ABSTRACT
Diseases are only the most perceptible, sometimes even spectacular, manifestations of microbial activity, however. We have discovered that an extraordinary variety of microorganisms have a beneficial action. The free or symbiotic nitrogen-fixing bacteria and the phosphorus-harvesting mycorrhizal fungi contribute directly to soil fertility. All the decomposers play a no less important role by ensuring plants a regular supply of mineral elements recycled from organic waste. Some microorganisms seem capable of stimulating plant growth and development by direct action on their
United States, 60 to 70% wheat plants may be infected by Pythium spp., resulting in a yield reduction of nearly 8% (Cook and Veseth, 1991). Nematodes such as Trichodorus can also attack root tips and arrest their growth.
