An aggressive vascular-inhabiting Phoma (Phoma tracheiphila f. sp. chrysanthemi nov. f. sp.) weakly pathogenic to chrysanthemum

A form of Phoma tracheiphila (Petri) Kant. & Gik., newly designated as f. sp. chrysanthemi Baker et al., massively invades the phloem and xylem and to a lesser extent the cortex and pith of chrysanthemum plants but causes only slight injury in the first season. However, infected plants either produce weak shoots the following year or commonly fail to resume growth. Injury appears to result from depletion of photosynthates and nutrients rather than from vascular plugging or toxins. Infection occurs through intact roots or through wounds of roots or stems, and the pathogen spreads to the top of 120-cm stems in 3 months. Infection occurs readily from 10–29.4 but most abundantly at 10–21° C. Mycelial development in the stems is retarded at 10 and is optimal at 21° C. This Phoma decline disease was prevalent in commercial and home chrysanthemum plantings in California in 1948–1956, but it has since been controlled by the annual planting of healthy cuttings in fumigated soil, as practiced for control of verticillium wilt. In home gardens the disease may cause severe losses if plants are grown as perennials, but if healthy cuttings are planted annually, the disease will be minimal even in plants grown in infested soil. The pathogen is indistinguishable morphologically from Phoma tracheiphila f. sp. tracheiphila Baker et al., cause of "mal secco" of citrus, but will not infect sour orange or rough lemon plants.