|Salgado-salazar, Catalina - Orise Fellow|
|Leblanc, Nicholas - Orise Fellow|
|Ismaiel, Ed - Ed|
|Rivera, Yazmin - Animal And Plant Health Inspection Service (APHIS)|
|Warfield, Colleen - Ball Horticultural Company|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2018
Publication Date: 5/8/2018
Citation: Salgado-Salazar, C., LeBlanc, N., Ismaiel, A.A., Rivera, Y., Warfield, C., Crouch, J.A. 2018. Genetic variation of the pathogen causing impatiens downy mildew pre-dating and including 21st century epidemics on Impatiens walleriana. Plant Disease. 102(12):2411-2420. http://doi.org/10.1094/PDIS-01-18-0077-RE.
DOI: https://doi.org/10.1094/PDIS-01-18-0077-RE Interpretive Summary: Downy mildew is a disease that causes impatiens to loose their leaves and die, due to an infection by a type of mold. The disease first showed up in the U.S. in 2004 and has been a problem ever since. This research describes the first use of DNA analyses to learn why this disease so rapidly became such a major problem for impatiens. The study shows that most of the impatiens molds living in the U.S. are members of three different DNA groups, but two of these groups were unknown before the 2004 epidemics. No distinction was made between the molds infecting impatiens and jewel weed plants. This research is significant because it shows closely related plants such as jewel weeds can be a source of infectious downy mildew mold that can infect impatiens, and that many DNA types of the mold can cause the disease. This research will be useful to growers, breeders, plant pathologists, extension personnel and quarantine officials who work to minimize the transmission and impact of downy mildew diseases in the United States.
Technical Abstract: Impatiens downy mildew (IDM) disease on cultivated Impatiens walleriana has had a significant economic impact on the ornamental horticulture industry in the U.S. and globally. Although recent IDM outbreaks started in 2004, downy mildews on non-cultivated Impatiens species have been documented since the 1880s. To understand the relationship between the pathogen causing recent epidemics and the pathogen historically present in the U.S., this work characterized genetic variation among a collection of 1000 samples on 18 plant hosts. Samples included collections during recent IDM epidemics and historical herbarium specimens. Ten major genotypes were identified from cloned rDNA amplicon sequencing and endpoint SNP genotyping. Three genotypes accounted for >95% of the samples, with only one of these three genotypes found on samples predating recent IDM outbreaks. Based on phylogenetic analysis integrating data from three markers and the presence of individual genotypes on multiple Impatiens species, there was some evidence of pathogen-specific infection of I. noli-tangere, but the distinction between genotypes infecting I. walleriana and I. balsamina was not upheld. Overall, this work provides evidence that the majority of genotypes recovered from recent IDM epidemics are different from historical U.S. genotypes, and that these genotypes can infect Impatiens spp. other than I. walleriana.