This summer, we have seen two similar but different diseases on blackberry samples: orange rust of blackberry and black raspberry and leaf and cane rust of blackberry. Orange rust is typically the more devastating disease because it can become systemic, moving from leaves into other parts of the plant. The orange rust fungus has two forms, Arthuriomyces peckianus (formerly Gymnoconia peckiana) and Gymnoconia nitens, which differ only in the number of spore stages produced. Pustules full of orange-yellow spores develop on the undersides of leaves in late May and early June. These spores are blown to healthy leaves and infect when humidity is high and leaves are wet. Heavily infected leaves may die and defoliate. Once the plant is infected, the rust fungus becomes systemic. It grows down the infected shoot, into the crown, and then can enter newly formed roots. Symptoms associated with shoot infections include proliferation of shoots, weak and spindly canes, and lack of spines on the shoots. In mid- to late summer, brownish black pustules that contain dark teliospores develop on the undersides of lower leaves. Teliospores do not infect, but germinate to produce basidiospores that can infect new buds or shoots, or the teliospores can overwinter on leaves before producing basidiospores the following year. Infected plants remain infected throughout their lifetime and do not recover.
|Orange rust pustules on underside of leaves. Note leaf distortion. (Photo: PDIC Database)|
Leaf and cane rust is caused by the fungus Kuehneola uredinis. Leaf and cane rust produces yellow spores in pustules that split the bark of infected canes, causing them to become weak. The pustules can also be found on the undersides of leaves. Diseased old canes should be pruned after fruiting. Alternate-year fruiting programs can help reduce disease pressure, and routine fungicide spray programs may be effective in preventing new infections.
|Leaf and cane rust. Note yellow spores bursting from cane. (Photo: NCSU Database)|
|Leaf and cane rust. Infected leaves maintain shape unlike |
those infected with orange rust. (Photo: NCSU Database)