If you grow red raspberries, you need to monitor leaves early in the season for yellow rust.
Like its cousin, the bright orange rust seen on the underside of rose leaves, yellow rust is a fungal disease. Unlike many other fungal diseases, this rust only occurs on the outside of plants. This is not the same yellow rust seen on wheat, rye, and barley, which is called stripe rust. Stripe rust is caused by Puccinia striiformis.
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 and a black spot 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 the more severe orange rust (Arthuriomyces peckianus). In any case, infected leaves wither and die, reducing the plant's ability to perform photosynthesis. This can reduce crop size significantly.
Yellow rust control
Pruning for good air flow helps leaves and stems dry out, making life more difficult for this 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. The canes of summer-bearing raspberries won’t produce any more fruit anyway, so you might as well. Just be sure to dispose of the trimmed canes in the trash, and not the compost pile.
Left in place, these spores then spread the infection to the next season’s primocanes, or vegetative growth. Also, keep the area around the plants 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, the root system will put out new canes pretty quickly. That first growth is the most likely to have become infected.
Fixed copper sprays and lime sulfur are recommended for severe outbreaks. Otherwise, you can simply remove infected leaves by hand and improve the air flow 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.
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