Integrating resistance from wild relatives against downy mildew in Impatiens

Integrating resistance from wild relatives against downy mildew in Impatiens

Summary

 The advent of a virulent new race of downy mildew has defoliated and decimated impatiens across the United States, as well as worldwide. Often a key early season crop for small greenhouse growers and nurseries, and a fixture in landscapes and home gardens, susceptibility appears to be near-universal and severe in the common species, I. walleriana. While the New Guinea types (I. hawkeri) appear resistant, they have drastically different cultivation requirements and methods. Unfortunately, they also do not form viable hybrids with the common species. However, preliminary data suggests that other, compatible species may be resistant or highly tolerant, and useful for breeding new, more diverse forms. We have begun screening a wide range of impatiens species for their reaction to downy mildew, and have also attempted hybrids between these and the common species. Concurrently, we are also investigating the potential of other, easy-growing species of impatiens to fill the garden niche held by I. walleriana, diversifying the range of species cultivated and providing more options for growers and gardeners. We have also attempted hybrids between these species, as well as having induced diversity through mutation and changes in ploidy, with the hopes of developing more commercially appropriate forms. In addition, we have been collecting seeds and plants from native jewelweed impatiens populations, and looking at their degree of susceptibility to assess the risk that they might become reservoirs for the disease.

Objectives/Performance Targets

1. Screen approximately 30 species for reaction to impatiens downy mildew

• • Acquired approximately 150 impatiens accessions from commercial sources, private collections, and wild populations.

• • Conducted the first field trial for resistance at the Long Island Horticultural Research & Extension Center (LIHREC) on 38 accessions

• • Recorded quantifiable resistance data from the field trial

2. Observe reaction of different populations of native jewelweed species to downy mildew

• • Collected or acquired native jewelweeds from four locations in three states.

• • Collected one naturalized species from two different locations.

• • Evaluated one accession for resistance in the field trial

3. Hybridize identified resistant species with common impatiens to introgress resistance

• • Attempted over 175 crosses involving I. walleriana with a resistant species

• • Had seven rescued ovules from four different crosses develop into vegetative tissue

• • Had one rescued ovule differentiate into a plantlet

4. Asses the success of hybridization process and the resistance and consumer appeal of interspecific hybrids.

• • Met with a commercial nursery marketing an interspecific impatiens population

• • Grower discussions of commercial cultivar trials at the LIHREC field day

• • Planting of different impatiens species and hybrids for a local field day

• • Planting of different impatiens species and hybrids in a public garden bed

• • Informal discussions about desirable/undesirable qualities in cultivated and underutilized species

5. Hybridize various impatiens species to identify hybrids with potential for seed-propagation as an alternative to common impatiens.

• • Attempted over 160 crosses not involving I. walleriana & obtained several putative hybrids

6. Identify and create mutants for various impatiens species and evaluate how they may expand and benefit cultivation of the species.

• • Performed three mutation induction trials on a total of four different accessions

• • Identified a wild-growing mutant of I. balsamina with potential commercial appeal

• • Made a preliminary test of oryzalin on in vitro cultures

7. Present information on resistant species, and their use as landscaping alternatives, to gardeners, growers, extension personnel, and representatives from industry.

• • Presented preliminary background and results to:

    • • The National Association of Plant Breeders Conference

    • • The Independent Plant Breeders Conference

    • • The International Plant Propagators Society Conference

    • • A field day for the LIHREC

    • • An undergraduate course at Cornell University

    • • The Horticulture Department of Cornell University

    • • The Plant Breeding Department of Cornell University

• • Discussed project with commercial growers and researchers

• • Distributed posters on the pathogen from the USDA Ornamental Pathology Lab to interested growers, Master Gardeners, and extension agents

Accomplishments/Milestones

This year, the second year of the grant, has seen an expansion of our work and some concrete results. We started the year by expanding the cultivation of our existing species and acquiring new ones. Through a trip to several West Coast nurseries and personal connections from the private collector mentioned in last year’s report, we added many new accessions to our collection. Several of these were existing interspecific hybrids among the Madagascar impatiens species, reputed to be resistant to downy mildew. We propagated these, as well as existing accessions in our collection, in order to have duplicates as we determined growing conditions and to produce plant material for experiments. There are still issues propagating the native jewelweed species, but we have had some limited success getting them to germinate after approximately five months of stratification. We also made collections of native jewelweed seedlings from populations on both the East and West Coasts. Recent contact with the curator of the USDA-NPGS Ornamental Plant Germplasm Center has also opened avenues to procure seeds from these species from a wider geographic range, as well as some unique floral forms. Acquiring a Small Seed Lots Permit from USDA-APHIS let us import seeds from a collector in Austria of several species which are difficult to find in the United States.

