Hosta Virus X - The Latest AHS Research


Rob Mortko, VP Genus Hosta
   The date and location of the 2010 AHS annual convention could not have been more timely for an update on the subject of Hosta Virus X. At the Thursday evening banquet Dr. Ben Lockhart from the University of Minnesota Plant Pathology Department was the keynote speaker. Dr. Lockhart and Grace Anderson recently completed a two year Hosta Virus X (HVX) study at the University of Minnesota. While the study was sponsored by the AHS, it was funded entirely by donations from interested individuals, local and regional hostas societies, and the American Hosta Growers Association.
   The theme of Dr Lockhart’s HVX presentation was to “expect the unexpected”. Research is like that. You answer a few questions but generate new questions in the process.
   Dr.  Lockhart started with a general overview of plant viruses and a history of Hosta Virus X. HVX was first discovered and documented by Dr. Lockhart in 1996. A key in the early understanding of HVX was its classification as a Potexvirus (a genus of plant viruses). As such, HVX was expected to exhibit similar behaviors.

       

Benham E. L. Lockhart, PhD 

Dr. Lockhart summarized what we know about HVX:

  • HVX seems to only infect hostas. It is not known to infect any other plants besides hosta.
  • HVX is transmitted readily by leaf-wounding using an abrasive (mechanical or rub-inoculated).

Dr. Lockhart then summarized what we assumed about HVX based on its classification as a Potexvirus:

  • HVX is not spread by insects or nematodes.
  • HVX is not transmitted through pollen or seed.
  • HVX is not known to be spread in soil.
  • HVX is transmitted only by plant division (vegetative propagation) and by contact involving surface wounding.
 
   This background set the basis for the AHS sponsored study. The goal of the HVX research project was to clarify methods of HVX transmission and to understand the dynamics of HVX spread so that we might reduce exposure to and perpetuation of the virus.
   The research project commenced in the fall (September) of 2007. The first surprise of the research was that the timing of exposure to HVX is critical in virus transmission. Out of 100 Hosta ‘Honeybells’ exposed to HVX, not a single plant (0%) became infected. Conversely, 100 out of 100 plants (100%) became infected when exposed to HVX the following spring. HVX infection develops readily in actively-growing plants and much less so in plants after flowering as they approach dormancy.
There are no hosta cultivars that are more (or less) resistant to HVX as had been previously reported. Any hosta exposed to HVX while actively growing will likely become infected.       

   HVX was transmitted by contaminated scissors during pruning (2 of 20 tests) but not by knife (0 out of 20 tests) during division of crowns. HVX was not transmitted (0 out of 20 tests) on sap-contaminated fingers during leaf removal.
   Dr. Lockhart reported that thorough cleaning of pruning/cutting tools with dishwashing detergent, 70% alcohol, or a 10% solution of household bleach (1 part bleach and 9 parts water) were all effective in preventing HVX spread by eliminating HVX in sap or plant debris contamination.  However he also stressed that simply dipping the cutting tool in these solutions is not sufficient. The tool must be wiped to physically remove any residual sap or plant debris. Thorough scrubbing is required.

Garden tools can be easily disinfected, but dipping in a solution is not sufficient

Thorough scrubbing is required.

   The second major surprise of the research project occurred when it was observed that virus-free hostas became infected with HVX when planted in the same location where an HVX-infected plant had previously been growing. Soil with HVX plant debris and root material was found to remain infective for at least two years. (Note that two years was simply the duration of this research project. The actual time of remaining infectivity could be longer.) Subsequent testing showed that 2 out of 20 hostas planted in soil from which HVX-infected plants had been removed became infected with the virus. This method of virus spread had not been previously reported for Potexviruses and was totally unexpected.

  Prior recommendations that you could replant after waiting about 4 to 6 weeks are not valid. 

   Until more research is conducted, do not plant another hosta in the same location where an HVX- infected plant has been removed.

Dr. Lockhart concluded with some new questions that have arisen based on this research project:

  • What are the most effective and practical means of decontaminating cutting tools in order to avoid spread of HVX?
  • For how long does HVX-infected residual root tissue remain infectious to roots of healthy replacement hosta?
  • How much virus-infected residual root tissue is needed to cause root infection?
  • Are any other shade perennials susceptible to HVX?
  • Can root-to-root spread of HVX occur during post-harvest washing of roots for removal of soil?

 

   It is specifically these questions that the AHS intends to address in a proposed Phase 2 HVX Research Project. The AHS plans to continue to work with Dr. Lockhart and Grace Anderson at the University of Minnesota. This will maintain continuity while building on their considerable existing knowledge base. A research proposal has recently been requested by the AHS and received from Dr. Lockhart for this next phase of research. The proposed budget for this next phase of research is comparable to the initial phase of work. The intent is to confirm available funding this year (2010) with the research to begin in the spring of 2011. The research project is expected to be completed within two calendar years. 
   All individual AHS members, local and regional hosta societies, and hosta growers are encouraged to consider a generous contribution to this project. A donation form is posted on the AHS site. A summary brochure of the recently completed virus research project is also posted on the AHS site. These brochures have been mailed to all local and regional hosta society clubs to share with their membership. The brochures can be also be viewed, downloaded and printed from the AHS site.

Dr. Lockhart in his University of Minnesota lab 

Please support the Phase 2 HVX Research Project with a generous donation.
Details are available on the AHS site.

 

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