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- Author or Editor: Heinz K. Wutscher x
Abstract
Diseases caused by viruses and mycoplasmalike organisms are among the most serious citrus production problems. Mycoplasmas are, of course, much larger than viruses and contain both RNA and DNA. Many are obligate parasites, but some are not (54, 55). From a horticultural point of view, mycoplasmalike diseases of citrus are much like virus diseases. The first citrus disease shown to be caused by a virus was psorosis in 1933 (32). About 20 virus and viruslike diseases of citrus are recognized now (53, 55, 56); to cite an exact number is difficult because, like taxonomists, citrus virologists disagree on which diseases should be lumped together and which should be considered separately. Recent summaries of citrus virus research have been published (7, 54, 55, 56, 80, 89, 126). Table 1 summarizes the more important virus and viruslike diseases of citrus.
Abstract
One-year-old ‘Hamlin’ and 2-year-old ‘Valencia’ orange [Citrus sinensis (L.) Osbeck] trees on rough lemon (C. limon Burm. f.) rootstock were grown in solution culture for 7 months. The solutions of the two treatments were identical, except for Si. The KNO3 in the –Si solution was substituted by K2SiO3 and NH4NO3 to supply 66 ppm Si in the +Si solution. Solution pH was initially adjusted with HNO3 and NH4OH and maintained at 7 ± 0.5 by addition of dolomite. Plant weight at 28-day intervals showed significant differences in fresh weight increase between treatments only in the first 2 months. Analysis of eight different tree tissues for Si and 14 other elements showed strong correlations between Si levels and levels of P, S, Mg, Fe, Mn, Zn, Cu, and Mo, especially in the leaves, bark, and feeder roots. Si accumulated mostly in the leaves and the feeder roots, a pattern that was also found in field-grown, 17-year-old ‘Hamlin’ on rough lemon trees.
Abstract
Roots have a strong influence on plant composition, but differential ability to absorb nutrients is only part of a maze of interactions affecting mineral concentrations in plant tissues. Most reports on rootstock effects on mineral nutrition are based on leaf analysis, but all other tissues are involved. Rootstock and scion effects are reciprocal, and the influence of the scion can be as strong as that of the rootstock. Variation in distribution pattern, capability of nutrients to move across bud unions, environmental and soil factors, fruit load, and, above all, the genetic makeup of stock and scion are intimately involved. There is a large amount of literature on rootstock effects on scion composition, but generally other horticultural properties outweigh nutritional effects when a rootstock is chosen. This is especially true for the major elements. The main use of nutritional properties of rootstocks has been to avoid toxicities, especially those of Cl and B, which are difficult or impossible to avoid by other means, and to avoid deficiencies.
Seven-year-old `Hamlin' orange on Swingle citrumelo rootstock were sprayed with 30% methanol and 0.05% Silwet surfactant. There were four treatments: one spray application 48 days, two spray applications 48 and 32 days, and three spray applications 48, 32, and 20 days before harvest on December 2, 1993, with five untreated control trees. The treatments were arranged in five replications of randomized, complete blocks throughout the orchard. There were no significant differences in fruit weight, fruit diameter, rind color, rind thickness, juice content, soluble solids, total acids, solids/acids ratio, and juice color of 30 fruit samples collected from each tree. Leaf samples collected at harvest and analyzed for 12 elements showed higher Na and Cl levels in the leaves of the trees treated with methanol once than in those treated three times.
An 8-ha block of 9-year-old Valencia orange trees, surrounded on three sides by drainage ditches, was divided into four equal-sized plots. A 4-m deep sampling well was drilled in the middle of each plot and a short piece of perforated pipe was placed above the water level in the bank of one of the drainage ditches to intercept seepage water. Water from the well in the third plot and the corresponding seepage pipe contained consistently NO3-N in the 20-ppm range, in contrast to the other sampling points, ranging from 0.1 to 9 ppm. Electrical conductivity was higher in plots 3 and 4, downstream from plots 1 and 2, in the ground water flow. Sodium in the water followed the same pattern P and K were the same, and pH was higher in plots 1 and 2 than in 3 and 4. Soil pH (5.2–5.8) and water-extractable NO3-N showed no patterns, organic matter (0.79% to 0.12%) and soil moisture (5.5% to 6.3%) were higher in plots 3 and 4. Leaf nitrogen (2.60% to 2.90%) was highest in the high-nitrate plot 3. The soil on the east side of this plot showed a higher nitrate-holding capacity compared to the other plots in an anion-exchange capacity procedure.
