Why Do My Raspberry Canes Have Purple Blotches Near the Buds?

Dark purple or brown discolouration centred around the buds and nodes of raspberry canes — spreading up and down the cane from each bud junction — is the classic appearance of spur blight, caused by the fungus Didymella applanata. It is one of the most common and widespread raspberry diseases in temperate climates, particularly in wet seasons and in overcrowded plantings. Heavily infected buds fail to open or produce only weak growth, which directly reduces next year's fruit crop.

How spur blight affects the plant

The fungus infects the node tissue around buds in summer, producing the characteristic purple-brown discolouration. In autumn and winter the infected tissue produces spores. When buds try to break in spring, those in severely infected nodes may fail entirely or produce only a short, weak lateral rather than a productive fruiting shoot. Yield is reduced in proportion to the number of infected nodes on the fruiting canes. The cane itself is not usually killed outright — that distinguishes spur blight from cane blight.

Managing spur blight

Thin the fruiting canes to no more than six to eight per metre of row after autumn pruning — proper spacing allows air to circulate around each cane, preventing the wet conditions in which spores germinate. Remove all pruned cane material from the site rather than leaving it to decompose (composting is acceptable if the heap reaches high temperature, otherwise bin it). A copper-based fungicide applied in early spring as buds begin to swell and again after leaf emergence has good efficacy against spur blight.

Effect on next year's crop

Spur blight infects the current year's canes in summer but the impact is primarily felt the following spring when those canes would normally fruit. This is why a bad spur blight year leads to a disappointing harvest the following year. Managing the disease in summer prevents the impact from carrying forward.