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Late leaf rust

Late leaf rust is caused by the fungus Pucciniastrum americanum, and only infects red and purple raspberries. Late leaf rust infects both leaves and fruit. In rare cases, late leaf rust can cause premature defoliation of both summer-bearing and fall-bearing raspberries, reducing the yield and quality of the fruit while making the canes more susceptible to winter injury. Defoliation from late leaf rust is rare in Minnesota. In fall-bearing raspberries, late leaf rust directly infects the fruit, making the fruit unappealing.

Identification

late-leaf-rust

Late leaf rust on fall-bearing raspberry fruit.

Michelle Grabowski, UMN Extension

Late leaf rust is most commonly seen on the fruit of fall-bearing raspberries. Typically, one to three drupelets will be covered with bright orange powdery spores in September. The disease can also cause spots of bright orange powdery spores to form on the underside of the mature leaves.

Important biology

Like many rust diseases, the late leaf rust fungus has two hosts: the white spruce and raspberries. In areas with white spruces, spores are blown from infected spruce to nearby raspberry plants. Infection usually starts on mature lower leaves, where higher humidity aids in spore germination. Infected raspberry leaves and fruit produce abundant powdery orange spores that start new infections throughout the growing season. In areas without spruce trees, late leaf rust appears to be able to survive from one season to the next on infected canes.

Management

Crop losses from late leaf rust are rare in Minnesota, and control measures are rarely needed. The summer-bearing red raspberry Nova has shown to be resistant to late leaf rust, while the fall-bearing red raspberry Heritage is susceptible. New cultivars of fall-bearing raspberries are continually being released, but their susceptibility has not yet been determined.

Cultural practices that reduce humidity within the raspberry patch will help to reduce late leaf rust. Maintain narrow rows, use drip irrigation or water early on sunny days. Remove all old floricanes as well as any primocanes showing symptoms of disease. Removal of infected canes will improve air movement through the patch and will also remove an overwintering site of the fungal pathogens.


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