Abstract
NO3-N concentrations in 35-year-old ‘Hamlin’ orange (Citrus sinensis L. Osbeck) and ‘Marsh’ grapefruit (C. paradisi Macf.) trees on rough lemon (C. limon Burm. f.) rootstock were highest in the feeder roots (212-962 ppm), followed by the leaves (160-300 ppm) and trunk wood (0-304 ppm). Only in 3 of 10 orange trees and in 1 of 10 grapefruit trees was NO3-N detected in the bark. Nitrate-N concentration in the leaves and the wood and the percentage of NO3-N in total N in the wood were higher in orange than in grapefruit trees.
Abstract
Healthy ‘Valencia’ and blight-affected ‘Pineapple’ orange [Citrus sinensis (L.) Osbeck] trees, both on rough lemon [C. limon (L.) Burnì, f.] rootstock, had a seasonal pattern of higher Zn in the outer 2.5 cm of the trunk wood in the winter and lower Zn in the summer. The 5-year means for the ‘Valencia’ trees were 5 ppm Zn in February and 3 ppm Zn in July; the blighted ‘Pineapple’ trees contained 21 and 16 ppm Zn (2-year means), respectively. There was no clear seasonal pattern for water-soluble phenolics.
Abstract
Leaf NO3 concentration in ‘Hamlin’ orange [Citrus sinensis (L.) Osbeck] and ‘Marsh’ grapefruit (Citrus paradisi Macf.) declined in midsummer when temperature and rainfall were at their maximum. Total N concentration remained steady after declining in the spring. The decrease in NO3-N concentration during the hottest time of the year contrasts with a peak at this time in dry citrus-growing areas.
Abstract
One-year-old ‘Valencia’ orange [Citrus sinensis (L.) Osbeck] trees on rough lemon (C. limon Burm. f.), Carrizo citrange [Poncirus trifoliata (L.) Raf. × C. sinensis], and sour orange [C. aurantium (L.)] rootstocks were transplanted into 4 soil media and grown in the greenhouse for 2 years. Treatments were 1) the entire pot filled with 2 sand: 1 sphagnum peatmoss: 1 perlite (v/v/v); 2) the entire pot filled with 3 red clay: 1 sand (v/v); 3) the lower half of the pot filled with sand: sphagnum peatmoss: perlite and the upper half with clay: sand; 4) the lower half of the pot filled with clay: sand and the upper half with sand: sphagnum peatmoss: perlite. The trees were grown from February to October 1979 when the tops were cut off. A single shoot was allowed to regrow and the trees were harvested again in October 1980. The largest trees were on rough lemon rootstock; trees on Carrizo were slightly smaller, those on sour orange distinctly smaller. Trees on sour orange grew very poorly in treatments 1 and 4 when the whole medium or the upper layer was sand: sphagnum peatmoss: perlite. Trees on all rootstocks grew best in clay-sand over sand-sphagnum peatmoss-perlite. Rootstocks induced significant differences in 12 elements, the media in 10 of 14 elements determined in the leaves. They also affected K, Mg, Zn and water-soluble phenolics in wood and bark. There were rootstocks × medium interactions for growth, 13 elements in the leaves, and the 3 elements and phenolics in the wood and bark.
Abstract
Citrus trees affected with blight, a disease of unknown cause, were associated with soils having higher pH and Ca levels than soils under healthy trees. A detailed study of soil samples taken in 5-cm increments to a depth of 45 cm under healthy and blight-affected trees from five sites showed that pH, determined separately for each increment, showed significantly lower mean pH values under healthy trees. Double-acid (0.05 N HCI and 0.025 N H2SO4) extracts of soil under blighted trees contained more Ca at all five sites. Higher P and K were associated with blight in three of six and four of six comparisons, respectively. In one of five sites, Mn and Zn were higher and Fe lower under blighted trees. The soil under trees in very early stages of blight in the heavily affected part of a block compared with soil under trees in the healthy part of the block was higher in pH and Ca. Soil where lime was incorporated deeply by mixing or deposition of dredged calcareous subsoil on the surface was associated with the most severe incidence of blight.