<P> Nematodes are small, multicellular wormlike animals . Many live freely in the soil, but there are some species that parasitize plant roots . They are a problem in tropical and subtropical regions of the world, where they may infect crops . Potato cyst nematodes (Globodera pallida and G. rostochiensis) are widely distributed in Europe and North and South America and cause $300 million worth of damage in Europe every year . Root knot nematodes have quite a large host range, they parasitize plant root systems and thus directly affect the uptake of water and nutrients needed for normal plant growth and reproduction, whereas cyst nematodes tend to be able to infect only a few species . Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle . </P> <P> There are a few examples of plant diseases caused by protozoa (e.g., Phytomonas, a kinetoplastid). They are transmitted as durable zoospores that may be able to survive in a resting state in the soil for many years . Further, they can transmit plant viruses . When the motile zoospores come into contact with a root hair they produce a plasmodium which invades the roots . </P> <P> Some colourless parasitic algae (e.g., Cephaleuros) also cause plant diseases . </P> <P> Parasitic plants such as mistletoe and dodder are included in the study of phytopathology . Dodder, for example, is used as a conduit either for the transmission of viruses or virus - like agents from a host plant to a plant that is not typically a host or for an agent that is not graft - transmissible . </P>

History of plant pathology with special reference to indian work