The Dissemination of Plant Pathogens

Abstract

A cloud of reddish spore dust arose from a rusted wheat field in southern Mexico and was borne northward on the prevailing winds. It settled finally on some fresh green wheat leaves in Texas. In a few days a billion rust spores were produced on the Texas wheat and carried aloft, again northward. These in turn found some wheat leaves and regenerated their kind. In this hop, skip fashion the wheat stem rust pathogen travelled up through the wheat belt of the central United States and all the way to Canada, some 2000 miles in two months. In its wake lay destruction, the wheat kernels failing to fill and the yield of grain drastically reduced. This dramatic event could be repeated every year were it not for the fact that resistance to this menace has been built into most wheat varieties by our plant scientists, and that a combination of ideal weather conditions must prevail for optimum disease development. It illustrates the ease with which microscopic organisms that cause disease in plants may be disseminated. Countless billions of fungal spores, bacterial cells, viral particles, and nematodes, many with the potential for destruction of plants, are carried literally all over the globe. And many are the ways by which they may be disseminated-by the wind, by splashing rain and running water, by insects, birds, and other animals, and even, unknowingly, by man himself in his daily transport of plant products from place to place. Wind is perhaps the most common means by which plant pathogens, particularly the spores of fungi, are carried. Years ago black clouds of smut spores arose from the combine as it harvested diseased wheat heads along with healthy ones. (Fig. 13) The most gentle of breezes can sweep the tiny spores from their mooring places. A little cloud of blue mold spores, for example, may swirl upward from a rotted orange or lemon as the housewife picks out fruit in the market place. They settle on fresh fruit which in turn may become rotted. Smut spores may shower down from a black gall on a corn plant as the home gardener picks the ripe ears. These spores may produce a new gall or fall to the soil and survive the winter to menace the next year's crop. A grower care-