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Optimizing ‘Hass’ avocado (Persea americana Mill.) tree nutrient status is essential for maximizing productivity. Leaf nutrient analysis is used to guide avocado fertilization to maintain tree nutrition. The goal of this research was to identify a ‘Hass’ avocado tissue with nutrient concentrations predictive of yields greater than 40 kg of fruit per tree. This threshold was specified to assist the California avocado industry to increase yields to ≈11,200 kg·ha−1. Nutrient concentrations of cauliflower stage inflorescences (CSI) collected in March proved better predictors of yield than inflorescences collected at full bloom (FBI) in April, fruit pedicels (FP) collected at five different stages of avocado tree phenology from the end of fruit set in June through April the following spring when mature fruit enter a second period of exponential growth, or 6-month-old spring flush leaves (LF) from nonbearing vegetative shoots collected in September (California avocado industry standard). For CSI tissue, concentrations of seven nutrients, nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), sulfur (S), zinc (Zn), and copper (Cu) were predictive of trees producing greater than 40 kg of fruit annually. Conditional quantile sampling and frequency analysis were used to identify optimum nutrient concentration ranges (ONCR) for each nutrient. Optimum ratios between nutrient concentrations and yields greater than 40 kg per tree were also derived. The high nutrient concentrations characterizing CSI tissue suggest current fertilization practices (timing or amounts) might be causing nutrient imbalances at this stage of avocado tree phenology that are limiting productivity, a possibility that warrants further investigation. Because CSI samples can be collected 4–6 weeks before full bloom, nutritional problems can be addressed before they affect flower retention and fruit set to increase current crop yield, fruit size, and quality. Thus, CSI nutrient analysis warrants further research as a potential supplemental or alternative tool for diagnosing ‘Hass’ avocado tree nutrient status and increasing yield.

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Agriculture is a major industry in California, with cool-season crops grown along the state’s coasts, warm-season crops grown in the hot deserts, and many temperate crops grown in the state’s valleys. In coastal communities such as Ventura County, the Calleguas Creek and the Santa Clara River watersheds have 50,000 and 60,000 irrigated acres of farm crops, respectively. These watersheds are considered impaired by nutrients, salts, pesticides, and other agricultural contaminants. Mitigation of chemical and sediment runoff through grower-implemented best management practices (BMPs) is therefore one of the highest priorities in the Los Angeles Regional Water Quality Control Board Basin Plan. A 3-year project was designed to assist Ventura County growers in meeting regional water quality objectives. The University of California Cooperative Extension Ventura County and the University of California, Riverside, collaborated with the Ventura County Resource Conservation District and the Ventura County Agricultural Irrigated Lands Group (VCAILG) to address three project goals: increase grower and landowner understanding of local agricultural water quality issues; identify gaps or deficiencies in current management practices in agricultural operations; and reduce the contribution of nutrients, pesticides, and other pollutants to impaired water bodies. To achieve these goals, 469 surveys of agricultural water quality management practices were collected to assess the extent of current adoption of BMPs. Over 160 growers who farm more than 14,000 acres that drain into Calleguas Creek and 7,000 acres that drain into the Santa Clara River watersheds were assisted. Using the survey, growers developed site-specific farm water quality plans and received on-farm recommendations for BMPs. Additionally, 12 water quality educational programs, “including demonstrations of successful BMPs,” were developed and more than 2500 copies of educational materials published by the University of California, the Resource Conservation District, and the Natural Resources Conservation Service of the U.S. Department of Agriculture were distributed at on-farm visits, workshops, and other grower events. The project resulted in improved understanding, by growers and landowners, of water quality issues and significantly increased the implementation of appropriate on-farm BMPs to protect water quality. Nearly 100 new BMPs primarily aimed at managing erosion, sediment movement, and irrigation runoff were identified and documented through annual reassessments for more than 8000 acres draining into Calleguas Creek. A total of 518 people attended the educational programs, and over 90% of participants who completed evaluations rated the programs highly. In the final year of the project, 75% of attendees indicated they plan to implement new BMPs within the next 5 years, especially in the areas of irrigation, erosion, and pest management.

Despite problems of low fruit set, small fruit size and alternate bearing, the Hass cultivar dominates commercial avocado production worldwide. To increase yield and fruit size, gibberellic acid (GA 3) (25 mg L −1) was applied at different stages of 'Hass' avocado tree phenology: (i) mid-late April (flower abscission), end of June-beginning of July (fruit abscission and beginning of the exponential phase of fruit growth), and mid-January (beginning of pre-harvest fruit drop); (ii) end of June-beginning of July; and (iii) mid-September (near the end of the major fruit abscission period; period of exponential fruit growth). In both years of the research, applications of GA 3 in April and June-July were within the periods of intense flower and fruit abscission, respectively; fruit abscission was low in September and January. Maximum air temperature was not related to flower or fruit abscission. In the on-crop year (391 fruit per untreated control tree), a single application of GA 3 at the end of June-beginning of July significantly increased total yield (kilograms only) and yield of commercially valuable fruit (178-325 g/fruit) (as kilograms and number per tree) compared with the control (P < 0.0001). GA 3 applied in September increased total yield (kilograms only) and yield of commercially valuable fruit (kilograms and number per tree) to values intermediate to and not significantly different from all other treatments, except trees receiving multiple applications of GA 3. This treatment reduced total yield and yield of commercially valuable fruit (kilograms and number per tree) relative to all treatments (P ≤ 0.0002). In contrast, during the off-crop year (32 fruit per control tree), no GA 3 treatment had a significant effect on yield or fruit size compared with the control and all other GA 3 treatments. For 'Hass' avocado, there was no negative effect from applying GA 3 at the end of June-beginning of July in both the off-and on-crop years; 2-year cumulative total yield and yield of commercially valuable fruit were increased by 27 kg (128 fruit) and 22 kg (101 fruit) per tree, respectively, above the yield of untreated control trees (P < 0.0001).

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