1a. Objectives (from AD-416):
With the decline in citrus production, many citrus growers and homeowners are looking for viable alternatives to citrus in East-Central Florida. Avocado production has significant potential, but no cultivars have been selected for their suitability in our region. In addition, the disease laurel wilt is a significant threat to avocado. This project will investigate disease resistance, fruit quality and other horticultural traits for production in East-Central Florida and will couple these investigations with a genetic mapping study to determine which genes are linked to key traits for use in future marker assisted selection.
1b. Approach (from AD-416):
About 600 trees have been planted from a population of Hass x Bacon and Bacon x Hass for a detailed mapping study and as a source for avocado cultivars suitable for East-Central Florida production. Extensive data on tree growth, disease resistance, and fruit quality will be collected over the next five years. In addition, seedlings are being generated from every divergent seed source genotype we can identify to be challenged with the Laurel Wilt fungus (Raffaelea lauricola). These seedlings will be inoculated and monitored for disease development and aspects of resistance response. All material will be assessed using a newly developed Avocado SNP Genotyping Chip and resulting data will be used to assess which gene variants are associated with critical traits of interest.
3. Progress Report:
This project is related to Objective 1: Create new genetic combinations of citrus, Objective 2: Screen germplasm for important traits and select superior individuals, Sub objective 1 D: Create new scions and rootstocks with potential resistance to huanglongbing (HLB) and citrus bacterial canker (CBC) by genetic transformation. After receiving $54,000 from National Plant Disease Recovery System (NPRDS) last year, in support of these projects, the opportunity arose to leverage additional resources for this project using $70,000 in USDA/ARS funds to get matching funds from the University of Florida. These funds are being used to support the research of a doctoral student and will provide 4 years of support to work on projects from this proposal. The Hass x Bacon mapping study/cultivar identification was expanded by an additional 240 seedlings. With 590 members to the population, being grown in the Indian River area, there is an excellent probability of finding a high quality transgressive segregant well-suited to local commercial production. The first group will crop significantly this year and there will be numerous unique hybrid trees providing samples for fruit evaluation, which will be led by University of Florida and a new Ph.D. student. Seedlings of the greatest accessible range of avocado diversity continue to be assembled to be challenged with laurel wilt. In 2010, populations of 22 seed parents were acquired and 20 germinated to yield 673 seedlings, and were planted in the field at Ft. Pierce USDA/ARS last summer. Additional seed were obtained in 2011 and germinated at Ft. Pierce, bringing the population to 38 half-sib families. In 2012-2013 we propose to expand the effort to identify resistant material in the inoculation trials. Reports indicate that the Mexican and Guatemalan material and hybrids may offer significantly greater resistance or tolerance to laurel wilt than is observed in West Indian or West Indian x Guatemalan material. A large number of seeds will be sent from the California South Coast Field Station to be germinated by our colleagues in Miami (funding under a separate proposal). Seed of these selections and additional material from families from Florida collections (primarily National Clonal Germplasm Repository (NCGR)-Miami) will be established in Ft. Pierce to provide a total of ~50 seedlings per seed source with around 100 different half-sib families represented. When they reach suitable size they will be inoculated in collaboration with Univeristy of Florida. Initial efforts suggested that laurel wilt inoculations are most effective when applied to trees of 2 plus years of age. This requires that trees be grown for a considerable period before they can be assessed for evidence of resistance/tolerance, adding greatly to the time, expense, and complexity of these assessments. There is evidence that laurel wilt disease spreads to green tissue from adjoining infected woody tissue, suggesting that techniques may be developed to infect young tissue directly. Ability to screen younger material will permit more rapid identification of useful resistant material. If cuttings or in-vitro shoots can be assessed, it would permit assessment of existing genotypes rather than requiring seedlings or grafted trees. In addition, screening of smaller tissues would facilitate evaluations under more controlled conditions, using fungal cultures and culture filtrates, possibly contributing to understanding of disease development and ultimately mechanisms of resistance. A trial of existing avocado cultivars will be established in Ft. Pierce using trees purchased from Florida and California sources. Many of these trees will be challenged by inoculating with the laurel wilt fungus. Enough trees will be purchased so that some can be grown to maturity to also determine which cultivars perform best in our environment. The most promising cultivars will ultimately be placed in limited commercial trials and will be used as parents in crosses with the most resistant material to produce the next generation of better adapted, laurel wilt tolerant cultivars.