Yellow Rust

If you grow red raspberries, monitor leaves early in the season for yellow rust.

Yellow rust pustules on raspberry leaf (James Lindsey) CC BY-SA 3.0

Like its cousin, the bright orange rust seen on the underside of rose leaves, yellow rust is a fungal disease. Unlike many other fungal infections, this rust only occurs on plant exteriors. When yellow rust occurs on wheat, rye, and barley, it is called stripe rust, a Puccinia striiformis infection.

Symptoms of yellow rust

Plants infected with yellow rust (Phragmidium rubi-idaei) will initially have yellow pustules, called aecia, on the tops of the lower leaves. These symptoms are usually only seen in spring and early summer. In early to mid-summer, yellow to orange pustules, called uredinia, are found on the underside of leaves. 

As summer progresses, these growths darken. Black spots can be seen in the middle if you look closely. You may also see orange spots on the fruit. Similar infections that occur later in the season may be late leaf rust (Pucciniastrum americanum) or more severe orange rust (Arthuriomyces peckianus). Either way, infected leaves wither and die, reducing the plant’s ability to perform photosynthesis, which cuts crop size significantly.

Yellow rust control

Pruning for good airflow helps leaves and stems dry out, making life more difficult for these fungi. Since yellow rust spores (teliospores) overwinter in fruiting canes or floricanes, pruning those canes out at the end of the growing season can break this disease triangle. Canes of summer-bearing raspberries won’t produce any more fruit, so you might as well. Just dispose of these canes in the trash and not the compost pile.

If unproductive canes stay in place, any spores they contain will spread the infection to the next season’s primocanes or vegetative growth. Keep the ground around cane fruit clear of dead leaves and other plant debris. If your raspberries are especially prone to yellow rust, you may want to cut the first spring growth of new canes back to ground level. Don’t worry. Their root systems will quickly put out new stems. That first growth is the most likely to have become infected.

Fixed copper sprays and lime sulfur can treat severe outbreaks. Otherwise, remove infected leaves by hand and improve the airflow between plants to keep this disease in check and protect your delicious raspberries.

Resistant varieties are available, so check with your local Department of Agriculture or Master Gardeners for recommendations for your area.