Leaf rust (blueberry) (Thekopsora minima)

Leaf rust (blueberry): biology

Leaf rust is a fungus that usually impacts blueberries in the sprout year more than in the fruit year, as it appears later in the season – typically in late July.

Leaf rust has a complicated life cycle that includes several different spore types and infection of both blueberry and hemlock. Although it has minimal effect on fields in the fruit year, which are typically harvested in August, it is considered to be a major disease in the sprout year, causing extensive leaf spotting and leaf drop. Infected leaves that have overwintered on the field produce spores that can infect new leaf growth. There may be several cycles of infection throughout the season.

Symptoms of leaf rust are similar to other diseases and consist of leaf spots with brown centres and diffuse red margins. This disease can, however, be distinguished easily from other diseases by examining the lower leaf surface for rust pustules.

Leaf rust can cause extreme leaf defoliation during the sprout year. Leaf drop can be extensive by early September and the production of fruit buds in severely affected fields in the sprout year can be reduced by 30 percent or more the following year.