Yellow Rust

Of the major caneberry crops, yellow rust infects only red raspberry and is not a systemic pathogen, meaning the pathogen does not spread internally through the plant. In spring, yellowish orange pustules (aecia) form on the tops of raspberry leaves close to the ground. Early season observation of aecia on the tops of leaves is a general way to distinguish this rust from late leaf rust, which also infects red raspberry.

Severely affected leaves can dry out and die. Later in June and July, orange to yellow pustules (uredinia) appear on the undersides of leaves; these structures later darken as black teliospores develop from the middle of July to fall. The yellow rust fungus overwinters as teliospores on the bark of remaining floricanes (fruiting canes). Such canes are the sources of inoculum that affect emerging leaves and primocanes (vegetative canes) the following spring.

If possible and horticulturally sensible, complete removal of floricane and first flush of primocane is useful in controlling this disease, because it removes most sources of inoculum. Any method of pruning that improves air circulation is helpful in reducing yellow rust, as this allows leaves, flowers, and fruit to dry more quickly, subsequently reducing plant susceptibility. The dry conditions in a macrotunnel greatly limit the infestation of yellow rust, provided the tunnels are constructed before ideal conditions for infestation begin. In the Monterey Bay area, this generally means mid- to late July.

Cultivating to bury old crop debris, removal of fruiting canes after harvest, and sprays of lime sulfur or some fixed copper products are acceptable methods in an organically certified crop.

Treatment for all occurrences of yellow rust are not necessary because it has been shown that light-to-moderate infections of yellow rust do not affect raspberry plant productivity. Fungicide treatments should be made with an eye to preventing large outbreaks, rather than eliminating the disease. Thus, low levels of disease late in season probably do not merit treatment, while it might be recommended to apply fungicide if signs of yellow rust are seen early in the season, for example in June.