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Tree Crops


152 - Development of Molecular Markers in Avocado

Principal Investigator: Dr. Mary Lu Arpaia, Botany and Plant Sciences Department, University of California - Riverside. For more project information, click here.

Avocado is a crop with enormous commercial potential, yet breeding practices have not changed for centuries. We intend to accelerate the breeding process with molecular tools. Our goal is to develop molecular markers that track high or low levels of biochemical components contributing to fruit nutritional value. As consumers are becoming more concerned with the nutritional value of foods, avocado has been singled out repeatedly in recent years as having beneficial effects on human health by virtue of an array of antioxidants, vitamins, lutein, and the cholesterol-lowering and anti-carcinogenic properties of β-sitosterol and other constituents. Given this important health advantage of avocado over many other crops it is vital to gain an understanding of the genetic underpinnings of these compounds so that future breeding efforts can keep pace with the expanding health demands by today’s consumers. We propose to use an approach that detects associations between levels of particular fruit constituents (nutritional phenotypes) and molecular markers. Markers found to associate with nutritional phenotypes in preliminary rounds of selection will be deployed within the Scion Breeding Program located at UC Riverside. By laying the foundations for the development of new, high-quality cultivars, we hope to secure the long-term future of the California avocado industry in the face of low-cost foreign imports and its almost complete reliance on cultivar ‘Hass’.

199 - USDA Interregional Project 4, Avocados

Principal Investigator: Dr. Peggy Mauk, Botany and Plant Sciences Department, University of California – Riverside. For more project information, click here.

Project 199 Avo IR4
The purpose of this project is to conduct pesticide residue trials with the goal of securing registration of additional agrochemicals for use on avocado through the USDA Interregional Project 4 (IR-4). IR-4 is a publicly and privately funded USDA program that conducts research and submits petitions to the Environmental Protection Agency (EPA) for registration of pest control agents on specialty crops including avocado. Since 2003, the UCR IR-4 Center has conducted twelve trials in avocado at SCREC testing seven agrochemicals.

 Related Work:

UC Riverside Scientists Working Towards A Solution To Invasive Ambrosia Beetles In California Avocados (Frank Byrne, Akif Eskalen and Joseph Morse, University of California, Riverside)

208 – Pruning Wound Susceptibility and Protection

Principal Investigator: Dr. Akif Eskalen, Plant Pathology and Microbiology Department, University of California – Riverside. For more project information, click here.

California is the leading producer of avocado fruit in the United States.  High-density planting is becoming more common in California avocado groves and this necessitates more frequent pruning to manage tree growth.  Fungi of the Botryosphaeriaceae family are known to cause branch cankers on avocado which are initiated by spores entering the host plant through fresh wounds such as pruning wounds.  More frequent pruning, such as would occur in a high density grove, could increase the transmission rate of this pathogen from tree to tree.  Past spore-trapping studies show that Botryosphaeria spores are trapped throughout most of the year within California avocado groves, with a sudden increase in spores trapped soon after or coinciding with a rain event.

There are currently no effective control strategies of branch canker on avocado.  The chemical treatment of any open wounds on the tree would be the primary means of protecting against Botryosphaeria spores entering and initiating infection.  No current literature was found which evaluates the chemical treatment of pruning wounds for avocado, therefore, the objective of this study is to evaluate the effectiveness of 8 to 11 chemical treatments on fresh pruning wounds of avocado trees on reducing the initial infection by Botryosphaeriaceae spores.

212 - Use of Foliar-and Irrigation-Applied Plant Growth Regulators to Mitigate Alternate Bearing of the ‘Hass’ Avocado

Principal Investigator: Dr. Carol Lovett, Department of Botany and Plant Sciences, University of California – Riverside. For more project information, click here.

Alternate bearing cycling of heavy ON and light OFF crops reduces income to California ‘Hass’ avocado growers. ON-crop trees produce numerous small fruit of reduced commercial value; OFF-crop trees produce too few fruit to provide growers with a good income. The ON crop causes correlative inhibition (auxin > cytokinin) of buds that produce the summer and fall vegetative shoots, which reduces the number of nodes that bear inflorescences the next spring. In addition, the ON crop through correlative inhibition combined with high abscisic acid inhibits spring bud break. Trunk injecting (to avoid the complication of poor leaf uptake) ON-crop trees with a proprietary cytokinin with the auxin transport inhibitor tri-iodobenzoic acid (TIBA) in July and again in January increased the number of summer vegetative shoots that developed and the number of apical indeterminate floral shoots and axillary determinate floral shoots borne on summer vegetative shoots at return bloom the following year. We are now ready to test the efficacy of this strategy applied to the foliage or through the irrigation to ON-crops trees to increase yield the year following the heavy ON crop. Our goal is to develop a cost-effective technology for ‘Hass’ avocado orchards that successfully mitigates the effects of the ON crop of fruit and increases return bloom and yield in the putative OFF-crop year, such that 2-year cumulative total yield, 2-year cumulative yield of commercially valuable large fruit, and 2-year cumulative net income is significantly greater than that of untreated control trees.

213 - Evaluation of Rootstock Material from Israel

Principal Investigator: Dr. Mary Lu Arpaia, Botany and Plant Sciences Department, University of California – Riverside. For more project information, click here.

