What is Quince Rust?
Quince rust is a fungal disease caused by Gymnosporangium clavipes that infects apples, quince, hawthorn, cedar, and other Rosaceae species. While quince rust may be less well known than cedar apple rust (CAR), it is still a significant threat to commercial apple growers and can lead to 50-100% yield loss in ideal conditions.
What does Quince Rust look like?
Quince rust initially presents as a purple raised lesion on the sides or underside, also known as the calyx region, of susceptible apple fruit. As this lesion matures, it continues to cause an external dark green-purple distortion of the fruit, and internally exists as spongy brown rust lesions penetrating all the way to the core. If basidiospores penetrate the stem of the developing fruit, it can cause an abscission and the fruitlet will fall off.
Infected fruit may also produce secondary aeciospores once mature. These powdery, rust-colored spores appear in tubes along the calyx region and are spread by wind to infect new hosts.
Where does Quince Rust come from?
The disease cycle for quince rust requires a pomaceous host such as apple or hawthorn, and a juniper/cedar alternate host. In the late summer and fall, aeciospores from infected pomaceous hosts land on young bud and twig tissue of cedars and juniper trees and form a gall. Once mature, the gall produces telial horns in the spring under wet conditions, and the basidiospores from the horns are spread by wind to spread the infection to the pomaceous host.
Quince rust requires specific environmental conditions to devastate apple crops. In the spring, infections are more likely to occur when there is a wet period of more than 48 hours and the temperature is above 10°C (50°F), while the fruit is between the tight cluster and petal fall developmental stage. These conditions maximize infection potential and spore dispersal, leading to large scale losses. However, these conditions lining up is not a frequent occurrence at this development stage, and individual cultivar bloom dates can have a significant impact on disease susceptibility. As climate change continues to cause warmer and wetter springs, it is possible that Quince rust outbreaks may grow more severe as optimum conditions become more frequent.
How do I prevent and control Quince Rust?
- Plant cultivars resistant to apple rust species, including quince rust and cedar apple rust
- Not all cultivars are resistant to both species (i.e., McIntosh is resistant to CAR but susceptible to quince rust)
- Remove alternate hosts from orchard vicinity, including hawthorn, juniper, and quince