Why Do My Blackberry Canes Have Lumpy Growths or Galls?

Abnormal swellings, warty growths or irregular lumps on blackberry canes have two main causes: crown gall disease caused by the soil bacterium Agrobacterium tumefaciens, and the gall-like swellings produced in response to raspberry cane midge attack followed by secondary fungal infection. Distinguishing between them helps you decide whether the plant can be saved or needs replacing.

Crown gall

Crown gall produces rough, corky, irregular swellings on the lower cane, crown or roots — most commonly at or below the soil line, but occasionally on canes above ground. The galls are caused by the bacterium inserting its genetic material into the plant cells, forcing them to proliferate uncontrollably. Small galls on established plants do not usually cause significant problems — the plant can continue to produce fruit. Large galls that encircle a cane restrict the flow of water and nutrients above the gall, weakening the cane. There is no treatment; affected canes should be cut out below the gall. If the whole crown is galled, remove the plant.

Cane midge and secondary fungal damage

Raspberry cane midge (Resseliella theobaldi) lays eggs in splits in the cane bark in spring. The hatching larvae feed under the bark and open a wound through which cane spot and spur blight fungi enter, producing swellings and purple-black lesions that can look gall-like from a distance. Close inspection reveals the distinctly purple and spotted coloration of a fungal infection rather than the corky, irregular surface of a true gall. Cutting out affected canes is the appropriate response.

Prevention

Avoid creating wounds on canes during routine work — crown gall and cane midge entry points are almost always wounds. Disinfect tools between plants. For crown gall, purchase certified disease-free planting stock from reputable suppliers rather than using divisions or cuttings from plants with an unknown disease history.