Wilt and Collapse of Cuphea ignea Caused by Phytophthora tropicalis in Italy

Susceptibility of Greenhouse Ornamentals to Phytophthora capsici and P. tropicalis

The susceptibility of fabaceous (Lupinus and Lathyrus spp.) and solanaceous (Calibrachoa, Browallia, Nicotiana, Nierembergia, and Petunia spp.) ornamental plants compared with straightneck squash (Cucurbita pepo) inoculated with Phytophthora capsici and P. tropicalis was investigated in greenhouse studies. Four P. capsici isolates and one P. tropicalis isolate were evaluated. Flowering tobacco (Nicotiana × sanderae), sweet pea (Lathyrus latifolius), lupine (Lupinus polyphyllus), squash, and million bells (Calibrachoa × hybrida) were susceptible to P. capsici and P. tropicalis. Bush violet (Browallia speciosa) and cup flower (Nierembergia scoparia) were not susceptible to either pathogen. Petunia (Petunia × hybrida) was susceptible to P. capsici but not P. tropicalis. Area under the plant growth curve (AUPGC) was also affected in some susceptible plants. AUPGC was significantly different in inoculated plants compared with the untreated controls of Nicotiana and Calibrachoa. In addition, six Calibrachoa cultivars were evaluated for susceptibility to P. capsici and P. tropicalis in a separate experiment. Although all Calibrachoa cultivars were susceptible to P. capsici and P. tropicalis, 'Celebration Purple Star' displayed limited disease following inoculation with either pathogen. Calibrachoa 'Cabaret' and 'Can-Can' inoculated with P. capsici or P. tropicalis displayed significant differences in AUPGC compared with the uninoculated controls. 'Callie', 'Million Bells Cherry Pink', and 'Superbells' had significantly reduced AUPGC compared with the controls when inoculated with P. tropicalis but not P. capsici. These results expand the host range of P. capsici to include Calibrachoa spp., L. polyphyllus, and Lathyrus latifolius, and P. tropicalis to include L. latifolius, Nicotiana spp., and straightneck squash.

Interspecific hybridization and apomixis between Phytophthora capsici and Phytophthora tropicalis

Phytophthora capsici and the closely related Phytophthora tropicalis infect different hosts that have documented overlapping geographical distributions. Phytophthora capsici attacks annual vegetable hosts whereas P. tropicalis has been recovered from woody perennial hosts. Our objective was to test whether interspecific hybridization is possible and to characterize the resulting progeny. Crosses were made between P. capsici (LT263) from pumpkin to P. tropicalis from rhododendron (LT232) and to P. tropicalis from Theobroma cacao (LT12). The wild type isolates were analyzed for mitochondrial and nuclear DNA sequence diversity and progeny were tested for mating type (MT), AFLP marker profiles and mitochondrial DNA haplotype (mtDNA type). All oospore progeny from LT263 x LT12 were identical to LT263 whereas progeny from LT263 x LT232 were parental as well as hybrid. Hybrid progeny had either one or the other parent mtDNA type and there was no correlation between MT and mtDNA type. Attempts to generate an F2 population from the hybrids proved unsuccessful while a backcross to the P. capsici parent produced hybrid progeny. These results demonstrate that apomixis might play a significant role in species separation and that hybridization between P. capsici and P. tropicalis is possible beyond the F1 generation.