Characterization of Verticillium dahliae and V. tricorpus isolates from lettuce and artichoke

Verticillium isolates collected from lettuce and artichoke were characterized for morphology, growth and pathogenicity. Several isolates were identified as Verticillium tricorpus based on morphological and cultural characteristics, including the production of dark resting mycelia, chlamydospores, microsclerotia, and yellow to orange pigmentation in culture. Compared with isolates of V dahliae, these isolates also produced microsclerotia and conidia that were significantly larger and exhibited a distinct growth pattern at varying temperatures. Using database sequence information, primers were developed from the internal transcribed spacer region to produce a diagnostic 337-bp product specific to V tricorpus and used to confirm the identification of isolates. Pathogenicity tests indicated that isolates of V tricorpus were weak pathogens, causing a median disease severity (DS) of < 1 (0-to-5 scale) on lettuce and artichoke. In contrast, isolates of V dahliae consistently caused severe wilt with a median DS of > 3.5 on lettuce and 5.0 on artichoke. Although lettuce and artichoke inoculated with isolates of V tricorpus exhibited reduced height and fresh foliar and root weight, the reductions were not statistically significant, unlike in plants inoculated with isolates of V dahliae. Lettuce co-inoculated with isolates of V tricorpus and V dahliae exhibited reduced symptoms of Verticillium wilt and improved growth relative to those inoculated with V dahliae alone. The early introduction of V tricorpus in soil-drench inoculations appeared to provide better relief from subsequent V dahliae inoculation than when the two species were co-inoculated simultaneously using the root-dip method, suggesting competitive exclusion as a plausible mechanism. A spore-polymerase chain reaction assay developed using cultured spores directly as template and primers specific to V tricorpus confirmed the presence of V tricorpus on inoculated roots. This work demonstrates the potential use of V tricorpus to directly reduce the effect of V dahliae on lettuce and artichoke and, to our knowledge, is the first reported characterization of V tricorpus isolates collected from lettuce and artichoke.