Nematodes

Lesion and dagger nematodes on bramble canes and roots

Nematodes are elongate, tubular organisms that looks like very small snakes or worms. They live in the soil and water. They are one of the most successful and adaptable of animal groups, being rivalled only by insects as regards range of habitats or number of species.
Several nematode species are parasitic to blackberry plants. Soil populations of these parasites may increase considerably as plants age, resulting in dwarfed fruiting canes and smaller fruit. Damage generally is most severe in light, sandy soils. During hot summer months or under severe moisture stress, foliage of heavily infested plants may turn yellow and drop early.
The lesion nematode (Pratylenchus species, most importantly P. penetrans) feeds on roots and contributes to plant decline. Lesion nematodes enter and destroy the cortex of feeder roots, creating the lesions that they are named for. These wounds interfere with proper root function and can serve as an ingress for soil-borne fungal pathogens. Above ground, raspberry plants may appear stunted, wilted, and yellowed. Plants often have reduced vigor, produce fewer canes, and are less resilient when stressed by factors such as winter injury, drought, and insect feeding. Decline caused by nematode infestation may occur gradually over a number of years.
Dagger nematodes (Xiphinema spp.) are most important in raspberries as vectors of viruses. Some species of dagger nematode also cause direct damage to raspberry by feeding on root tips. 
A practical approach to nematode control in commercial fields and in home gardens consists of planting clean root cuttings in soil where only grasses or small grains have been allowed to grow for 3 to 4 years. In the absence of suitably clean land, soil may be treated with an approved pre-plant soil fumigant to reduce nematode populations to acceptable levels.