The pathogenicity and phytogeny of Fusarium oxysporum isolates on Coreopsis verticillata 'Moonbeam'

In the summer of 2006, Fusarium wilt of Coreopsis verticillata cv. Moonbeam was first reported and the causal agent was identified as Fusarium oxysporum (Fo). Given the recent appearance of this serious disease and the potential for widespread distribution via the horticultural trade, we investigated the host range of the pathogen. Ground millet colonized by Fo from coreopsis was used to infest potting mix (2.5 g millet/L potting mix). No wilt symptoms or reductions in dry weight were observed in 3 cultivars of aster, 8 cultivars of chrysanthemum, and one cultivar of echinacea when compared to controls. Similarly, no symptoms were observed on ageratum, basil, eggplant, gomphrena, scabiosa, and snapdragon. In contrast, symptoms of stunting, wilt, and vascular discoloration were observed on C. verticillata and C. lanceolata. These data suggest that a forma specialis designation may be applicable to this pathogen. To assess the level of genetic diversity in the pathogen, the tef-alpha gene genealogy from 17 Fo isolates was developed, including isolates from other formae speciales of Fo. All of the coreopsis isolates were closely related and placed in a single group within Clade 2 of the Fo species complex, indicating that host specialization might have occurred from a single ancestor. We propose the pathogen be called Fo f. sp. coreopsii.