Downy mildew research findings released

WASHINGTON, DC and COLUMBUS, OH — Downy mildew is a term used to define a collection of similar pathogens caused by water molds, or Oomycetes. While not technically fungi, these microorganisms can cause devastating disease problems and have increased in prevalence in recent years. Impatiens downy mildew (IDM), caused by Plasmopara obduscens, has, perhaps, received the greatest attention in the green industry. Today IDM is considered the biggest limiting factor in I. walleriana production and landscape beds. 

USDA’s Animal & Plant Health Inspection Service (APHIS) allocated over $425,000 in special funding provided through the Farm Bill to continue studies on the detection and mitigation of downy mildew on impatiens, cucurbits, hops, and basil in FY 2016. Several researchers across the U.S. are involved in this project and convened a phone call in early May to share key findings from 2015. Researchers included: 

· Dr. JoAnne Crouch, USDA-ARS,

· Ms. Margery Daughtrey, Cornell University,

· Dr. Mary Hausbeck, Michigan State University,

· Dr. Aaron Palmateer, University of Florida,

· Dr. Cristi Palmer, IR-4 Project,

· Dr. Lina Quesada-Ocampo, North Carolina State University, and

· Dr. Nina Shishkoff, USDA-ARS. 

As a stakeholder, AmericanHort was invited to participate as well.

This group of researchers has formed a comprehensive collaboration that includes a breeding program for IDM resistance, discerning the life cycle and disease cycle of P. obduscens, developing an early detection method for greenhouses, and evaluating fungicide control options. IDM has already been identified this year in Georgia, Florida, Oklahoma, and Texas.

Key findings from 2015 include the following.

• Dr. Quesada-Ocampo’s group and collaborators developed a new, small, roto-rod spore trap that could be used for early detection of downy mildew pathogens in the greenhouse or field. The trap is user-friendly, inexpensive and it should retail for less than $500. This could improve early detection of downy mildew pathogens and ultimately aid in timely disease management to improve control outcomes. Early detection can improve control, reduce fungicide reliance, and delay pathogen resistance. 

• Impatiens encompass a diverse group of plants. Over 1,000 species are known worldwide, and vary greatly in their susceptibility to impatiens downy mildew (IDM), caused by Plasmopara obducens. Margery Daughtrey’s group is screening different species for IDM resistance for breeding purposes. Impatiens hawkeri, I. briartii, and I. cinnabarina are three species with relatively high resistance to IDM. Many species showing resistance originated in Africa and south and southeast Asia.

• Preliminary studies suggest that the pathogen can survive cold temperatures in infested plant litter and then infect living plants, indicating that P. obduscens may be able to overwinter in some parts of the country. Observations in Maryland and Michigan landscape sites also indicate that the pathogen is overwintering. If validated in ongoing outdoor experiments, these results could have significant implications for landscapes and disease management programs.

• The fungicide Adorn (active ingredient = fluopicolide) reportedly caused plant injury by way of flower damage and overall plant stunting on impatiens. This phytotoxicity worsened when Adorn was tank-mixed with Subdue Maxx. Since Adorn is an effective IDM control product, Dr. Hausbeck’s group is evaluating the effect of substrate on phytotoxicity development. Organic matter content is inversely proportional to phytotoxicity caused by Adorn; substrates with increasing organic matter resulted in less observed phytotoxicity.

• Researchers overwhelmingly reported excellent efficacy with the new fungicide Segovis (active ingredient = oxathiapiprolin). Segovis was granted federal registration in February; state registrations are pending. All researchers noted that drench applications were more effective than foliar sprays, and that it has a good plant safety profile so far. For now, the mode of action of oxathiapiprolin is unknown and is likely unique. There is a great concern over preventing fungicide resistance development to Segovis and thus continued research in the area of fungicide management is needed.

• IDM has already been reported this year in Georgia, Florida, Oklahoma, and Texas. AmericanHort and its research affiliate, the Horticultural Research Institute, in conjunction with other interested organizations, such as the Society of American Florists, supported funding of this research through the Farm Bill Section 10007. This collaboration is coordinated by the IR-4 Project, a USDA-sponsored entity that works to expand the specialty crop industry’s access to pest management tools. Industry leaders such as Ball Horticultural Company and Syngenta are also involved.

AmericanHort, in conjunction with other interested organizations, such as the Society of American Florists, supported funding of this research through the Farm Bill Section 10007. This collaboration is coordinated by the IR-4 Project, a USDA-sponsored entity that works to expand the specialty crop industry’s access to pest management tools.

AmericanHort will host a webinar later this year to discuss this research in greater detail. Additional research details can be found www.AmericanHort.org/DownyMildewResearch.

Photo: Jill Calabro, AmericanHort