The spores of the fungus -Leptosphaeria coniothyrium - normally enter the cane at damages points. The damage need not be extensive for entry to take place - slight bruising of the stem seems to allow entry to the Raspberry Cane blight. Pruning - essential in raspberry fruit production, is sometimes the entry point for the fungal spores.
The fungus spores can also enter healthy stem tissue. With badly affected outbreaks, there have been no significant sign of raspberry midge damage, so the boring by this and other 'borers' can be largely discounted as a means of the disease spreading or taking hold!
A sudden wilting of cane or foliage is the first sign of infection. This wilting of the raspberry cane can either be with small areas of foliage or whole cane batches.
The affected areas of the raspberry canes sometimes flatten and cracks will be seen to otherwise healthy looking raspberry canes. Scraping the bark tissue will show moist green healthy stem tissue where there is no problem, but where infected with cane blight, the scraped bark will show as an orange-red tissue which is the general confirmation that it is cane blight and not another disorder. This discolouring will be noted above and below the visible affected areas.
It is good practice to prune out the old canes in dry weather conditions. This will help the pruning wound to heal quickly and possibly deter entry of fungal spores.
try not to get the foliage and stems wet by overhead watering or drenching. Watering should be carried out early morning in order that the foliage and stems can dry off quicker than evening watering. Good air circulation helps drying of the raspberry canes, so try not to overcrowd your planting.
We do not feel that we are able to suggest any chemical sprays for control. Burn affected stems. Do not try to compost or otherwise dispose, for the fungus spores of Cane blight can live for up to four years on dead tissue.
The good side of all this, is that even badly affected crops of raspberries can recover the following year.