Robert H. Beede

Regulated deficit irrigation effects on yield, nut quality and water-use efficiency of mature pistachio trees

Summary Regulated deficit irrigation (RDI) was evaluated on deep rooted, mature pistachio trees grown under high evaporative demand in the low rainfall southern San Joaquin Valley of California. The focus of this work was to assess the impact of deficit irrigation during various parts of the season with the goal of determining which period was most stress tolerant in terms of nut production. Pistachio nuts have a unique fruit growth pattern in which rapid kernel growth does not begin until about six weeks after full shell size has been attained. Our hypothesis was that irrigation could be reduced during this period with limited negative effects on production. The season was divided into three preharvest periods: leafout to full shell expansion (Stage 1), full shell expansion to the onset of rapid kernel growth (Stage 2), and rapid kernel growth to harvest (Stage 3). Water deprivation during Stage 1 applied 7.2% less water than the near-fully irrigated Control without any yield loss, based on mean values for the last “on” and “off” alternate bearing years of this study. Shell splitting (endocarp dehiscence) at harvest (a positive impact) was significantly higher (10.4% relative to the Control) but this was offset by a nut weight reduced by 9.4%. Stage 3 water deprivation (53% less applied water than the Control) significantly reduced nut size, shell splitting, mechanical nut removal by tree shaking, and yield, while increasing kernel blanking and abortion. Postharvest water deprivation (5.0% less applied water than the Control) had no significant negative effects on yield components. Six irrigation regimes that applied water at various rates were used to investigate Stage 2 behaviour. While there were no significant differences in yield components among these regimes, the best production occurred with deficit irrigation during Stage 2 at 50% of near-potential ETc during Stage 2 and 25% of near-potential ETc after harvest. This RDI regime saved about 180 mm of water (23.2% of the Control) and water use efficiency was significantly higher (4.69 versus 3.61 kg marketable fruit per mm water, for this RDI regime and the Control, respectively). We believe RDI during Stage 2 and postharvest is a viable irrigation strategy to save water while maintaining top yields of high quality pistachio nuts. Further work is needed to determine if the improved shell splitting harvest that occurred with Stage 1 stress can be exploited to improve grower profits.

Seasonal phenology of Ferrisia gilli (Hemiptera: Pseudococcidae) in commercial pistachios.

ABSTRACT The mealybug Ferrisia gilli Gullan is a serious new pest of pistachios in California. It was first found near the town of Tulare in the late 1990s and has since spread to orchards in most pistachio-producing regions of the state. The seasonal phenology of F. gilli was evaluated in a commercial pistachio orchard in Tulare County during 2005 and 2006. During both seasons F. gilli overwintered as small nymphs and had three complete generations per year. Mealybug population densities were low and remained as immatures in March and April; by late May adult females formed and averaged (±SE) 1.3 ± 0.3 and 1.2 ± 0.3 per 0.75 m of sample branch in 2005 and 2006, respectively. The first in-season generation occurred from early June through mid-July, with mealybug densities ranging from 17.6 ± 5.6-26.4 ± 6.2 mealybugs per 0.75 m sample branch. The second in-season generation occurred from late July through September and had peak densities of 408.6 ± 93.9 and 182.0 ± 34.2 mealybugs per branch. In March and April mealybugs were located primarily on the buds and branch wood; in May the population was on branch wood as well as the rachis; from June through September the population was located primarily in the pistachio cluster. F. gillis seasonal phenology described herein was used to develop a management program.

Neoformed growth responses to dormant pruning in mature and immature pistachio trees grown on different rootstocks

Summary The relationship between pruning and the production of neoformed growth in ‘Kerman’ pistachio (Pistacia vera L.) trees on different rootstocks was investigated to determine if the increased neoformed growth common in trees on more vigourous rootstocks was related solely to rootstock vigour, or was an artifact of pruning practices. While some neoformed growth was inherent in mature trees, pruning increased the amount produced on each tree regardless of rootstock or crop load. However, regardless of pruning severity, the least vigourous rootstock never produced as much neoformed growth as the two more vigourous rootstocks. Pruning cuts made into 2-year-old wood were the most stimulatory, while pruning cuts into 1-year-old wood resulted in growth similar to unpruned controls. The data presented indicate that the invigorating response to pruning in young trees is highly localised, and that pruning can be varied depending on the stage of tree development and the desired growth response. Overall, the data suggest that hand-pruning, while more costly in the short-term than mechanical pruning, may be useful in creating a more desirable canopy architecture, thereby potentially reducing long-term pruning needs and overall management costs.

Crop Loss Relationships and Economic Injury Levels for Ferrisia gilli (Hemiptera: Pseudococcidae) Infesting Pistachio in California

ABSTRACT Ferrisia gilli Gullan (Hemiptera: Pseudococcidae) is a new pest in California pistachios, Pistacea vera L. We conducted a 3-yr field study to determine the type and amount of damage caused by F. gilli. Using pesticides, we established gradients of F. gilli densities in a commercial pistachio orchard near Tipton, CA, from 2005 to 2007. Each year, mealybug densities on pistachio clusters were recorded from May through September and cumulative mealybug-days were determined. At harvest time, nut yield per tree (5% dried weight) was determined, and subsamples of nuts were evaluated for market quality. Linear regression analysis of cumulative mealybug-days against fruit yield and nut quality measurements showed no relationships in 2005 and 2006, when mealybug densities were moderate. However, in 2007, when mealybug densities were very high, there was a negative correlation with yield (for every 1,000 mealybug-days, there was a decrease in total dry weight per tree of 0.105 kg) and percentage of split unstained nuts (for every 1,000 mealybug-days, there was a decrease in the percentage of split unstained of 0.560%), and a positive correlation between the percentage of closed kernel and closed blank nuts (for every 1,000 mealybug-days, there is an increase in the percentage of closed kernel and closed blank of 0.176 and 0.283%, respectively). The data were used to determine economic injury levels, showing that for each mealybug per cluster in May there was a 4.73% reduction in crop value associated with quality and a 0.866 kg reduction in yield per tree (4.75%).