Studies are being conducted which aim to improve the efficiency of irrigation and N fertilization for lemons produced on sandy soils in the low desert The first experiment evaluates the response of 'Lisbon' lemons to various flood irrigation intervals. Irrigation intervals are based on soil moisture depletion (SMD) as calculated from frequent neutron probe soil moisture measurements. Individual treatments were irrigated when total SMI) was 25 %, 40 %, 55 %, and 70 %, respectively. The second experiment compares the performance of young lemons produced under flood, trickle, and micro-spray irrigation systems. The third experiment evaluates the response of young lemons to water and N combinations (3 by 3 factorial) under micro - spray irrigation. The three irrigation rates were targeted for 30 cnbar, 20 cnbar, and 10 cnbar tension. The three N rates were 0.1, 0.2, and 0.4 kg N/tree. One flood irrigation treatment was added for comparative purposes. Overall, results obtained in experiment 1 during 1994, 1995, and 1996 indicate optimal fruit growth and yield is obtained at approximately 40% SMD. The results of experiment 2 show that after 3 years, only micro-jet irrigation produced less tree growth than flood irrigation. In 1995, first year fruit yields were significantly greater for pressurized irrigation compared to flood irrigation. However, by 1996 there were no differences in yield to irrigation treatment. Results from experiment 3 show a linear response in tree growth to irrigation. In 1994 and 1995, tree growth at the high micro - spray soil moisture regime was significantly greater than trees irrigated by flood. However, in 1996 where we failed to increase the micro-spay irrigation time to meet the increased water demand by the trees, the flood irrigation regime was superior. Yields were also increased to irrigation. There were no significant differences in tree growth to N fertilization rates in 1994 and 1995. However, there was increased tree growth in 1996 and a yield increase to N fertilizer rate at the highest soil moisture regime.

Commercial and University citrus groves were sampled over a two year period in an attempt to develop mathematical models capable of predicting fruit scarring based on the population of immature citrus thrips on susceptible fruit. Five predictive models were derived. One model correlated used citrus thrips populations from fetal fall to 2.0 in. diameter fruit. While in the other models, thrips populations were divided into four distinct fruit size cohorts. Four of the five models were statistically valid. Based on these models, lemons ½ in. in diameter, should be treated with insecticides when the number of immature CT reaches 1.5 per 10 pieces of fruit. While fruit > ½ should be treated if immature CT reach or exceed 2.0 per 10 fruit.

Three small plot efficacy trials were conducted evaluating different insecticide rotation regimes using commercially available insecticides and the effectiveness of new insecticide chemistries to control citrus thrips. Because of its long residual activity, and ability to control post- application egg hatches, Carzol appears to be the product that best fits the petal fall application window. Agri-Mek, Baythroid, Dimethoate or Vydate are probably good follow -up insecticides. However, Agri-Mek and Baythroid will probably provide greater control, especially under hotter conditions. If temperatures are cool, Agri-Mek looks good at reduced rates. The best insecticide for subsequent applications depends on temperatures and what was previously applied. Avoid making back -to -back applications of the same materials, and Dimethoate or Vydate applications should probably be followed by Carzol to catch post- application egg hatches. Overall, Vydate appears to be very similar to Dimethoate in efficacy and residual activity, while Baythroid appears to be slightly better. Although the addition of Lannate to Dimethoate does slightly enhance thrips control, the additional cost probably does not justify the tank mix. Of the new chemistries (Alert, Success, Ni-25, and M-96-015) evaluated, Success and M-96-015 appeared to offer the best fruit protection. However, M-96-015 does not appear to be very effective in killing the thrips, but is very effective in repelling them. Also, M-96-015 will need to be applied at a high gallonage, i.e. 500 gal/A. None of the new products tested appear to fit the petal fall application window very well. Ni-25, Alert and Success appear to lack the residual activity of Carzol, and M-96-015 should not be used as a clean-up material but preventively following Carzol at petal fall.

M-96-015 did not appear to effectively control woolly whitefly but does appear to kill citrus mealybug. However, as with other insecticides coverage is a problem. The real benefit of M-96-015 towards citrus mealybug would occur if it prevented their spread. However, we were not able to measure this in this study. As with previous trials, M-96-015 is an effective citrus thrips material.

Lemons appear to be most susceptible to damage by citrus thrips from petal fall until they reach 1.0 inch in diameter. Correlation analysis suggests that fruit greater than 1.0 inch in diameter may not be highly susceptible to thrips scarring and thus may not require protection. 1f this relationship can be verged with additional data, late- season thrips sprays may be avoided.

