Late Leaf Rust
Late leaf rust can be a problem on fall-bearing raspberries. The disease infects red and purple raspberries but not black raspberries or blackberries. This rust, unlike orange rust, is not systemic.
Yellow masses of spores are noticed primarily on fall fruit of primocane-bearing varieties, making the fruit unmarketable. Because symptoms on the fruit do not usually develop until late in the season, infections in plantings of summer-bearing varieties may go unnoticed. Powdery yellow spores also form on the undersides of leaves, causing badly infected leaves to drop prematurely, but this symptom is generally not noticed until infected fruit is seen.
There are several species of late leaf rust fungi worldwide. In our region, Pucciniastrum americanum is believed to be the causal agent. White spruce and Engelmann spruce serve as alternate hosts, and their closeness to a planting may increase the likelihood of occurrence. Spores are produced on infected spruce needles in early summer and can infect raspberries. High humidity is necessary for infection to take place. The raspberries will show symptoms shortly afterward. However, spruce are thought not to be necessary for the rust to survive in a planting once infected since this disease has occurred in successive years in plantings with no spruce in the vicinity. Spores are disseminated by wind but may also be physically moved from infected to uninfected plantings by people or machinery.
Clean nursery stock is important since planting stock can be the initial source of inoculum. Control is aided by cultural practices that increase air circulation within the planting such as thinning canes, keeping rows narrow, and practicing good weed control. Prune fall bearers to the ground; do not keep canes around for a summer crop and rake and dispose of all old leaves. Removing floricanes and infected primocanes in winter will reduce the amount of inoculum. This disease has been especially problematic on summer-bearing Festival and fall-bearing Heritage and Jaclyn. Fall-bearing Josephine and spring-bearing Nova and Esta red raspberries tend to be resistant. Because this fungus is not systemic, eliminating the disease from plants is possible. Refer to Table 7.5 for pesticide recommendations.