By Pam Fisher, OMAFRA
There are several rust diseases that attack raspberries. Rust diseases often have complicated life cycles that include alternate hosts, and most produce several types of spores. If you see rust diseases on your raspberry crops, it is important to identify which disease is present, so you can manage it effectively.
Late leaf rust (Pucciniastrum americanum)
- Crops attacked: Red and purple raspberries
- Alternate hosts: White spruce
- Symptoms: Pale orange powdery spores on lower leaf surface, upper surfaces develop small yellow areas that gradually turn brown. Severe infections may result in early leaf drop, reduced plant vigor and yield and increased winter injury to infected canes. On fruit, bright orange powdery spore masses develop on infected drupelets.
- Control: Prune and trellis raspberries to encourage air movement and rapid leaf drying. Prune out and remove old canes. If possible, remove nearby white spruce which are required for the disease to complete its life cycle. Prebloom applications of Ferbam to raspberries might help reduce infections. Currently there are no other fungicides registered for control of late leaf rust in Ontario.
- Comments: Late leaf rust is common in Ontario. While summer-bearing cultivars often escape fruit infections, fall-bearing raspberries tend to develop fruit infections if weather conditions are favourable for disease development. The cultivars Heritage, Jaclyn, and Caroline seem especially susceptible.
Orange rust (Arthuriomyces peckianus and Gymnoconia nitens)
- Crops attacked: All raspberry species except red raspberry
- Alternate hosts: none
- Symptoms: Plants develop symptoms the year following infection. New leaves are stunted, pale and spindly. Bright orange waxy pustules develop on the lower leaf surface, later becoming bright orange and powdery. Infected plants are unproductive.
- Control: This disease is systemic – once a plant is infected it is always infected. Remove infected plants, preferably before orange pustules break open and spores spread to more plants. Remove wild black raspberries from adjacent woods and hedgerows.
- Comments: Very common on wild black raspberries and blackberries.
Yellow rust (Phragmidium rubi-idaei)
- Crops attacked: Red raspberries
- Alternate hosts: none
- Symptoms: In spring and early summer, yellow pustules appear on the upper leaf surface of young leaves. As summer progresses orange pustules are produced on the lower leaf surface, eventually turning black as the overwintering spores are produced. If infections are early and severe, the disease can cause early leaf drop and reduced winter hardiness.
- Control: Purchase clean plants from a certified plant grower. Reduce inoculum by pruning out old canes and cultivating to reduce leaf debris. Several group 3 fungicides, such as Nova, Tilt and Bumper, are registered for control of yellow rust. Apply before symptoms appear.
- Comments: This disease is not prevalent in Ontario, although it is common in the Pacific Northwest. Yellow spots on the upper leaf surface can help growers distinguish between yellow rust and late leaf rust, which does not produce symptoms on the upper leaf surface.