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Systemic spread of Plasmopara obducens in Impatiens plants with roots exposed to sporangia or oospores.

Nina Shishkoff: USDA ARS FDWSRU

<div>Since 2011, Impatiens has been threatened by a downy mildew disease caused by <em>P. obducens</em> that defoliates and kills the plant. It is not yet clear whether the pathogen spreads primarily through airborne inoculum or whether it can also overwinter in soil. We studied the soil-borne spread of the disease in <em>Impatiens balsamina</em>, a seed-planted annual, and<em> I. walleriana</em>, an annual bedding plant. In two experiments, immersing the root system of six 2-3 week old seedling of <em>I. balsamina</em> in a suspension of sporangia (2 x 10<sup>3</sup> sporangia/ml) for 48 hr and then planting led to a systemic infection of some plants (2-3 plants out of six). In three experiments with <em>I. walleriana</em>, adding a sporangial suspension to potting media of eight plants at the 2-leaf stage led to infection of some plants (6-8 plants out of 8). Oospores of <em>P. obducens </em>that had been cold-conditioned by exposure to 0 C for one month were added to potting media, and then germinated seed were placed on top, in three experiments for each plant species. Both <em>I. balsamina</em> (in 19-44 plants out of 50) and <em>I. walleriana</em> (8-18 plants out of 38) became systemically infected. Learning to effectively manage this disease is crucial to the health of the bedding plant industry. In particular, a better understanding of the life cycle of the pathogen is necessary. Seed-borne oospores have been reported for <em>I. balsamina</em> in India; this species might become systemically infected from seed-borne inoculum, by oospores overwintering in infested litter or by roots coming into contact with vegetative sporangia washed into the soil by rain.</div>