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 SMD 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 and 1995 indicate optimal fruit growth ,and yield is obtained at approximately 40% SMD. The results of experiment 2 show that after 18 months micro -spray irrigation produced significantly more tree growth than flood and drip irrigation methods. Additionally, first year fruit yields were significantly greater for pressurized irrigation compared to flood irrigation. Results from experiment 3 show a linear response in tree growth up to 10 cnbar soil moisture tension. Furthermore, tree growth at 10 cnbar tension was significantly greater than trees irrigated by flood. Yields were also increased to irrigation regime. There were no significant differences in tree growth to N fertilization rates. However, there was a yield increase to N fertilizer rate at the highest soil moisture regime.

Granular (Rugby 10G) and liquid (Rugby 100ME) formulations of Cadusafos were evaluated for the control of Tvlenchulus semipenetrans on mature lemon trees in a commercial citrus orchard at Yuma, Arizona. Three applications of cadusafos, with two months between applications, at the rate of 2 g a.i. /m2 reduced nematode populations to undetectable levels and increased the yield and rate of fruit maturity of 'Rosenberger' lemons. Yields were increased 12,587 kg per hectare with Rugby 100ME and 8,392 kg per hectare with Rugby 10G. Nematode populations were suppressed for at least 12 months after the last application.

Biological agents have been employed in a series of experiments to evaluate their efftcacy,in control of the citrus nematode and Liohippelates eye gnat in the Yuma mesa area. Steinernema riobravis, an entomopathological nematode, considered climatically adapted to western Arizona temperatures, was selected from commercially available sources. Three field trials and a greenhouse study utilizing the nematodes at population ratés of one and two billion juveniles per acre were each unsuccessful in reducing the nematode or insect pest. Poor viability and survival were attributed to the negative results in each of the investigations. The microbial nematicide, DiTera, which was included in two limited trials, was found to be highly effective in suppression of citrus nematode populations infecting Yuma citrus. These results have prompted Abbott Laboratories, manufactures of the product; to establish two demonstration plots in the Yuma mesa area.

The effects of several preemergence herbicides at various rates were tested on bearing Redblush' grapefruit trees and non-bearing Navel orange trees. Some of the preemergence herbicides caused foliar injury symptoms on the trees including Hyvar X Diuron, and Krovar I although they did not cause significant yield reductions in the short time period of this experiment. Several herbicides including Solicam, Surflan, and Prowl did not cause foliar injury or reduce yield and provided good weed control for various lengths of time. The Prowl treatments provided the longest period of weed control in the experiment on bearing grapefruit trees.

Studies were conducted to evaluate potential chemical disease management tools for Alternaria fruit rot on navel oranges and Coniophora brown wood rot on lemon trees, to investigate the possible effect of branch diameter on development of Coniophora wood rot on lemon trees and to summarize our evaluations of citrus rootstocks with respect to relative resistance to root rot and stem canker development when challenged with Phytophthora citrophthora and P. parasitica. We were unable to reduce the level of Alternaria fruit rot on navel oranges with single applications of Abound or copper hydroxide following significant rainfall events. Wood decay in lemon branches inoculated with Coniophora eremophila was significantly suppressed by Abound and a thick formulation of sodium tetrathiocarbonate. The degree of Coniophora brown wood rot in lemon branches of different diameters was variable, although the level of disease in 10 mm diameter branches was significantly smaller than the amount of wood decay in 30 mm diameter branches. Root loss due to Phytophthora citrophthora and P. parasitica in Citrus macrophylla, rough lemon, C. volkameriana and Troyer citrange was lower than most of the 36 different rootstocks tested. On the other hand, root loss on Carrizo citrange, C-35 citrange and sour orange was among the higher values of disease recorded. Stem canker development due to both species of Phytophthora on Troyer citrange, Carrizo citrange, sour orange and Citrus macrophylla was lower than most of the 36 rootstocks tested. Stem cankers on rough lemon and Citrus volkameriana were among the higher values of disease recorded.

When applied early-postemergence, Milestone at 0.75 and 1.0 lbs-ai/A provided excellent control of summer annual grasses and broadleaf weed for as long as 4 months, equivalent to Karmex. Princep and Krovar also provided good weed control but appeared to offer slightly shorter residual against some broadleaf species. Solicam, although it provided good weed control, particularly of the grasses, was the weakest material evaluated in this trial. Although Karmex is notorious for causing crop injury on the sandy soils of the Yuma Mesa, we did not detect any injury from this herbicide or any others in this trial.

