The oospore stage of Plasmopara obducens, impatiens downy mildew

ABSTRACT

Impatiens downy mildew is caused by Plasmopara obducens, a pathogen known in the United States for over a hundred years, but newly attacking ornamental Impatiens walleriana in production and in the landscape. Little is known about the life cycle of P. obducens; thus, in this study an attempt was made to determine whether the pathogen is homothallic or heterothallic. Fourteen single-sporangium isolates and three single-zoospore isolates were used in single and dual inoculations of stem tissue to see whether 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 1 mo induced oospores to germinate and produce primary sporangia. Inoculation of plant tissue with germinating oospores resulted 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 other temperatures. Evidence suggests that oospores can serve as an overwintering stage.

KEYWORDS:

• Downy mildew
• epidemiology
• Impatiens
• oospores
• ornamentals
• Plasmopara obducens

ACKNOWLEDGMENTS

I thank Dr. Farivar Eskandari, Elaine Cerchin, Bianca Hoch, and Kristine Johnson for technical assistance. In memory of R. P. Korf. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities. USDA is an equal opportunity provider and employer.

Additional information

Funding

Funding for this research was partially supported by a grant from the United States Department of Agriculture-Animal and Plant Health Inspection Service-Center for Plant Health Science and Technology through the 2014, 2015 and 2016 Farm bills received in collaboration with Cristi Palmer and the Interregional Research Project No. 4. Project..