The morphology of Verticillium albo‐atrum, V. dahliae , and V. tricorpus

Important taxonomic differences between Verticillium albo‐atrum, V. dahliae , and V. tricorpus occur in the morphology of the conidiophores and the conidia. These differences are additional to the morphological differences in the resting mycelium, and justify the recognition of different pathogenic species of Verticillium , causing wilt diseases in a wide range of hosts. Verticillium albo‐atrum on natural host material typically has larger conidiophores with a dark swollen base; larger conidia. mainly 3.5–8 × 2–3µ. occasionally one septate; dark thickened resting mycelium and dark swollen hyphae; hyphal “knots” but no microsclerotia; a lower temperature range for growth on media and pathogenicity; a more restricted host range; a greater degree of pathogenicity than V. dahliae at 20°c towards many hosts (including tomato, cucumber, and strawberry). Verticillium dahliae on natural host material typically has fewer. smaller. completely hyaline conidiophores. smaller conidia. mainly 3–6 × 1.5–2µ; abundant black microsclerotia; a higher temperature range for growth on media and pathogenicity; a very extensive host range; a lower pathogenicity than V. albo‐atrum on many hosts. Verticillium tricorpus on inoculated tomato stems typically produces hyaline conidiophores with a swollen light tan base; large conidia, mainly 6–15 × 2.5–4µ; dark thickened hyphae; dark torulose hyphae; black microsclerotia. and has a high temperature preference. and is usually a weak pathogen. As in other countries, there are host and geographic distribution differences for the two Verticillium species found in New Zealand. Verticillium albo‐atrum has been found causing economic losses only on potatoes, tomatoes. lucerne. and strawberries in the southern (colder) districts of New Zealand. Verticillium dahliae . however, causes economic losses on tomatoes, tobacco, eggplant. strawberries. gooseberries. apricots, and roses. particularly in the northern (warmer) districts and in hot summers. V. tricorpus has not yet been isolated in New Zealand. but was described from a culture first received in 1949 from Dr S. Wilhelm. University of California. Cultures typical of these three species were examined by Dr I. Isaac. He stated that they corresponded with his interpretation of these three species.