Pesky Blackberry Foes

Here in North Carolina, blackberry season is in full swing! For many North Carolinians, that means it is the perfect time to stock up their freezer or pull out those ball jars and can up some tangy black berry jam. Blackberry season brings back sweet summertime memories of riding my bike down the road to the black berry thicket at the edge of the woods, gorging myself with fresh, juicy berries (probably picking up a few chiggers in the process), and heading home with purple stains on my hands, face, and clothes. Whether you prefer to pick your own berries or buy a pack from the local Farmer’s Market, we can all agree that blackberries are a signature snack for a late summer’s day. Unfortunately, blackberries, like all things delicious, come with their fair share of pests and diseases that impact fruit production.

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)

Orange rust does not kill the plant outright, but infected plants are completely lost to production due to their inability to produce blossoms and berries. Controlling orange rust is largely achieved through cultural practices. Plant disease-free stock plants, eradicate diseased plants and wild berries in the surrounding area, and completely remove and destroy the entire plant as soon as symptoms develop on canes or leaves. Thin healthy plants to promote air circulation and to reduce leaf wetness.

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)

Care must be taken to differentiate systemic orange rust from leaf and cane rust because leaf and cane rust does not require drastic removal methods to control disease. Identification of the rust pathogen requires a microscope and considerable knowledge in rust morphology. Suspect samples should be sent to the Plant Disease and Insect Clinic for a formal diagnosis.