If you grow raspberries, you should be on the lookout for spur blight. For some reason, blackberries and other bramble fruit are not affected by the Didymella applanata fungi. Red raspberries are more likely to become infected by this fungal disease than other raspberry species, but all raspberries can get spur blight.
Currently found most often in Scotland, Oregon, and Washington, spur blight can significantly reduce your raspberry crop. Knowing what to look for can help you nip this disease in the proverbial bud.
Spur blight symptoms
Spur blight first appears in mid to late summer on new growth, on leaves and around buds. Initial symptoms of spur blight look a lot like anthracnose, fireblight, and cane blight, with brown and purple lesions on leaves, around buds, and on the lower area of stems. These lesions cause buds to shrivel up. By spring, those lesions will look ashy grey and the buds will be weak or dead. Stems that grow from these infected buds will be wilted and weak.
Infected leaflets have triangular-shaped brown areas and they may fall off, leaving the petiole (leaf stem) in place. You might also see dead spots on the canes near petioles. This infection causes premature leaf drop, which weakens the plant overall. As the blight progresses, splits and cracks may form in the bark.
If you look closely, you might see tiny black dots emerging from those cracks. Those dots are fungal fruiting bodies, called pycnidia. If you look at pycnidia with a microscope, they are flask-shaped. Pycnidia formation is followed by a different type of fruiting body, called perithecia. Perithecia are also black, but they are medium-sized and, if you watch, erupting with spores.
Spur blight lifecycle
Spur blight is spread by spores on wind, rain, and irrigation water. These spores are released each time wet weather occurs, even if that wet weather is your garden hose. If that infected water falls on or near young canes, newly forming buds, leaves, wounds, or stoma, infection can occur. Once inside, this disease spreads throughout the plant, overwintering in the lesions.
How to manage spur blight
Proper sanitation and pruning methods will go a long way toward preventing spur blight on your beloved raspberries. In late winter or early spring, before new canes emerge, remove all dead, diseased, or weak canes and put them in the trash, rather than the compost pile. Be sure to keep canes properly spaced for good air flow, and train them up trellising that allows canes to dry quickly and to make it easier for you to water your raspberry plants at ground level. You should also keep weeds away, as they complete for water and nutrients and reduce air flow around the canes. Unless a soil test has indicated a genuine need for fertilizer, avoid feeding your raspberries. The presence of too many nutrients causes plants to produce an abundance of vulnerable tissue.
As always, only buy certified, disease-free plants and place new plants in quarantine. In the case if severe infection, fixed copper or lime sulfur treatments may reduce lesion size and control internal infection if applied when new shoots are 8-10” long.
Believe me when I tell you that it is far easier to prevent spur blight than to treat it. And raspberries are worth the effort.
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