessive nitrogen fertilization.Use white reflective paint on young trunks to reduce sunscald wounds.
  • Paint the southwest side of tree trunks with white latex paint to avoid winter injury.


The Doromaki procedure is commonly practiced in commercial Japanese orchards. In the spring, cankers are packed with a paste of wet soil (1" thick) from the orchard floor. Vinyl film is wrapped around the packed canker to retain moist soil on the canker. In dry climates the soil may have to be sprinkled to retain moisture during the hot dry months of July and August. The vinyl film is removed the next spring and the soil pack is removed. Trichoderma spp. (Planters box) can be painted onto the cankers and kept moist with the Doromaki procedure. 

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Abiotic Ailments