Using the material in our collection, and from our recent acquisitions, we now have a breadth of germplasm to test for resistance. We shared last year’s in vitro resistance screens with a collaborating pathologist, who helped us identify the infecting pathogen as a non-target one (eg. not impatiens downy mildew, Plasmopara obducens). They helped us develop a field screening program, where plants of I. walleriana already infect with downy mildew were interspersed among different replications of a variety of other impatiens germplasm from our collection. In all, we tested 9 cultivars of I. walleriana, 6 interspecific hybrids involving I. walleriana, 8 cultivars of I. balsamina, and 15 other impatiens species or hybrids. Each accession was replicated five times and the order was randomized. From this work, we identified varying degrees of resistance, both within and among species (eg. Figure 1), including several species with complete resistance. We are in the process of the analyzing the data from our resistance screen, and plan to repeat it next year with a wider range of germplasm.

Informed by the data from our resistance screen, we have been making crosses to transfer this resistance into I. walleriana. While several of the existing I. walleriana interspecific hybrids procured from other sources show decent resistance, we wanted to take advantage of one less-susceptible accession of I. walleriana we identified and use it to make hybrids. To this end, we have made crosses combining this accession with species identified as mostly- or fully-resistant in our screens. Through rescuing ovules, we now have five interspecific hybrid plantlets involving one source of resistance and the less-susceptible plant of I. walleriana. Many of these did not develop normally even in culture, but one eventually differentiated into a viable plant. We are now propagating that plant through tissue culture and growing it in the greenhouse, in hopes of seeing it flower and using it to make further crosses. We also have several seedlings from another accession of I. walleriana crossed with the same source of resistance, but these are still young and in the process of developing. Our breeding work has also resulted in an interspecific hybrid between two other impatiens species, one of which showed decent resistance during the field screen. We are propagating this plant in the greenhouse and also making crosses between it and other closely-related species.

Information from the field resistance screen has also informed our mutagenesis work. We have now tried a variety of protocols for ethyl methanesulfonate (EMS) treatment of seeds and cultures to induce mutations. The accessions we selected for these were based on a variety of factors, including seed production or ease of culture (depending on the tissue treated), pigmentation (an easy phenotype to observe changes in), and data on resistance. So far we have treated cultures from four species and seed from two. One of the seed treatments included two accessions of I. balsamina: the one ranked most resistant in our screen and the one ranked most susceptible, but with excellent pigmentation. Unfortunately, the treatment of the more resistant accession had some procedural issues which resulted in death of most of the treated seeds. However, the treatment of the susceptible accession produced a population of mutated seedlings, which have been slowly recovering from the treatment and we are growing up to observe for unusual phenotypes. Preliminary observation shows distinct differences from the controls (Figure 2), including one plant possibly exhibiting variegation.

We are also working to build on last year’s successes with colchicine treatment to induce polyploidy by substituting the more effective and less toxic chemical oryzalin, but these experiments have not yet yielded definitive results. Serendipitously, we also observed an off-type plant of I. balsamina in a commercial planting with an unusual phenotype of producing flowers at the end of its branches, rather than along its central stem. Seed sown from this plant produced seedlings with a similar phenotype (Figure 3); we are in the process of propagating them further and making crosses to determine inheritance.

• Figure 1. Examples of the range of resistance shown in our downy mildew screen. A) Resistant check plant of I. hawkeri B) Completely susceptible cultivar of I. walleriana [remaining tissue marked with a star] C) Less susceptible cultivar of I. walleriana
• Figure 2. EMS treatment plants approximately three weeks after sowing. Controls are in the left row, all others were treated.
• Figure 3. One seedling from the mutant plant of I. balsamina, exhibiting characteristic flowering.

Impacts and Contributions/Outcomes

This year we were able to use the results of our research to educate and inform a wide range of audiences. We presented the preliminary data from our screening work and other project information at three professional conferences, one guest lecture, two department presentations, and a field day with representatives from industry and the public sector. In addition, we had many informal discussions with professional growers, nurserymen, private collectors, and Master Gardeners. These allowed us to gain feedback on our work, share information and resources, and remain current on their concerns. Through the generosity of the USDA Plant Pathology Laboratory we were also able to pass along posters with information on impatiens downy mildew, which included a link on how to send samples of the pathogen to the USDA for research and evaluation.

The interest generated from this project has led to several invitations to present both our plants and our research. This past summer we received requests from two local groups for plants to showcase in their gardens, as well as information about our research. We supplied plants (see Figure 4 for an example) and visited both sites to receive feedback from growers as well as the public. Recent contact with the curator of the USDA-NPGS Ornamental Plant Germplasm Center has given us an avenue to conserve germplasm of the seed-propagated species in our collection and also distribute them to a wider audience. This will be particularly important as we further publish and disseminate our results on resistant species. Perhaps most excitingly, we have just been asked to present our research and participate in conversations next year at the First International Symposium on Impatiens Research in Bonn, Germany. As our research progresses, we hope to be able to have more information to share with an even wider audience.

• Figure 4. A portion of a green-wall and test garden planting with two species from our collection: I. repens (center, red) and I. balfourii (right, lower).

Collaborators: