Raspberry leaf spot is perhaps the most common raspberry disease we see in the clinic. Early infections look like dark green circular spots on new leaves. As leaves get older, spots become light tan to gray (see picture below). Severe infections cause leaves to fall off in late summer and early fall and this may not only reduce your raspberry harvest, but it also makes plants more prone to winter injury.
Raspberry leaf spot is caused by a fungus called Sphaerulina rubi. This fungus overwinters on leaves and canes which then serve as sources of infection in spring. Young leaves are highly susceptible to this disease, but older spots or lesions (see picture) produce spores that are readily spread by rain or wind to new tissues throughout the entire season.
Cultural measures that increase air circulation are probably the most effective way to control this disease. Since this fungus thrives under high humidity conditions, promoting faster drying of leaves and canes after rain can reduce the chances of infection. The easiest tactic is to start off with properly spaced plants between and within rows, and in established plantings, avoid too many canes per plant. Also, reduce the sources of infection by pruning out old fruiting canes and removing them from the field.
Infected raspberry leaves showing gray-tan spots caused by S. rubi.
Infected raspberry leaves with raspberry leaf spot. Lesions may fall out creating a shot-hole appearance.
S. rubi spores produced in lesions are spread by rain and wind-driven rains to young tissues.