S.C. Nelson, Department of Plant & Environmental Protection Sciences, UH-CTAHR
Acacia koa is severely affected by soil borne disease problems in Hawaii. This project examines grafting technology to produce potentially disease-resistant Acacia koa plants.
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Contact
Dr. Scot Nelson (snelson@hawaii.edu)
Phone: 808-981-5207
Fax: 808-981-5211 |
FUNDING has been provided to CTAHR for this research from the USDA Cooperative State Research, Education, and Extension Service Project HAW00957-H.
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OBJECTIVES
1) Evaluate Acacia koa for graft compatibility with several Acacia species found in Hawaii.
2) Determine resistance of Acacia rootstocks and grafted plants to Fusarium oxysporum, Meloidogyne incognita, and M. javanica.
APPROACH
A. koa seedlings will be grafted below the cotyledons using the cleft grafting method described by S. C. Nelson in United States patent and technology application number 60,684,359 (Nelson, 2005). Approximately 2 weeks after grafting, plants will be moved to a shade house (with 60% shade) for growth and development. When plants are moved to the greenhouse, they will be inoculated with appropriate nitrogen-fixing bacterial species to induce nodulation. Grafted plants will be allowed to grow for 4-6 months to determine their rootstock-scion graft compatibility. Grafted plants will be transplanted as needed to larger pots containing a mixture of peat, perlite and cinder. The economic feasibility of grafting koa will be measured by tracking the labor and material costs associated with it and factoring in the success rates to arrive at an estimated per-plant cost to the timber or nursery industry.
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Pathogens. Two soilborne pathogens of Acacia koa (the fungus, Fusarium oxysporum f. sp. koae, and the root-knot nematode, Meloidogyne incognita; Meloidogyne javanica) will be isolated from naturally diseased roots of Acacia koa on the island of Hawaii. Some isolates may be obtained from exisiting collections of pathogens at UH-Manoa or the Hawaii Agricultural Research Corporation. Pure cultures of the koa wilt pathogen, Fusarium oxysporum f. sp. koae, will be grown on potato dextrose agar. The fungus will also be maintained on living plants of Acacia koa in a greenhouse. Populations of Meloidogyne incognita and M. javanica will be reared on tomato (Lycopersicon esculentum) and/or Acacia koa seedlings grown in sterile soil in a greenhouse. Pathogenicity of Fusarium isolates to be used in these trials will be confirmed by inoculating the roots of 6-wk-old koa seedlings with aqueous suspensions of Fusarium spores and mycelium and examining plants after 2-3 weeks for the development of root rot. Only confirmed pathogenic isolates will be used to test grafted plants and Acacia species for resistance to koa wilt.
Testing rootstocks and grafted plants for disease resistance. For koa wilt disease, 6-wk-old plants of each of the scion-host combinations and each of the Acacia rootstock species (e.g., A. koa; A. mangium; A. confusa; A. melanoxylon) will be inoculated individually with a pathogenic isolate of F. oxysporum f. sp. koae, by drenching root systems with standardized spore suspensions in sterile distilled water. Inoculated plants will be held in humid chambers for 1 week after inoculation, after which they will be moved to a 60% shade house and placed randomly on benches until disease symptoms appear. Plants will be rated after 1 month for percentage of root rot and foliar wilt. Fungal isolations on water agar from any symptomatic or diseased root systems will be done to recover the pathogen. For root-knot disease, 6-wk-old plants of each of the scion-host combinations and each of the Acacia rootstock species (e.g., A. koa; A. mangium; A. confusa; A. melanoxylon) will be inoculated individually with 2,000 eggs of the two Meloidogyne species.
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PROGRESS
2005/12 TO 2006/09
Scions of Acacia koa Gray were grafted onto rootstocks of A. koa, Acacia mangium Willd., and Acacia confusa Merr., using cleft or approach grafts applied to very young seedlings. This is the first report of grafting success between A. koa and any Acacia species. A. koa scion plants at the second true leaf stage of development were joined with the Acacia rootstocks of similar phenology just below the cotyledons. Following seedling decapitation by scalpel, graft unions were secured with foam rubberized grafting clips. Grafted plants were held in plexiglas humid chambers at room temperature in a laboratory next to a window for 7 days, after which clips were removed. Two weeks later, surviving plants were transferred from the plexiglass chamber to a 60% shadehouse for further growth and development. The initial grafting trials resulted in success rates from 20-70%. Graft unions were completely healed within 8 wk and plants ready for transplanting at about 3 mo. More than 25 grafted plants were produced. Grafted plants appear to perform very well upon transplanting outdoors. One plant was outplanted near Hilo, Hawaii, and is performing exceptionally well for an Acacia koa plant at that altitude, reaching approximately 4 meters in height in the first year. Plans are being made to challenge rootstocks and grafted plants in controlled inoculation studies with the pathogen, Fusarium oxysporum f. sp. koae.
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IMPACT
2005/12 TO 2006/09
This is the first report of grafting success between A. koa and any Acacia species. The new grafting method allows for the production of grafted A. koa seedlings with the potential disease resistance, improved horticultural performance and potentially wider adaptation to Hawaiian ecosystems and landscapes.
PUBLICATIONS (not previously reported):
2005/12 TO 2006/09
Nelson, S. C., 2006. Grafting of Acacia koa Gray onto young seedlings. Native Plants Journal 7(2):137-140.
Click here to link to Koa Wilt Webpage.
Click here to link to Koa Pest and Disease Image Gallery.
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