By Greg Welfing
Pest Profile is a recurring feature in this magazine. Periodically we pick an agricultural pest to feature and learn more about its biology and control methods.
Yellow rust is a plant disease caused by the fungus Phragmidium rubi-idaei. It is a very common disease affecting red raspberries (Rubus idaeus). It is very prevalent in the Fraser Valley. The only host plant for this disease is red raspberry.
The fungus overwinters as spores (a specific type of spore called a teliospore) on leaf tissue and in the bark of dormant canes. It will progress through a series of spore-producing stages throughout the season. In early spring through early summer, yellow pustules (raised bumps) will appear on the upper surface of the leaves. These start off small and then mature into raised ring-shaped structures, called the aecia stage. This stage is the most conspicuous and is the easiest to notice in the field (figure 1). As the summer progresses, a third spore stage (uredinia) will develop on the underside of the leaf. These spore spots are usually more orange in colour than the yellow aecia stage. In this stage the spread of the disease can be quite rapid. The spread is always faster in cold and wet conditions. The uredinia stage will then develop into the black-coloured overwintering (teliospore stage), completing the life cycle.
Yellow rust is predominantly a foliar disease. It can infect succulent canes as well, but the primary damage is done to the leaves. In warm years, it can be solely a cosmetic problem, but in wetter years like this one, it can defoliate canes completely if left uncontrolled. If infections happen early and are severe, yellow rust can cause leaves to drop early and reduce the winter hardiness of the plant.
Fruiting laterals can also be infected early in the spring. If this is the case the fruit will die on the plant before maturing.
Proper disease control always starts with having a clean field. Make sure that any new plants that you purchase are from a certified disease-free nursery. Different varieties vary in their susceptibility to yellow rust. If yellow rust is a concern in your fields, try to pick a variety that is less susceptible. In the fall make sure to reduce inoculum by pruning out old canes and cultivating the prunings to reduce leaf debris. While tying up the primocanes in the fall, remember to strip off the leaves. When leaves are left on the plant over the winter, they serve as a source of inoculum for the disease. During the growing season, strive to maximize air flow between the plants. This will help keep the plants dry and help reduce the spread of yellow rust.
There are some chemical control options as well. There are a number of FRAC group 3 fungicides registered for use on red raspberries for control of yellow rust. It is best to apply these products preventatively in spring.