Air layers from 6 blight-affected and 6 healthy grapefruit, Citrus paradisi Macf., trees were grown in a greenhouse with low N, low S, and lime and high N, high S, and no lime regimes, one air layer from each tree in each treatment. Shoot fresh weight of air layers from healthy trees was 25%, and root fresh weight was 38% greater than that of air layers from blighted trees, after 8 months of treatment (shoot/root ratios of 1.59 and 1.90). Shoot weight was the same with both nutrient treatments; root weight was 40% smaller with high N, high S, and no lime than with low N, low S, and lime (shoot/root ratios of 2.12 and 1.37). Air layers from blighted trees had higher concentrations of N, P, and water-soluble phenolics, and lower Ca and Na in the wood; more S, Fe, Zn, Cu, and Mo in the bark; more N and K, and less Mg, Na, and Cl in the roots, and more P and less B and Cl in the leaves than air layers from healthy trees. Low N, low S, and lime induced higher K and Mo in the wood, higher K in the bark, and lower Na and Cl in the roots of air layers from blighted trees; high N, high S, and no lime increased Mg and Zn in the roots, Fe in the wood, and Zn in the leaves of air layers from blighted trees above the levels of healthy air layers. There were curvilinear relationships between evapotranspiration and root weight and the shoot/root ratio; air layers from blighted trees transpired more water than those from healthy trees on a per unit shoot and root weight basis.
The shape of ‘Redblush’ grapefruit, Citrus paradisi Macf., grown in controlled environments was affected by the difference between day and night temperature. Fruit grown under a 32°/30°C day/night temperature regime had creased stem ends; a 32°/24° regime resulted in normal fruit, and 32°/7° induced severe sheepnosing. Reducing daylength from 14 to 11 hours had no influence on fruit shape.
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.
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.
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.
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.
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.
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.
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.
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.