A field survey of mature lemon trees showed an average of 30% of trees with symptoms of brown heartwood rot caused by Coniophora sp. In vivo growth of Coniophora inoculated into branches of different types of citrus (Valencia orange, Marsh grapefruit, Orlando tangelo or Lisbon lemon) on rough lemon rootstock was significantly higher in lemon while Coniophora inoculated into Lisbon lemon wood branches on trees established on rough lemon, volkameriana, macrophylla, Cleopatra mandarin, sour orange or Troyer citrange rootstocks showed no significant differences in growth. Vegetative incompatibility trials from one mature orchard demonstrated that isolates from different trees are incompatible. In vitro fungicide trials showed that only NECTEC paste effectively reduced decay on lemon blocks 15 weeks after inoculation with Coniophora. Field fungicide trials showed that NECTEC P paste as well as the blank paste without fungicides, propiconazole at 10,000 μg /ml, imazalil at 20, 000 μg /ml or propiconazole plus imazalil in combination at 10,000 and 20,000 μg/ml, respectively, significantly inhibited the advance of fungus 7 mo. after inoculation. A second fungus isolated from brown rot in branches in younger orchards was identified as Antrodia sinuosa, a native decay fungus on conifers in Arizona.

The incidence and severity of Alternaria fruit rot on navel oranges has increased in Maricopa County. The objectives of this study were to test the efficacy of two fungicides, Kocide 101 and Rovral, for disease control and to determine an application schedule that will result in optimum control of disease. In 1994, compared to nontreated trees, a significant reduction in the number of dropped oranges occurred on trees sprayed one to five times with Rovral at monthly intervals from April through August. In the same year, monthly applications of Kocide 101 from April through August were no better than leaving trees untreated. In 1995, Rovral treatments from March through August provided no apparent beneficial effects on control of Alternaria fruit rot, while a single application of Kocide 101 in December, January, or February resulted in significantly less dropped oranges compared to nontreated frees. Because of the inconsistent activity of Kocide 101 and Rovral in these two studies, an additional trial is in progress during 1996. The additional data from 1996 may help identify when and what fungicide(s) could provide significant control ofAlternaria fruit rot of navel oranges in Arizona.

A physiological characterization has established that vascular changes in Macrophylla decline affected trees are not similar in character to xyloporosis affected trees. In addition, a survey of Macrophylla decline affected citrus did not establish any genetic similarity between Macrophylla decline and xyloporosis. We report diagnosis of either CCV or CEV by reverse transcription-polymerase chain reaction (RT-PCR), as well as diagnosis of Macrophylla decline or xyloporosis by Zn-distribution, water conductivity, accumulation of decline- specific proteins and examination of phloem morphology in lemon trees on the Macrophylla rootstock.

Deficiency of boron (B) in citrus has serious consequences for tree health and crop production. There is evidence that B deficiency may be a problem in Arizona citrus. Certainly, many symptoms of B deficiency are apparent, especially on the Yuma Mesa. A field trial was conducted at Yuma, Arizona to examine the effect of foliar boron application on fruit yield and quality of Citrus sinensis and C. limon. Boron was applied to 5 year old Citrus sinensis cv. Hamlin and C. limon cv. Rosenberger Lisbon trees at 5 different concentrations (0 ppm, 500 ppm, 1000 ppm, 2000 ppm, 3000 ppm) either before flowering or after flowering. At harvest, fruit yield and quality, and boron concentrations were determined. Foliar application appeared to increase leaf boron concentration (r= 0.50, p= 0.004). Fruit set was increased in Hamlin trees receiving bloom and post bloom applications of boron at the 1000 ppm level. Boron applications had no significant effect on lemon yield in these studies. This fruit set increase in Hamlin accounted for a 35% increase in overall yield relative to control trees. However, there was no significant difference in fruit weight, fruit pH, titratable acidity, peel thickness, juice volume, or soluble solid content of the fruits among treatments. Previous studies indicate that boron influenced in vivo and in vitro pollen germination in many crops. A plausible explanation for increased fruit yield may be that the applied boron was transported to the flowers where it exerted its influence of increased fruit set through an effect on pollen viability and/or pollen tube growth. However, clearly boron supplementation must be performed judiciously to avoid fruit drop from over -application of the element.