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

In October 2006, diseased impatiens (Impatiens walleriana) cv. ‘Fiesta’ were received from a commercial nursery near Melbourne, Victoria, in south eastern Australia. The undersurface of the leaves showed white downy growth typical of a downy mildew fungus. When viewed from above, infected leaves were mottled and yellowing; affected plants were wilting. Seedlings were 8 to 11 weeks old, and approximately 90% of the crop was affected. The disease was first noticed in that nursery in mid-September 2006.

Microscopic examination of the fungus revealed that it was Plasmopara obducens. The only other downy mildew reported from Impatiens is Bremiella sphaerosperma, which is known only from North America and eastern Asia (Constantinescu, 1991). Plasmopara obducens differs from B. sphaerosperma by its mainly monopodial sporangiophores with straight branches (Fig. 1 & 2), lack of apical thickening on the branchlets, ovoid sporangia and associated leaf lesions lacking well defined margins (Constantinescu, 1991). Voucher specimens have been deposited in the Victorian Plant Pathology Herbarium (VPRI). A partial nuclear large ribosomal subunit DNA sequence was obtained and found to be identical to the P. obducens sequence reported by Lane et al. (2003). Our sequence has been deposited in GenBank (Acc. No. EF095193).

On further investigation at the same nursery, the disease was found on other single and double flowered impatiens plants, in addition to ‘Fiesta’. Most affected plants were grown from seed, but some were from cuttings. No evidence of the disease was found in vegetatively propagated New Guinea hybrids (Impatiens x hawkeri). Surveys of other major impatiens nurseries in Victoria revealed no evidence of the disease.

This is the first report of Plasmopara obducens in Australia. It has been recorded previously in Europe, Asia and North America, infecting a number of wild and cultivated Impatiens species, including I. walleriana and I. balsamina (Jones & O’Neill 2004). Recent reports from the UK (Lane et al., 2003) and California (Welugo et al. 2004), indicate significant outbreaks on commercial impatiens (I. walleriana types). In Australia, further additional surveys found P. obducens in the states of Victoria, South Australia, New South Wales and Queensland.