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B.R. Gardner, R.L. Roth, and C.A. Sanchez

A study was conducted during four seasons to evaluate the nutrient requirements of `Valencia' oranges converted from flood to a pressurized spray irrigation system. The experiment was a 3×2×2 factorial with 3 N rates (0.34, 0.68, and 1.36 kg/tree/year), 2 P rates (0 and 0.11 kg/tree/year) and with and without added micronutrients (Fe, Zn, Mn, and Cu). There were no growth or yield responses to micronutrients. Phosphorus fertilization increased fruit yield, improved juice quality, and reduced peel thickness. There were trends for N to reduce juice quality and increase peel thickness when P fertilizer was not added. Tree growth increased by N fertilization only the first season after conversion. Fruit yield also increased by N but only when P was added. Leaf tissue N concentrations increased with time during the first two years within N treatments. These data suggest that the higher rates of N may only be needed initially after conversion as the tree roots adapt to the new irrigation system.

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R.L. Roth, B.R. Gardner, and C.A. Sanchez

A study was conducted during four seasons to evaluate the performance of mature `Valencia' oranges converted to pressurized irrigation systems. Trickle, bubbler, spray, and sprinkler systems were all compared to the traditional flood-border irrigation. During the second year after conversion, trees irrigated by flood grew significantly more than trees irrigated by any of the pressurized systems. However, there were no differences in tree growth during the third and forth year, suggesting that the trees adapted to the new irrigation systems. Effects of irrigation treatments on leaf concentrations of N, P, Fe, Zn, Mn, and Cu were minimal. There were significant differences in orange yields among the irrigation treatments within years. However, average or total yields over the four year period did not vary by irrigation treatments. Similarly, there were no consistent differences in fruit or juice quality. Overall, results from this study indicate the mature citrus can be converted to pressurized irrigation systems with minimal effects on fruit yield and quality.

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N.M. El-Hout, C.A. Sanchez, and S. Swanson

Potassium is often considered the nutrient element most limiting to crop production on organic soils. On Histosols in southern Florida, K2SO4, rather than KCl, is often used for lettuce (Lactuca sativa L.) production to minimize the risk of salt injury. However, recent soil-test calibration research suggests that current K fertilizer recommendations for lettuce may be too high. Four field studies were conducted from 1989 to 1991 to evaluate the response of five lettuce types to K rate and source. The five lettuce types evaluated were leaf, bibb, boston, romaine (cos), and crisphead. Two sources of fertilizer K (K2SO4 and KCl) were evaluated at rates ranging from 0 to 600 kg K ha-1. Lettuce showed a minimal or no response to K fertilization. Potassium chloride had detrimental effects on lettuce only when applied at rates in excess of those required for optimal production. These studies showed that K fertilizer recommendations for lettuce produced on Histosols in Florida can be reduced. Furthermore, KCl, a more economical source, is suitable when the K is applied at appropriate rates.

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C.A. Weber, K.E. Maloney, and J.C. Sanford

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C.A. Weber, K.E. Maloney, and J.C. Sanford

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J.A. Pattison, W.F. Wilcox, and C.A. Weber

A hydroponic method was developed and tested for screening red raspberry genotypes for resistance to Phytophthora fragariae var. rubi, the most common causal agent of Phytophthora root rot in raspberry. Plants of `Titan' and `Encore' exhibited typical disease symptoms, with the latter developing significantly smaller stem lesions and fewer petiole lesions. The resistant cultivar, `Latham', regenerated healthy root tissue from the crown and older-order roots after initial infection and necrosis of young roots and exhibited no other symptoms beyond minor leaf chlorosis. This component of the resistance reaction has not been documented previously. A segregating F1 population from the cross of `Latham' × `Titan' had a survival rate of 56% with 42% classified as resistant, exhibited minimal symptoms, and produced varying amounts of healthy root tissue. This screening method allows multiple observations of all plant tissues, including roots, under repeatable and definable growth chamber conditions. It should be useful for classifying the phenotype of individuals in segregating red raspberry populations to investigate the inheritance of Phytophthora root rot resistance using molecular markers.

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C.A. Brown, D.A. Devitt, and R.L. Morris

Research was conducted to assess the response of tall fescue (Festuca arundinacea Schreb.) to water deficit conditions. Different leaching fractions (LF = drainage volume/irrigation volume) and irrigation frequencies (IF) were imposed over a 119-day summer period in Las Vegas, Nevada, followed by a 71-day recovery period. Plots of tall fescue contained 120 cm deep × 51 cm diameter draining lysimeters. Irrigations were based on an evapotranspiration (ET) feedback system to establish LFs of +0.15, 0.00, -0.15, -0.25, and -0.40. Plots were irrigated on a daily or twice per week schedule. N was applied to subplots at a rate of 0, 12.2, or 24.4 kg·ha-1 per month. As LF decreased, relative soil water in storage declined in a linear fashion (r 2 = 0.97, P = 0.001). Storage depletions for the four lowest LFs at the end of 119 days of imposed water deficits were about 15%, 40%, 60%, and 70% compared to the +0.15 LF treatment. Canopy temperature, soil matric potential (Ψm), leaf xylem water potential (ΨLX), leaf stomatal conductance (gs), clipping yield, color and cover ratings all statistically separated (P < 0.05) based on LF but not on IF. However, irrigation amount (I), ET, tissue moisture content and total Kjeldahl N (TKN) separated based on LF and IF with a significant LF by IF interaction for I (P < 0.05) and TKN (P < 0.001). An irrigation savings of 60.4 cm was realized during the 119-day water deficit period at the -0.40 LF. However, at the lower LFs, plant stress increased (all parameters) with color ratings declining below an acceptable value of 8.0. An Irrigation/Potential ET (I/ETo) threshold of 0.80 was determined for both color and cover. After a 71-day recovery period both color and cover returned to pre experimental values at the two higher N rates. Results of this experiment indicate that implementing a twice weekly irrigation strategy at a -0.15 LF on tall fescue during summer months in an arid environment would lead to savings of 37.5 cm of water while still maintaining acceptable color and cover ratings.

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W.L. Mountain, C.A. Powell, and L.B. Forer

A trough system was developed to study rates of plant virus transmission by plant parasitic nematodes. Perforated plumber's polyvinyl chloride pipe, 5 cm in diameter, was cut into 48-cm lengths, split longitudinally, and fashioned into troughs to hold soil and common dandelion (Taraxacum officinale Webber) transplants. The first plant in each trough was infected with tomato ringspot virus (TmRSV), followed by 10 uninfected plants spaced at 4-cm intervals. The soil contained a high concentration of Xiphinema rivesi (199 per 100 cm3), a low concentration (16 per 100 cm3), or none. Plants were assayed biweekly for TmRSV. After 42 weeks, transmission rates between the low and high concentrations of nematodes were not significantly different. The subirrigated trough system provided excellent soil conditions for plant growth and sufficient nematode survival to detect virus transmission through 36 weeks.

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R.J. Henny, R.T. Poole, and C.A. Conover

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R.J. Henny, R.T. Poole, and C.A. Conover