I believe I have spread the infestation with shovels and by other means.
Nematodes are microscopic roundworms that live in the soil or as parasites in plant tissue.
Nematodes don't do much traveling on their own but are transported when water or digging moves the soil. Harmful nematodes, such as root-knot nematodes, cause plants to decline, become stunted and wilted, even though the soil is moist. Infected roots swell where infected and form knots or galls. Sometimes numerous infections occur, and this can cause large fleshy galls. Galls vary in appearance depending on the plant infected and the nematode species.