Wilt of lucerne caused by species of Verticillium

SUMMARY All isolates of Verticillium albo‐atrum from lucerne are virulently pathogenic to this host, whereas V. dahliae is only slightly pathogenic. Isolates of both fungi from other crop plants do not produce symptoms when inoculated into lucerne, suggesting a degree of specialization in the ‘lucerne strains’ of V. albo‐atrum and V. dahliae. However, both these strains can infect broad bean, strawberry, Italian clover (Hedy‐sarum coronarium) and pea, while V. albo‐atrum may also attack potatoes and runner beans. V. albo‐atrum spreads rapidly underground from diseased to healthy lucerne plants, particularly when they are closely planted. Resting mycelium remains viable for 9 months in pieces of moribund lucerne buried 12 in. in soil, for 7 months at 6 in. and for only 5 months at the surface. V. albo‐atrum spreads from buried infected lucerne debris to the adjacent soil and can be re‐isolated for up to 9 months from 12 in. and for 5 months from 6 in. levels. In pot experiments a correlation was found between the speed of symptom development and the number of conidia in contact with the root system. Isolates of the fungus from lucerne can infect a number of common weeds (Plantago lanceolata, Polygonum persicaria, Sinapis arvensis, Rumex obtusifolius, Solanum nigrum, Papaver rhoeas, Capsella bursa‐pastoris, Senecio vulgaris and Anthemis cotula) and the fungus is still pathogenic to lucerne when re‐isolated. Field observations in Norfolk show that harvesting machinery may be the carrier of both the initial infection and the later spread throughout the crop. The incidence of disease increases with the age of the stand; a significant decrease of dry‐weight yield of diseased lucerne occurs during the second harvest year: this often necessitates the ploughing‐in of badly wilted crops before they reach their third harvest.