Virus diseases often are spread between plants by vector organisms, some of which live below ground (e.g., fungi and nematodes) and feed on the plant root system. Tobraviruses are one of only two groups of plant viruses that have nematode vectors. They are primarily viruses of weed plants but can cause significant economic damage in a range of cultivated crops including potato, peas, beans, and many ornamental species. Identifying these viruses and their nematode vectors is a very important part of the struggle to combat disease in crop plants, and requires specialized techniques that will be discussed further in this unit.