Investigating the Evolutionary Relationship of Pseudoperonospora cubensis and P. humuli Through Phylogenetic and Host Range Analyses
Mancino, Leah E.
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Mancino, Leah E.
Downy mildew on hop, caused by Pseudoperonospora humuli, and downy mildew on cucurbits, caused by P. cubensis, are two distinct pathogens. Growers protect these crops against the mildew by planting resistant cultivars and supplementing with fungicides and cultural techniques. The disease on cucurbits has been managed by planting moderately resistant cultivars and didn’t require much fungicide applications until a recent resurgence of P. cubensis in 1984 in Europe and 2004 in the U.S. This reemergence of cucurbit downy mildew caused an estimated loss of ~$20 million and 40% loss in cucumber yields for growers in the Eastern United States alone. Recent research has indicated that there may have been a genetic bifurcation among cucurbit downy mildew species responsible for the recent epidemics. It has been suggested that cucurbit downy mildew has arisen from hop downy mildew via a host jump from Japanese hop, Humulus japonicus, due to P. humuli on H. japonicus from Korea being placed basal to the P. humuli – P. cubensis phylogenetic complex. Phylogenetic analyses were conducted utilizing 59 P. cubensis isolates from 6 countries and 31 P. humuli isolates from 7 countries by Bayesian Inference, Maximum Likelihood and Minimum Evolution methods based on two nuclear (internal transcribed spacer and ypt1) and one mitochondrial (cytochrome c oxidase II) loci. The resulting phylogenies support the conjecture that the downy mildew species responsible for the recent epidemic on cucurbits form phylogenetic clusters that are separated from the pre-epidemic cucurbit pathogen. However, the ancestry of the P. cubensis – P. humuli complex cannot be resolved from the phylogenetic analyses employed in this study. Host range experiments on H. japonicus were conducted using 6 P. humuli isolates from the U.S., 2 P. humuli isolates from Japan and 1 P. cubensis isolate from the U.S.. Only a single sporangiophore could be observed in the course of the study and therefore indicates that a host barrier between the downy mildew pathogens found on H. japonicus, H. lupulus, and cucurbit species. This finding also suggests that H. japonicus may be a source of host resistance genes for introgression into common hop.