Corroboration that highly resistant impatiens cultivars are not immune to downy mildew disease: a report of crop losses from two California producers

Authors: DAUGHTREY, MARGERY - Cornell University; BECKERMAN, JANNA - Purdue University; DAVIS, WILLIAM - Orise Fellow; RANE, KAREN - University Of Maryland; Crouch, Jo Anne

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2020
Publication Date: 8/3/2020
Citation: Daughtrey, M.L., Beckerman, J., Davis, W.J., Rane, K., Crouch, J.A. 2020. Corroboration that highly resistant impatiens cultivars are not immune to downy mildew disease: a report of crop losses from two California producers. Plant Health Progress. 21(1):214-216. https://doi.org/10.1094/PHP-05-20-0040-SC.
DOI: https://doi.org/10.1094/PHP-05-20-0040-SC

Interpretive Summary: Impatiens are a popular annual bedding plant, prized for their brilliant colors and ease of care in the landscape. After severe outbreaks of downy mildew disease in 2011, these plants disappeared from grower shelves and garden beds worldwide. In 2019, new impatiens cultivars that resist infection by the downy mildew were released. However these new cultivars are not entirely immune to the disease. This research describes two destructive outbreaks of downy mildew affecting the new impatiens cultivars at two commercial production facilities in California. Both grower facilities used mefenoxam fungicides during production. Given these findings, fungicide programs used to supplement cultivar resistance should utilize rotations of effective fungicides from different mode of action groups. This information will be useful to growers working to minimize the impact of impatiens downy mildew.

Technical Abstract: Two new series of Impatiens walleriana (impatiens) cultivars, Beacon and Imara XDR, were released to commercial growers in the US in 2019-20. Field trials show these new cultivar series are highly resistant to impatiens downy mildew (IDM). However, neither of the two impatiens series is completely immune to the disease, and preventive fungicide programs are still recommended for use during propagation to maintain plant health. Here we report two destructive outbreaks of IDM from Imara XDR in two commercial production facilities in California, one in 2019 and one in 2020. The disease outbreaks were caused by a known rDNA genotype of Plasmopara destructor (synonym=P. obducens). Modified Koch’s postulates showed that the pathogen could infect and cause disease in both Beacon and Imara XDR plants. Mefenoxam applied by both growers may have been ineffective due to resistance in P. destructor populations, which has been demonstrated on several previous occasions. Given these findings, fungicide programs intended to supplement genetic resistance should not be overly reliant upon application of mefenoxam and should utilize effective materials from different mode of action groups, in rotation. Fungicides to supplement genetic resistance are particularly appropriate in frost-free areas or in any circumstances that provide a potential inoculum source.