The oospore stage of Plasmopara obducens

Authors: Shishkoff, Nina

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2019
Publication Date: 5/28/2019
Citation: Shishkoff, N. 2019. The oospore stage of Plasmopara obducens. Mycologia. 111:632-646. https://doi.org/10.1080/00275514.2019.1601986.
DOI: https://doi.org/10.1080/00275514.2019.1601986

Interpretive Summary: Impatiens downy mildew is caused by Plasmopara obducens, a pathogen known from the US for over a hundred years but newly attacking Impatiens walleriana, a bedding crop valued at more than $115 million in 2014. Downy mildew outbreaks on I. walleriana in the US were observed in greenhouse-grown Impatiens from 2004 to 2011; the first major landscape outbreaks were observed during the winter of 2011/12, in southern Florida. By 2015, IDM was reported from 34 states. The life cycle of the pathogen is not well understood, especially in regard to how the organism survives from year to year. In an attempt to better understand if an overwintering stage existed, experiments were done to understand whether the organism was self-fertile and whether thick-walled, potentially overwintering spores could be induced to germinate under laboratory conditions. The organism appeared to be self-fertile, readily producing thick-walled spores. Cold treatment at 0 C for at least a month induced spores to germinate. Inoculation of plant tissue with germinating spores led to infection, suggesting that the spores have a role in the life cycle of the pathogen. The difference to growers of a pathogen whose spores blow in each season and one whose spores overwinter which can overwinter is significant: a pathogen that must spread each year in the wind will be distributed unpredictably and be strongly dependent on conducive climatic conditions, allowing growers to gamble that it won't appear in their region, at least until late in the season. A pathogen that overwinters in infested soil is more likely to appear year after year in the same beds and to remain alive in soil for many years. For a pathogen that overwinters, control measures to eradicate it from beds is required.

Technical Abstract: Impatiens downy mildew is caused by Plasmopara obducens, a pathogen known from the US for over a hundred years but newly attacking Impatiens walleriana. Its life cycle is poorly understood. In an attempt to better understand the oospore stage, 14 single-sporangium isolates and three single-zoospore isolates were used in single and dual inoculations of stem tissue to see if the pathogen was homothallic or heterothallic; all isolates tested were able to produce oospores when inoculated singly, suggesting homothally. Cold treatment at 0 C for at least a month induced oospores to germinate and produce primary sporangia. Inoculation of plant tissue with germinating oospores led in infection. Other incubation temperatures (-10, 10, and 20 C) did not induce germination, but fluctuating temperatures (between -10 and 0 C or 0 and 10 C) induced some germination. Spores incubated at -10 C had significantly thicker walls than spores incubated at the other temperatures.