Control of Verticillium dahliae with soil amendments: efficacy and mode of action

摘要

Verticillium wilt causes severe reductions in yield in a variety of important crops worldwide including potato (Solanum tuberosum) and tomato (Lycopersicon esculentum). In temperate regions, the potato disease "early dying syndrome" is caused by thefungal pathogen Verticillium dahliae Kleb. in association with various nematode species (61). Early dying syndrome is prevalent in the major potato growing region of Ontario. The vast majority of fields sampled (>50) in South Central Ontario (near Alliston) had more than 80 percent disease incidence (Lazarovits, unpublished results). In the U.S., early dying is rated as the most important disease of both seed and commercial potato crops and as the second most important yield constraint to potato production (56). Productivity in tomato fields may be as severely impacted as for potato. A survey of five fields near Leamington (South West Ontario) in 1993 revealed that in excess of 50 percent of the plants were infected by a new race of V. dahliae for which resistant cultivars are not available (10). V. dahliae produces microsclerotia consisting of clusters of melanized, thick-walled and hyaline, thin-walled hyphal cells. Microsclerotia are well adapted to persist in soil and can remain viable for more than a decade (78). Infection is initiated from microsclerotia which overwinter in soil or in infected plant debris from previous crops. Crop rotation has little impact on inoculum density of such persistent fungi, particularly as millions of microsclerotia are reintroduced annually with infected plant debris. A successful program to control disease caused by V. dahliae therefore must include a) reduction in microsclerotial inoculum in soil, b) use of resistant/tolerant crop cultivars, and c) control ofroot lesion nematodes. Fumigation with chemical sterilants, such as methyl bromide, Vapam, or Chloropicrin, can kill either nematodes, V. dahliae, or both, and thus reduce disease incidence (2,12). Sterilants however, also lower populations of beneficial, non-target soil microorganisms. This decline in soil microbial diversity often leads to increased populations of pathogens due to reduced competition and antagonism. Thus, the cycle of fumigation is perpetuated (52). Our laboratory has tested other soil disinfestation procedures such as solarization as an alternative to fumigation. However, in the temperate climate of Southern Ontario solarization was not effective in controlling V. dahliae (38).