Reduce the risk of tar spot and apple scab now

M. Grabowski, UMN Extension

Apple scab at mid summer
M. Grabowski, UMN Extension 

Two common leaf spot diseases that blight Minnesota landscapes every year are tar spot of maple and scab of apple and crabapple. Both diseases are caused by fungi that survive winter on last year’s infected leaves.

As snow melts and tree buds begin to swell, gardeners have one last chance to remove these infected leaves before the fungal pathogens become active.

How do leaves become infected?

Both fungi produce new spores in response to warming temperatures and moist conditions created by snow melt and spring rain. Spores are ejected forcibly into the air where they are carried by wind to new emerging leaves. There they will initiate this year’s leaf spot epidemic.  

Although neither disease is a significant threat to the health of the tree, leaf spot diseases blight the appearance of landscape trees, reducing their value as ornamentals. Many gardeners look for management options late summer when symptoms are very obvious but it is too late to prevent disease.

Apple scab infected crabapple leaves
Photo: M. Grabowski, UMN Extension 

Apple scab

Task: Rake up and remove any remaining leaves that may have been infected with apple scab last year. Bury infected leaves in a backyard compost pile or take them to a municipal yard waste site.
Deadline: Before tree buds open and ¼ inch of green leaves can be seen emerging.

Tar Spot

Task: Rake up and remove any remaining leaves that may have been infected with tar spot last year.  

Bury infected leaves in a backyard compost pile or take them to a municipal yard waste site.

Deadline: Before new leaves reach full size.

Maple leaf debris with tar spot
Photo: M. Grabowski, UMN Extension