Role of Weeds in the Incidence of Virus Diseases

Abstract

From the standpoint o f control o f virus diseases, there is perhaps no phase of virology more important than epidemiology. The role of weeds in the occurrence and spread o f plant virus diseases is an integral part o f the ecological aspect o f virus transmission. L. H. Bailey (3) has commented that nature knows no plants as weed s. Indeed, at the dawn of agriculture some 8,000 years ago, man would have recognized all plants as natural components o f the flora. Man altered the earth's floral cover in his first efforts in agriculture, and weeds were born. King (61) has pointed out that perhaps the shortest definition of weed is that attributed to Professor W. J. Beal, who defined weed as a plant out of place. King has summarized some thirty definitions into ten commonly accepted characteristics of weed: growth in an undesired location; com­ petitive and aggressive habits; wild and rank growth; resistant to control; large p opulations, with abundant, rank and extensive growth; useless, un­ wanted, and undesirable; harmful to man, animals, and crops; spontaneous growth, appearance without being sown or cultivated; of high reproductive capacity, and unsightly with disfigurement of the landscape. Thus, weeds are plants, with harmful or objectionable characteristics, that grow where they are unwanted. Whether plant is considered weed depends not only on its characteristics but also on its relative position with reference to other plants and to man. The same plant may be weed in one environment and an economically important plant in another. Thus, in at least one incident, sugar beets were considered to be noxious weeds during certain times of the year in legislation proposed to enforce beet-free period. Numerous reports (5, 14, 32, 118) have indicated close correlation be­ tween incidence o f beet yellows virus and overwintering beets. In Europe, the sources of beet yellows virus include beet seed crops and infected man­ golds in clamps; in the United States, they include escaped beets growing in waste places and beets in overwintering fields. The beet mosaic virus (32, 118) has been shown to be even more closely associated with overwintering