The cause of decline in vigor without visible symptoms is very difficult to diagnose. Many possible causes exist, most of which will lead to visible symptoms over time. Viruses occur mainly in older plantings, or in plantings where clean nursery stock was not used. Special tests are required to diagnose the presence of virus, and these tests produce many false negatives.
Symptoms usually appear only after infections are severe.
Dig up plants and examine the roots for lesions and feeding damage, and conduct a nematode test . Raspberries do not exhibit distinguishing symptoms of nutrient deficiencies. The use of heavy equipment in the field can slowly compact certain soils, leading to a decline in plant vigor. Clean cultivated fields are most vulnerable.