First report of impatiens downy mildew outbreaks caused by Plasmopara obducens throughout the Hawai'ian islands

Authors: Crouch, Jo Anne; KO, MANN - Hawaii Department Of Agriculture; McKemy, John

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2014
Publication Date: 5/31/2014
Citation: Crouch, J., Ko, M., Mckemy, J.M. 2014. First report of impatiens downy mildew outbreaks caused by Plasmopara obducens throughout the Hawai'ian islands. Plant Disease. 98(5):696.

Interpretive Summary: Downy mildew diseases are caused by fungal-like parasites that cause significant damage to crop plants in the United States. 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 destroyed impatiens during 2011-2012, impatiens have disappeared from grower shelves and garden beds in the continental United States. This research describes multiple outbreaks of downy mildew disease throughout the Hawai’ian Islands during 2013, the first time this disease was observed from this location. Knowledge of this disease will be useful to plant regulatory officials working to control the spread of impatiens downy mildew in the United States.

Technical Abstract: Downy mildew of impatiens (Impatiens walleriana Hook.f.) was first reported from the continental U.S. in 2004. In 2011-2012, severe and widespread outbreaks across were documented across the U.S. mainland, resulting in considerable economic losses. On 05-May-2013, downy mildew disease symptoms were observed from I. walleriana ‘Super Elfin’ at a retail nursery in Mililani, on the Hawai'ian island of Oahu. Throughout May and June 2013, additional sightings of the disease were documented from the islands of Oahu, Kauai, Maui and Hawai'i from nurseries, home gardens, and botanical park and landscape plantings. Symptoms of infected plants initially showed downward leaf curl, followed by a stippled chlorotic appearance on the adaxial leaf surfaces. Abaxial leaf surfaces were covered with a layer of white mycelia. Affected plants exhibited defoliation, flower drop and stem rot as the disease progressed. Based on morphological and molecular data, the organism was identified as Plasmopara obducens (J. Schröt.) J. Schröt. Microscopic observation disclosed coenocytic mycelium and hyaline, thin-walled, tree-like (monopodial branches), straight, 94.0 –300.0 x 3.2-10.8 'm sporangiophores. Ovoid, hyaline sporangia measuring 12.0 – 14.6 x 12.2 – 16.2 (average 13.2 x 14.7) 'm were borne on sterigma tips of rigid branchlets (8.0 – 15.0 'm) at right angle to the main axis of the sporangiophores (1,2). Molecular identification of the pathogen was conducted by removing hyphae from the surface of three heavily infected leaves using sterile tweezers, then extracting DNA using the QIAGEN Plant DNA kit (QIAGEN, Gaithersburg, MD). The nuclear rDNA internal transcribed spacer was sequenced from each of the three samples bidirectionally from Illustra EXOStar (GE Healthcare, Piscataway, NJ) purified amplicon generated from primers ITS1-O and LR-0R (3). Resultant sequences (GenBank accessions KF366378-80) shared 99-100% nucleotide identity with P. obducens accession DQ665666 (3). A voucher specimen (BPI892676) was deposited in the U.S. National Fungus Collections, Beltsville, MD. Pathogenicity tests were performed by spraying 6-week old impatiens plants (I. walleriana var. Super Elfin) grown singly in 4-in pots with a suspension of 1 x 104 P. obducens sporangia/ml until runoff using a hand-held atomizer. Control plants were sprayed with distilled water. The plants were kept in high humidity by covering with black plastic bags for 48 hours at 20 oC, and then maintained in the greenhouse (night/day temperature of 20/24 oC). The first symptoms (downward curling and chlorotic stippling of leaves) and sporulation of the pathogen on under-leaf surfaces of the inoculated plants appeared at 10 days and 21days after inoculation, respectively. Control plants remained healthy. Morphological features and measurements matched those of the original inoculum, thus fulfilling Koch’s postulates. To our knowledge, this is the first report of downy mildew on I. walleriana in Hawai'i (4). The disease appears to be widespread throughout the islands and is likely to cause considerable losses in Hawai'ian landscapes and production settings.