Pathogen Biology

Pathogen morphology

Verticillium belongs to the fungal class Deuteromycetes (Fungi Imperfecti), a group of fungi which do not have a known sexual stage. The vegetative mycelium is hyaline, septate, and multinucleate. The nuclei are haploid. Conidia are ovoid or ellipsoid and usually single-celled. They are borne on phialides, which are specialized hyphae produced in a whorl around each conidiophore. Each phialide carries a mass of conidia (Figure 10 ). Verticillium is named for this "verticillate" (=whorled) arrangement of the phialides on the conidiophore.

Figure 10

As the diseased plant senesces, the fungus becomes saprophytic and colonizes the dying tissues. During colonization, the fungus forms microsclerotia, which are masses of melanized hyphae.

Host specificity

Within the species Verticillium dahliae strains with varying morphology and pathogenicity have been identified. Strains have been classified into several vegetative compatibility groups (VCG). Each VCG includes those isolates which can anastamose (Figure 11) and form stable heterokaryons Anastomosis is the fusion of two adjacent fungal hyphae, resulting in the exchange of cellular contents. Heterokaryons, products of anastomosis, are mycelia containing the nuclei from each of two genetically different parent mycelia. Since Verticillium does not exchange genetic information through sexual reproduction, anastomosis is the only means by which genetic diversity increases within the species.

Figure 11

Each VCG is somewhat specific to a host plant or related group of hosts and may or may not be as virulent on other hosts (Table 1). For example, VCG 2 is very virulent on peppermint and less virulent on watermelon. VCG 4 is virulent on potato and related Solanaceous species, but less virulent on maple and peppermint.

Table 1. Vegetative compatibility groups of Verticillium dahliae associated with maple, peppermint, and potato

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Copyright © 2000 by The American Phytopathological Society