Both kinds of weeds—wild plants and volunteer cultivated plants—
may show either inconspicuous symptoms of disease or none at all, and this aggravates the difficulty of getting farmers to pursue weed control with sufficient zeal. The term "symptomless carrier" is a perfectly correct description of many host plant: virus combinations, but fungal root in-fections may be equally difficult to detect by above-ground inspection in the field, and many leaf spot diseases are inconspicuous and easily escape notice. Volunteer cultivated plants are less likely to be symptom-less, or near-symptomsymptom-less, carriers than are truly wild plants because so many cultivars have been selected without much thought for disease
resistance. Races of wild plants, on the other hand, have tended to evolve through the natural selection of both host and parasite toward a toler-ance of their various parasites.
Often enough, however, it is the crop itself that functions as a population of symptomless carriers of one or more diseases. If such a crop is used for vegetative propagation, clones derived from it may eventually suffer severely in the fullness of time or under particular environmental conditions from the diseases thus carried. Hence the need for governmental inspection and registration of propagating material, which is particularly important for systemic virus diseases and vascular wilts. A crop that is carrying hidden infection may transmit disease in other ways than through living infected tissues, as, for example, through propagating sets and true seeds. Thus a wheat crop, showing no per-ceptible signs of disease above ground, may leave in the soil a sufficient number of dead roots infected by Ophiobolus graminis to cause a disastrous failure through the take-all disease in a second crop of wheat that is subsequently sown too soon. Few farmers make this mistake twice, yet such an occurrence has to be seen to be believed, and no one with first hand experience of this disease can afford to appear wiser than Kirby and Thomas (1920) when they reported, with no little urgency, the first known appearance of take-all in the United States. Nevertheless, the weight of subsequent evidence on the behavior of O. graminis and the cosmopolitan distribution of this fungus now make it seem probable that the take-all disease was endemic on native grasses in the United States long before the first crop of wheat was raised in the New World.
Many other diseases besides take-all have from time to time made sudden, spectacular, and disastrous appearances for the first time in living memory. The hypotheses evolved to explain such visitations have been almost as numerous. It may not be so exciting, but it will usually be more rewarding, to search for the inoculum close at hand—in the soil, among the wild plants of surrounding vegetation, or among the weeds growing in the very fields themselves.
ACKNOWLEDGEMENT
The author wishes to express his gratitude to Dr. G. G. Meynell, who read the entire chapter in manuscript and made valuable suggestions.
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