M. Grabowski, UMN Extension Educator
|Black Knot Galls|
A traditional recommendation in Minnesota is to prune trees in February and March. One goal of pruning during these months is to remove branches infected with cankers and galls. The fungi and bacteria that cause cankers and galls on trees overwinter in these infections. When weather becomes warm and wet many of these pathogens start to reproduce and spread the disease within the canopy and to neighboring trees.
Climate change is increasing Minnesota’s average temperature and adding days to Minnesota’s frost free season. Spring in Minnesota has been arriving earlier and is warmer. Weather data from the twin cities metro area over the previous five years reveals that in February an average of 7 days reached a maximum daily temperature over 32 F, and in March an average of 23 days reached a maximum daily temperature over 32 F.
There are several common canker and gall causing pathogen that can reproduce and infect at temperatures just above freezing, although infection rates are low at these temperatures. Black knot, a common fungal gall disease found on Prunus trees in Minnesota, was reported to begin spore release at temperatures as low as 37 F. The fungi that cause coral spot canker and perennial Nectria canker on a number of hardwood trees can begin to germinate and infect at temperatures just above 32 F. In 2012, March had 9 days with a daily maximum temperature greater than 68 F. At that temperature range, many different canker and gall causing pathogens are actively reproducing and spreading.
We cannot predict exactly when warm weather will arrive in 2016. To prevent canker and gall causing pathogens from reproducing and spreading, gardeners should prune out and destroy canker and gall infections while temperatures remain consistently below 32 F. When fresh new plant growth emerges from dormant buds, it will then have a clean healthy start to the growing season.
When pruning to remove cankers and galls, make the pruning cut just above a lateral bud or at a branch union that is 10-12 inches below visible symptoms of the infection. Remove the infected branches from the area and burn or bury them. Black knot galls left lying in an orchard after pruning were found to continue to release spores for four months.
If warm weather comes earlier than expected, pick a dry day with no rain in the forecast for the next several days to prune. Many fungal and bacterial plant pathogens release spores in response to rain, heavy dew or high humidity and need moisture on the plant surface to start a new infection. Dry weather provides a period where trees can heal the pruning wound under conditions that are less than ideal for infection.