First Report of Downy Mildew Caused by Plasmopara obducens on Impatiens in California

Abstract

During February 2004, diseased double impatiens (Impatiens walleriana) plants were received from a commercial grower in southern California. The upper surfaces of symptomatic leaves were pale yellow with no distinct lesions. Diseased leaves later wilted, and severely affected leaves abscised from the stem. At the nursery, only double impatiens plants in the Fiesta series were infected, and some cultivars were more heavily infected than others. Disease incidence in cv. Sparkler Hot pink was nearly 100%. The interior of infected leaves was colonized by coenocytic mycelium. A conspicuous white growth was observed only on the underside of leaves. Sporangiophores were hyaline, thin walled, emergent from stomata, and had slightly swollen bases. Sporangiophore branching was distinctly monopodial. Smaller sporangiophore branches were arranged at right angles to the supporting branches, and tips of branches measured 8 to 14 μm long. Sporangia were ovoid and hyaline with a single pore on the distal ends. Distal ends of sporangia were predominantly flat but occasionally had a slight papilla. Short pedicels were present on the attached ends. Sporangia measured 19.4 to 22.2 (-25.0) μm × 13.9 to 16.7 (-19.4) μm. Oospores were not observed in leaf tissue. On the basis of symptoms and morphology of the organism, the pathogen was identified as Plasmopara obducens J. Schröt. Pathogenicity tests were done on double type cvs. Fiesta, Tioga Red, and Tioga Cherry Red and on single type cvs. Cajun Watermelon and Accent Lilac. Plants were spray inoculated with sporangiospore suspensions (1 × 104 sporangiospores per milliliter), incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C). Symptoms and signs of downy mildew developed after 12 days only on inoculated cv. Fiesta plants, and the pathogen morphology matched that of the originally observed pathogen. Nontreated control plants did not develop downy mildew. To our knowledge, this is the first report of downy mildew on impatiens in California. P. obducens is one of two causal agents of downy mildew of impatiens (2,4). The other pathogen, Bremiella sphaerosperma, has dichotomous sporangiophore branching and causes lesions with well-defined margins (2,4). In the United States, the disease has been recorded in the eastern and northeastern states and in Indiana, Minnesota, Mississippi, Montana, and Wisconsin (3). In Canada, the disease has been recorded in Manitoba and Quebec (1). References: (1) I. L. Conners. An Annotated Index of Plant Diseases in Canada and Fungi Recorded on Plants in Alaska, Canada, and Greenland. Research Branch, Canada Department of Agriculture, Publication 1251, 1967. (2) O. Constantinescu. Mycologia 83:473, 1991. (3) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, 1989. (4) G. W. Wilson. Bull. Torrey Bot. Club 34:387, 1907.

Comparative Analysis of Impatiens Leaf Transcriptomes Reveal Candidate Genes for Resistance to Downy Mildew Caused by Plasmopara obducens

“…Since then, IDM has been problematic in these countries. In the USA, IDM appeared on I. walleriana plants in California in 2004 [ 13 ]. In 2011, IDM was reported in eleven states in the USA including Massachusetts, New York and Minnesota [ 14 ].…”

“…In 2011, IDM was reported in eleven states in the USA including Massachusetts, New York and Minnesota [ 14 ]. By 2012, this disease spread to 33 states [ 13 ]. In 2013, IDM was reported in Hawaii [ 15 ].…”

First record of Plasmopara obducens (downy mildew) on impatiens in Australia

“…It has been recorded previously in Europe, Asia and North America, infecting a number of wild and cultivated Impatiens spp., including I. walleriana and I. balsamina (Jones & O’Neill 2004). Recent reports from the UK (Lane et al ., 2005) and California (Welugo et al. , 2004), indicate significant outbreaks on commercial impatiens ( I .…”

The Effect of Daily Light Integral, Ethephon, and Node Position of Impatiens ×hybrida Stock Plants on Flowering of the Harvested Unrooted Cuttings

“…2019). SunPatiens have grown in popularity due to their exceptional landscape performance and their resistance to impatiens downy mildew (Plasmopara obducens), which causes rapid defoliation of bedding plant impatiens in the landscape (Wegulo et al, 2004).…”