Frederick S. Davies

Field evaluation of integrated pest management-compatible pesticides for the citrus leafminer Phyllocnistis citrella (Lepidoptera: Gracillariidae) and its parasitoid Ageniaspis citricola (Hymenoptera: Encyrtidae).

Abstract Potentially selective and integrated pest management (IPM)-compatible pesticides for the citrus leafminer Phyllocnistis citrella Stainton and its parasitoid Ageniaspis citricola Logvinovskaya were compared under nursery field conditions at Gainesville, FL. In 1996, replicated blocks of young grapefruit trees were treated with 2% petroleum oil and 1× the lowest recommended field rate (LRFR) of diflubenzuron + oil (0.4%). Untreated and treated controls (avermectin at 1× the LRFR + oil) were included. In 1997, blocks were treated with oil (3%), 1× the LRFR of azadirachtin + oil, 1× the LRFR of diflubenzuron + oil and 0.1× the LRFR of avermectin + oil. Untreated and treated controls were again included. Oil at 3%, azadirachtin at 1× the LRFR + 0.4% of oil, and diflubenzuron at 1× the LRFR + 0.4% of oil were shown to be IPM-compatible pesticides. In 1997, these blocks had fewer mines per leaf and P. citrella pupae parasitized by A. citricola per total leaves sampled compared with the untreated control but more than the treated control (α = 0.05). Avermectin at 0.1× the LRFR + 0.4% of oil was not considered an IPM-compatible pesticide because, while it reduced the number of P. citrella mines per leaf, it reduced the number of A. citricola to levels as low as the treated control. Actual P. citrella infestation levels had no detectable effect on tree growth and vigor. Pesticide applications were not justified when P. citrella infestations were <1 mine per leaf and the biological control agent A. citricola was present.

Characterization of soil-water retention of a very gravelly loam soil varied with determination method

Soil-water characteristic curves were determined for Krome calcareous very gravelly loam soil in the laboratory and in situ in an orchard. In the laboratory, soil-water retention was determined with a pressure plate and pressure Tempe cells for soil collected from vegetable fields. In the orchard, soil-water suction measured with tensiometers was compared to volumetric soil-water content (&thgr;) determined with neutron or multi-sensor capacitance probes. Before field measurements, calibration equations were developed for neutron and multi-sensor capacitance probes for this soil. Krome calcareous gravelly loam soil was found to have two distinct solid fractions with 51% coarse particles and 49% loam particles that resulted in a peculiar soil-moisture retention pattern. Two soil-moisture retention regions were identified, each corresponding to one of the soil solid fractions. As shown by a large number of observations, rapid drainage occurs in the gravel fraction corresponding to soil-water suction less than 75 cm. In an orchard, suction values rarely exceeded 125 cm even when there was no rainfall and irrigation was withheld for three weeks. In the orchard, &thgr; measured with a capacitance probe was considerably more variable and less correlated with soil suction than &thgr; measured with a neutron probe. In very gravelly loam soils such as Krome, results from capacitance sensors may be too variable and inconsistent for reliable monitoring of soil-water content.

Flooding, mineral nutrition and gas exchange of mango trees☆

Abstract Two-year-old cultivar ‘Peach’ mango trees (Mangifera indica L.) were grown in containers with (+Fe) or without (−Fe) chelated iron in limestone soil for 7 months and exposed to one of three flooding regimes: non-flooded (control) and 10 or 20 days of flooding. Prior to the imposition of flooding, and about 80 days later, total leaf chlorophyll content (Chl) and foliar concentrations of N, P, K, Ca, Mg, Fe, Mn, Cu and Zn were determined. Initially, Chl and foliar Fe and Mn concentrations were higher, but foliar K, Ca and Mg concentrations were lower, in the +Fe trees than in the −Fe trees. After the imposition of flooding treatments, significant interactions were observed between iron fertilization and flooding treatments with regard to Chl and certain foliar nutrient concentrations. Therefore, the effect of flooding on mineral nutrition was analysed separately for each iron fertilization regime. For +Fe trees, Chl was unaffected by flooding treatment, but for −Fe trees Chl increased with increased flooding duration. For both iron fertilization regimes, foliar Mn increased with flooding and tended to be greatest with increased flooding duration. For both iron fertilization regimes, there was no effect of flooding on foliar N, Fe or Zn concentrations, and no clear effect of flooding on foliar Cu concentration. For +Fe trees, foliar P concentration was reduced in trees flooded for 20 days, but there was no effect of flooding on foliar P concentration in −Fe trees. Flooding resulted in reductions in foliar K concentration in −Fe trees, but not in +Fe trees. For both iron fertilization regimes, flooding resulted in a reduction in foliar Ca concentration. For +Fe trees, flooding for 20 days resulted in increased foliar Mg concentration, but there was no effect of flooding on foliar Mg in −Fe trees. Prior to flooding, net CO2 assimilation (A) was greater for +Fe trees than for −Fe trees. Six months after the imposition of flooding, A of flooded −Fe trees was similar to that of +Fe trees. The results of this study indicate that short-term flooding may reduce certain micronutrient deficiencies in mango trees grown in limestone soils.

Root and Leaf Ferric Chelate Reductase Activity in Pond Apple and Soursop

Abstract Root and leaf ferric chelate reductase (FCR) activity in Annona glabra L. (pond apple), native to subtropical wetland habitats and Annona muricata L. (soursop), native to nonwetland tropical habitats, was determined under iron (Fe)-sufficient and Fe-deficient conditions. One-year-old seedlings of each species were grown with 2, 22.5, or 45 µM Fe in a nutrient solution. The degree of tolerance of Fe deficiency was evaluated by determining root and leaf FCR activity, leaf chlorophyll index, Fe concentration in recently mature leaves, and plant growth. Root FCR activity was generally lower in soursop than in pond apple. Eighty days after plants were put in nutrient solutions, leaf FCR activity of each species was lower in plants grown with low Fe concentrations (2 µM) than in plants grown with high (22.5 or 45 µM) Fe concentrations in the nutrient solution. Leaves of pond apple grown without Fe became chlorotic within 6 weeks. The Fe level in the nutrient solution had no effect on fresh and dry weights of soursop. Lack of Fe decreased the leaf chlorophyll index and Fe concentration in recently matured leaves less in soursop than in pond apple. The rapid development of leaf chlorosis in low Fe conditions and low root and leaf FCR activities of pond apple are probably related to its native origin in wetland areas, where there is sufficient soluble Fe for adequate plant growth and development. The higher leaf FCR activity and slower growth rate of soursop compared to pond apple may explain why soursop did not exhibit leaf chlorosis even under low Fe conditions.

Growth regulator and low-volume irrigation effects on grapefruit quality and fruit-drop

Abstract ‘Marsh’ ( Citrus paradisi Macf.) grapefruit trees on rough lemon ( C. jambhiri Lush.) rootstock received irrigated and unirrigated treatments. Half of each irrigation treatment received a GA 3 + 2,4-D spray at color break. Pre-harvest sprays of GA 3 and 2,4-D extended the grapefruit harvest season by increasing fruit removal and rind puncture force, delaying development of mature color and decreasing late-season and post-freeze fruit-drop. Although soil moisture content in the top 0.9 m of unirrigated blocks was reduced by approximately 40%, xylem pressure potentials of these trees and performance of GA 3 + 2,4-D were unaffected. Soil moisture content was not an accurate indicator of tree water status for large grapefruit trees.