I’ve seen a number of young orchards recently with trees suffering from die-back. While much of this has been related to cold damage, some trees are feeling the effects of a subtle problem that often goes unnoticed until it becomes severe. Botryosphaeria canker or “Bot canker” is caused by a fungus which can infect the tree through wounds like limb breakage, pruning sites, or small cracks. Symptoms of the canker are sunken, cracked areas in the bark which often appear pulled away from the underlying wood. The infected wood may appear black while the bark surrounding the wound site appears grey-white in color. The problem is easily taken care of when caught in time but can lead to serious problems when left un-checked.
Botryosphaeria fungi may initially colonize dead tissue and move downward on the branch or trunk into healthy bark and sapwood. Spread occurs through air movement or splash dispersal of spores, and can also occur through use of contaminated pruning tools.
In almost every case I have seen this on 2-4 year old trees pruned up high to the point that the recent pruning sites on the trunk are left unshaded by the canopy. The pruning wound fails to heal over properly on the side of the tree facing the intense heat of the afternoon sun, allowing the fungus to take hold and proliferate.
There are no effective fungicide controls for Botryosphaeria canker. I have found the best way to stop Bot canker is to paint the affected site with white outdoor latex paint, which reflects the heat and allows the wound to heal over when caught in time. I have seen trees almost completely girdled which healed over and grew normally after painting. Bot canker can be spread through pruning so it is advisable to bleach pruning tools between cuts when it is known that Bot canker is present.