Establish and increase budwood source planting for 22 rootstock selections from Israel that show promise for salinity and possibly root rot tolerance. Some of the selections also are believed to be tolerant to alkaline conditions.


155 – Evaluation of New Citrus Cultivars

Principal Investigator: Dr. Mikeal Roose, Department of Botany and Plant Sciences, University of California – Riverside. For more project information, click here.

Development of new citrus cultivars has considerable potential to improve the profitability and competitiveness of the California citrus industry. This project is part of a series of trials to evaluate potential new cultivars at various locations that represent citrus production zones in California.  Potential new cultivars are produced by hybridization-selection or mutation breeding at the University of California, Riverside and Lindcove Research and Extension Center.  Promising selections are then propagated and 12 trees of each selection are planted in trials at SCREC and five or six other locations.  As trees fruit, they are evaluated for tree growth, fruit quality, yield, and other characteristics.  Following several years of evaluation, a decision on release is made.  This trial at SCREC was begun in 2002 with additional trees being planted in subsequent years as selections were made and trial trees propagated.  About 850 trees of 61 selections have been planted so far, with additional plantings expected in the future. The first new variety from this program, Tango mandarin (patent pending) a very low-seeded selection of W. Murcott mandarin, was released in June 2006. Promising selections of several other varieties are nearing release.

184 – Citrus Tristeza Virus Tolerance of Rootstocks

Principal Investigator: Dr. Mikeal Roose, Department of Botany and Plant Sciences, University of California – Riverside. For more project information, click here.

Citrus tristeza virus (CTV) is a significant disease problem in many California citrus growing areas and threatens the remaining areas where it is not already widespread due to limited eradication efforts.  CTV isolates already common in California cause "quick decline" of sweet orange on sour orange rootstock and similar reactions in a few other rootstocks.  Trees on some rootstocks are stunted by certain CTV isolates even though trees do not die.  It is essential to determine the reaction of new rootstocks to tristeza so that susceptible stocks are not released or recommended.  CTV is common at SCREC and many trials for CTV tolerance have been conducted there in the past. The specific objectives of the project are to evaluate 50 rootstocks, mostly new selections, for tolerance to four CTV isolates.  The longer-term objective is to identify superior rootstocks with CTV tolerance for use in California.  Seedlings of standard and experimental rootstocks were grown in a greenhouse at UCR, grafted with Valencia orange scion and, just before planting, inoculated with one of four CTV isolates selected with assistance from Dr. Marylou Polek, Central California Tristeza Eradiation Agency.  The trees were planted in the field at SCREC in June and July 2006.  CTV infection of each tree was confirmed by ELISA tests and uninfected trees were reinoculated. Trees were evaluated annually for tree health and tree size, CTV symptoms, and fruit counts were recorded in late 2008.  By 2008 near all trees tested CTV positive. By 2008, there was clear evidence of decline and stunting in trees on sour orange and some other rootstocks, including African shaddock x Rubidoux trifoliate. There were only slight differences in average effects of the different CTV isolates. Several additional years will be needed to reach conclusions about CTV tolerance of all rootstocks in this trial.  

Stone Fruit

188 Evaluation of Moderate-Chill Pluots

Principal Investigator: Dr. John Kabashima, Environmental Horticulture Advisor, UC Cooperative Extension. For more project information, click here.

Pluots are complex interspecific hybrids of plum and apricot (Prunus sp. L. X Prunus armeniaca L.). The sugar content of a pluot fruit is reported to be much higher than either of its antecedents. These delicious new type of fruit provide new opportunities for tree fruit growers. We are evaluating 5 moderate-chill pluot cultivars on Myrobalan 29C rootstock in a trial at South Coast Research and Extension Center. A second trial evaluates the performance of Flavorich cultivar on Citation rootstock compared to Flavor Grenade cultivar on Lovell rootstock. The cultivars continue to be evaluated for adaptability, yield, fruit quality and for susceptibility to pests and diseases.

Trees were planted in 2-tree plots, replicated five times in a randomized complete block design and the orchard managed following recommended cultural practices typical for plums. Specific data to be collected from the trials include tree trunk cross sectional area, fruit set, total marketable fruit number and weight, average fruit weight, fruit diameter, % TSS (total soluble solids) and panel taste scores. All data will be subjected to ANOVA and mean separation between treatments done by an appropriate test.

The results of this research will be disseminated to members of the California Rare Fruit Growers Association, production nurseries and on field days and group tours of SCREC. Poster and oral presentations at various meetings as well as papers in professional publications will be additional avenues for extending results from this trial.


203 – Jatropha for Biodiesel Production in California

Principal Investigator: Dr. Shrinivasa Upadhyaya, Department of Biological and Agricultural Engineering, University of California – Davis. For more project information, click here.

Jatropha curcas L. is a small subtropical tree / shrub that grows on marginal lands and produces oil suitable for cost effective production of biodiesel. The plant is being commercially developed in many nations such as Brazil, India, and Indonesia for producing biodiesel. This project proposes to test the possibilities of growing the crop in California, with research being focused on development of germplasm resources, adaptation to various California growing conditions and environments, and development of a cropping and mechanized harvest systems.