WILT OF LUCERNE CAUSED BY SPECIES OF VERTICILLIUM

A wilt disease of lucerne caused by species of Verticillium is described: at twenty-eight disease areas in England and Wales the pathogen was V. albo-atrum, while at one site it was V. dahliae. Both pathogens form superficial conidia on the basal regions of infected stems. It is shown that V. albo-atrum is introduced into a new area in contaminated plant material harvested with the seed from an infected crop. Rapid secondary spread of the disease follows the dissemination of spores from infected stems, and by contact of these and transported fragments of diseased tissues with the wounded surfaces of recently cut lucerne plants. It is recommended that seed should be collected only from healthy crops and that machinery, footwear, etc. should be disinfected before leaving a site of infection. Manurial trials showed that the incidence of wilt induced by V. albo-atrum was very severe under all soil conditions tested, whereas V. dahliae is a virulent pathogen only to plants in soil rich in superphosphate. Generally the more vigorous the growth of the lucerne-in soils rich in potash and hoof and horn-the more rapid is both the onset of wilt and the resulting secondary spread of the disease throughout the crop.