| Summary: | Daphne is a widely dispersed genus with large variation in morphology, native habitats, and use. Unfortunately, broader acceptance of Daphne in the ornamental trade has been limited due to Daphne Sudden Death Syndrome (DSDS), a disease that kills the plant without warning. The results of this research identified Thielaviopsis basicola (Berk. etBr.) Ferr. as the causal agent for this disease. Pure cultures of the pathogen were developed and used in a germ plasm screen.
To evaluate Daphne germ plasm in vitro, species-specific protocols were developed that alleviated two common problems in Daphne micropropagation, browning and hyperhydricity. Optimizing the concentrations of both PGRs and charcoal was able to control these problems. Selected species were evaluated for resistance against Thielavipsis basicola in both, in vivo and in vitro, conditions. The results of both methods displayed a strong correlation and indicated significant differences among the taxa. However, there were differences in disease progression rates. Typically, the in vitro challenge produced a comparable level of disease as the in vivo challenge but in two to three weeks less time. Across both screening methods, the most resistant species evaluated were D. tangutica and D. retusa, while D. cneroum was the most susceptible.
Based on ITS sequences, phylogenetic relationships among selected Daphne species were established and associated with their resistance against T basicola. The phylogeny indicated that Daphne is possibly a monophyletic group. However, placement of D.genkwa remained problematic. The analysis of ITS sequences data resulted in a parsimony consensus tree with two well-supported major clades and one Glade with less support. In general, the evolutionary tree for resistance, inferred from the phylogenetic data and the results of the screening project, indicate that resistance is a derived character and that plants recently evolved this ability.
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