If you grow raspberries, you should be on the lookout for spur blight.
Blackberries and other bramble fruit are not affected by the Didymella applanata fungi, but red raspberries are especially vulnerable.
Currently found most often in Scotland, Oregon, and Washington, spur blight can significantly reduce your raspberry crop.
Spur blight symptoms
Spur blight first appears in mid to late summer. Initial symptoms of spur blight look similar to 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, lesions will look ashy gray, 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 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.
Look closely to 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. A different type of fruiting body, perithecia, comes next. Perithecia are also black, but they are medium-sized and, if you watch, erupt with spores.
Spur blight lifecycle
Spur blight spores travel in wind, rain, and irrigation water. These spores are released each time wet weather occurs, even if that wetness is your garden hose. Infected water that lands on or near young canes, newly forming buds, leaves, wounds, or stomas, can result in infection. Once inside, this disease spreads throughout the plant, overwintering inside 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.
Keep canes properly spaced for good airflow, and train them up trellising to allow canes to dry quickly. It makes watering your raspberry plants at ground level easier, too. Keep weeds away, as they compete for water and nutrients and reduce airflow around the canes. Unless a lab-based soil test has indicated a need for fertilizer, avoid feeding your raspberry plants when spur blight is a problem. 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 of severe infection, fixed copper or lime sulfur treatments may reduce lesion size and control internal infection, but only if applied when new shoots are 8-10” long.
Spur blight is easier to prevent than treat. And raspberries are worth the effort.
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