Red Raspberry Leaves: Late-Stage Rust Infection
Late Leaf Rust in Red Raspberries: Understanding and Managing the Disease
Red raspberries are under threat from a fungal disease known as late leaf rust, caused by the fungus Aculeastrum (Pucciniastrum) americanum. This disease, common in raspberry farms, can lead to significant economic losses due to fruit infection and premature defoliation.
Causes
The causal agent is a rust fungus that infects raspberry leaves, causing late leaf rust. The fungus thrives under favourable conditions of moisture and moderate temperatures, allowing spores to germinate and infect leaves.
Common Symptoms
Early signs of late leaf rust include light yellow or light-colored spots appearing on the leaves. As the disease progresses, leaves wilt and eventually drop or deteriorate, with symptoms typically arising later in the growing season.
Management Strategies
Managing late leaf rust requires a combination of cultural controls, fungicide application, sanitation, and varietal resistance.
Cultural Controls
Prompt removal of infected leaves, good air circulation, and avoiding overhead irrigation are key cultural controls. Pruning infected leaves as soon as spots are noticed helps reduce inoculum, while promoting good air circulation around plants through proper spacing and pruning aids in faster drying.
Fungicide Application
Fungicide sprays should begin at the first sign of infection and be repeated as necessary to protect healthy tissues. It's essential to use fungicides registered for rust control on raspberries, with timing and frequency depending on weather and disease pressure.
Sanitation
Sanitation involves removing and destroying fallen leaves and plant debris to reduce overwintering spores.
Varietal Resistance
If available, planting rust-resistant raspberry varieties can help minimize the impact of the disease.
Identification and Prevention
Because late leaf rust symptoms can be confused with other leaf diseases, careful monitoring and accurate identification are crucial. Site selection with good air movement and full sun exposure is essential for disease management. Late leaf rust overwinters on infected raspberry canes as urediniospores or teliospores, but it apparently does not need the aeciospore stage to survive on raspberries.
The disease affects red raspberries but not black raspberries or blackberries. Symptoms on mature leaves may start as small chlorotic areas that turn brown before leaves die in the fall. Figure 1 shows early season symptoms of late leaf rust on infected red raspberry leaves, while Figure 2 depicts small pustules filled with yellow to orange powdery spores on the underside of infected leaves. Figure 3 shows late leaf rust symptoms on red raspberry fruit, and Figure 4 depicts pustules on individual drupelets on infected fruit, with masses of yellow spores.
In severe cases, badly infected leaves may drop prematurely, leaving canes bare by September. Economic losses occur from fruit infection and premature defoliation. The fungus is heteroecious, meaning that it attacks two different hosts at different stages of its life cycle, with the alternate host being white spruce (Picea americanum).
To avoid the disease, using healthy, disease-free planting stock is recommended. Implementing an integrated disease management strategy combining pruning, sanitation, and fungicide use can help control late leaf rust effectively.
- Incorporating science into pest management strategies for late leaf rust in red raspberries can help with accurate identification, effective fungicide application, and the adoption of rust-resistant varieties.
- The health-and-wellness of red raspberry plants can be impacted by medical-conditions such as late leaf rust, resulting in significant economic losses due to fruit infection and premature defoliation.
