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Extension > Garden > Yard and Garden > Fruit > Integrated pest management for home raspberry growers > Raspberry leaf spot

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Raspberry leaf spot

raspberry-leaf-spot

Leaf spot on 'Killarney' summer red raspberries at the end of August.

Thaddeus McCamant, Central Lakes College

Raspberry leaf spot (Sphaerulina rubi) is a common disease in certain raspberry cultivars across Minnesota. Leaf spot can be a devastating disease in susceptible summer-bearing raspberry cultivars. During humid summers, up to 75% of the leaves can be lost from the primocanes (first year canes). When leaves fall off due to leaf spot, the primocanes stop growing and become more susceptible to winter injury. In some cases, primocanes with leaf spot are half the size of uninfected primocanes. Small canes produce less fruit, and severe leaf spot can reduce yield by over half.

Leaf spot tends to be rare in the first two years after planting. In susceptible varieties, the disease gradually becomes worse each year unless the disease is controlled by cultural controls or fungicides.

Identification

Raspberry leaf spot starts as small spots on the upper surface of young leaves. As the lesions grow, the infected tissue may fall out, leaving holes in the leaves. Badly infected leaves curl downward at the edges and drop prematurely.

Important biology

The fungus that causes leaf spot overwinters in infected leaf debris and in cane lesions. Only young, expanding leaves are susceptible to infection. Spores are spread by splashing water, and leaf spot is common in years with frequent rain. The disease continues to spread all summer, as long as susceptible leaves are present. Leaf spot causes the most damage following summers with heavy rains in June and July.

Management

When assessing leaf spot, first determine if the diseased leaves are on primocanes or floricanes. Leaf spot is a more serious disease if it occurs on primocanes, because the leaves on floricanes naturally die towards the end of harvest.

The summer-bearing red varieties Latham and Nova are resistant to raspberry leaf spot. The summer-bearing red varieties Boyne and Killarney are susceptible and can be largely defoliated by leaf spot on years with high disease pressure. Royalty, a purple variety (a cross between red and black raspberry species), is highly susceptible. Royalty cannot be successfully grown in patches where leaf spot disease is present because the canes become weaker each year as the leaf spot becomes worse.

Leaf spot can be minimized by cultural practices that help the leaves dry out after rainfalls. Maintaining narrow raspberry rows at 18 inches wide or less will help the leaves dry faster. Thinning primocanes during the first summer will also reduce infections by helping leaves dry quickly. As with the cane diseases, removing floricanes after harvest is critical in reducing damage from raspberry leaf spot.

Fungicides are typically not necessary if good cultural practices are utilized. In patches with a history of severe leave spot, fungicides can slow the spread of the disease. Sprays should be directed towards the recently sprouted canes. Sprays need to be applied according to label instructions and repeated throughout the season as long as new leaves and shoots are developing. Fungicides with the active ingredient copper, captan or myclobutanil can help prevent leaf spot.


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