The agents that cause infectious disease in plants are pathogenic microorganisms, such as viruses, bacteria, fungi, protozoa, and nematodes. Several hundred species of nematodes, however, are known to feed on living plants, obtaining their food with spears or stylets and causing a variety of plant diseases worldwide. Almost all plant pathogenic nematodes live part of their lives in the soil. Many live freely in the soil, feeding superficially on roots and underground stems, and in all, even in the specialized sedentary parasites, the eggs, the preparasitic juvenile stages, and the males are found in the soil for all or part of their lives. Nematodes occur in greatest abundance in the top 15 to 30 centimeters of soil. A few nematodes that attack the aboveground parts of plants not only spread through the soil as described earlier, but they are also splashed to the plants by falling rain or overhead watering. All plant parasitic nematodes belong to the phylum Nematoda. In this chapter, considering the importance of plant diseases caused by nematodes, we review some of the most important diseases of some of the medicinal plants.