New chemistries (Alert, Success, Fipronil, Ni -25, Nexter, Danitol and Baythroid) were evaluated and compared with standard chemistries (Agri-Mek, Dimethoate and Carzol) for control of citrus thrips in lemons grown in the Yuma area. Additionally, Success was compared to Carzol in a large plot commercial demonstration. Under cool, early season conditions, all products appeared to offered good thrips control. However, under warmer conditions, Nexter, Danitol, Baythroid and Dimethoate appear weak. Among the new insecticides, Success and Fipronil appear most efficacious. Alert also appeared to have good activity at the high rate, but appeared to offer shorter residual control than Fipronil or Success. Fipronil was the only new product tested that flared mites. However, rotating sulfur into the Fipronil applications appeared to help prevent flaring. Growers can expect Baythroid and Danitol to behave similarly to Dimethoate for efficacy and residual control. Under commercial conditions, Success provided thrips control equivalent to Carzol at 1.38 lbs-ai/A.

Insecticides were evaluated for their efficacy to citrus mealybugs on lemons at three spray gallonages, 60, 240, and 600 gallons per acre. None of the products tested exhibited any activity at 60 or 240 gallons per acre. At 600 gallons per acre, Lorsban at 6 qt/A + NR-415 oil at 1.4% v/v, Supracide at 2 pt /100 gal + Kinetic at 0.25% v/v, and Applaud at 2.0 lbs -ai/A + NR -415 oil at 1.4% v/v all demonstrated the best activity. Provado at 0.1 lbs-ai/A + NR-415 oil at 1.4 %, Danitol at 0.4 lbs-ai/A + Lorsban at 4 qt/A + NR-415 oil at 1.4% v/v, and Nexter at 0.3 lbs-ai/A + NR-415 oil at 1.4% v/v showed good activity. Weaker treatments included Agri-Mek at 10 and 20 oz/A, Knack and Difenolan. For maximum control, growers should treat before the fruit is heavily infested, and use high gallonages of spray solution at a high pressure, the spray must penetrate the waxy coating to achieve activity. If applicable, a spray oil should be included to help break up the wax. However, if Supracide is used, use a high rate without oil.

The effect of Roundup on lemon trees was evaluated by repeatedly spraying 0.5, 0.75, 1, 1.25, and 1.5 lb. a.i./acre on the bottom 20 to 24 inches of the tree canopies over a three year period. The Roundup applications caused significant leaf injury in the sprayed area of the canopies and there was also significant defoliation of branches at the higher Roundup rates in all three years of the study. In 1996, flower and fruitlet counts were not affected by the Roundup applications and the 1998 data were inconclusive. However, flower and fruitlet counts in 1997 in the sprayed zone of the canopy were significantly reduced by Roundup and the effect increased with increasing Roundup rate. The 1996 and 1997 yield data indicated that Roundup applied to the bottom 20 to 24 inches of the tree canopies did not significantly affect lemon yield. The preliminary data suggest that accidental drift or misapplication of Roundup on to lemon trees when spraying weeds on the orchard floor has no short-term effect on grove productivity.

Four 'Lisbon' lemon selections from the University of Arizona Citrus Budwood Certification plot were selected for evaluation on Citrus rootstock 'Frost Nucellar', 'Corona Foothills', 'Limoneira 8A' and 'Prior' were selected because of their popularity among Arizona growers or because of the lack of information about their performance under Arizona climactic and edaphic conditions. Trees were planted in 1993. Early results indicate that the 'Limoneira 8A Lisbon' selection is outperforming the other selections in both growth and yield. In a similar trial, five rootstocks were selected for evaluation using 'Limoneira 8A Lisbon' as the scion. Carrizo citrange, Citrus macrophylla, Rough lemon, Swingle citrumelo and Citrus volkameriana were chosen. Trees were planted in 1993. Early results indicate that trees on C. volkameriana are superior to those on other rootstocks in both growth and yield.

The effect of Roundup on lemon trees was evaluated by repeatedly spraying 0.5, 0.75, 1, 1.25, and 1.5 lb a.i/acre on the bottom 20 to 24 inches of the tree canopies. Leaf injury symptoms, flower and fruit counts, and yield data were collected The Roundup applications caused significant leaf injury in the sprayed area of the canopies and there was significant defoliation of branches at the higher Roundup rates. In 1996, flower and fruitier counts were not affected by the Roundup applications. However, flower and fruitier counts in 1997 in the sprayed zone of the canopy were significantly reduced by Roundup and the effect increased with increasing Roundup rate. The 1996 yield data indicated that the Roundup applications did not significantly affect lemon yield, however, the effect of Roundup on the 1997 flower and fruitier counts suggests that there may be a yield effect in 1997. The preliminary data suggest that accidental drift of Roundup on to lemon trees when spraying weeds on the orchard floor has no short-term effect on grove productivity but this conclusion must be substantiated by further data collection.