The influence of paclobutrazol on the growth and cropping of sweet cherry cultivars.

Paclobutrazol (1.6 g a.i.), applied to the soil beneath nine year old trees of two sweet cherry cultivars in 1983, controlled shoot growth better in the year of treatment than foliar sprays (1500 followed by 750 mg l^-1) of the same chemical applied in late May and early June. Although no further treatments were applied, extension growth was strongly inhibited by both treatments in the two subsequent years and some inhibition persisted for four years after treatment. Inhibition of the shoot growth of both cultivars was more severe and persisted for several years longer for trees on ‘Colt’, compared with F. 12/1 rootstock. Paclobutrazol foliar sprays reduced yields in the year of treatment, and the residual effects of both soil and foliar treatments on yields were also negative in some years, particularly on the cv. Van. The reasons for these negative effects on yields were sought in studies of flowering density, flower quality and the efficiency of fruit set and retention.     

Morphology and anatomy of the burl disorder of mango (Mangifera indica L.) in India

Summary

Burl, typically manifest as a swelling of the trunk and lower branches, is a relatively unknown disorder of mango (Mangifera indica L.). We studied the incidence of the disorder in trees of each of three mango cultivars in India, and its effect on fruit yields over 2 years. Information was also collected on the relationship between the growth of the burl and the age of the trees, together with details of the anatomy of the affected tissues. ‘Langra’ had the highest incidence of the disorder (80.3% of 24 studied trees affected) and the largest burl (31.8 cm diameter), followed by ‘Chausa’ (17.5%; 16.4 cm), then ‘Gulab Jaman’(7.5%; 4.0 cm). Burl significantly reduced fruit yields in ‘Langra’ (121 kg tree^–1 in affected trees vs. 162 kg tree^–1 in normal trees), but not in ‘Chausa’ (110 vs. 129 kg tree^–1) or ‘Gulab Jaman’ (100 vs. 98 kg tree^–1). The sizes of the burls increased as the ages of the trees increased from 15 to 55 years, especially in ‘Langra’ and ‘Chausa’. No pathogens or insects were found to be associated with the affected tissues. The woody tissues in the burl lacked orientation and were not specifically aligned in a transverse, radial, or tangential direction. Further studies are warranted to determine the cause of this disorder and how it can affect fruit yields.     

The Influence of Simazine or a Straw Mulch on the Establishment of Apple Trees in Grassed Down or Cultivated Soil

The first three years’ growth of Cox’s Orange Pippin on M.26 was compared in a trial in which a four-foot square around the tree received either simazine, or straw mulch, or the same treatment as the surrounding area, which was grass or cultivation. Two rates of Nitrochalk were used.

The treatments influenced leaf area and nutrient composition; shoot number, length and weight; and root size and distribution.

Competition from a partial weed cover in the first year produced a severe growth reduction which persisted in the following years and was not reduced by the higher nitrogen rate.

A four-foot square kept weed-free with simazine enabled trees to be grassed in the year of planting and yet make growth comparable to those kept clean-cultivated. Less than 1% of the applied simazine remained active in the soil at the end of the third year.

Mulching produced the most vigorous trees. It is considered that soil moisture availability was the main factor influencing tree establishment.     

Verticillium Wilt of The Hop (HUMULUS LUPULUS) IV. Study of A Fluctuating Outbreak

Young trees of Cox’s Orange Pippin on M.VII were grown in sand for three-years and given two seasons of treatments. Residual effects were studied in soil in the fourth year. The trees made little new growth in the third and. fourth years.

Control trees received a continuous supply of complete nutrient solution containingi meq. NO₃/1. (low-N). Other treatments received the’ same solution but the nitrate level was increased to 16 meq./l. in January-February and March (spring-N) or in May, June and July (summer-N) or in September, October and November (autumn-N). The fifth treatment, was a continuous supply of 4 meq. NO₃/1. (moderate-N).

Summer-N gave greatest uptake and leaf and shoot growth, and. increased the proportion of the total nitrogen going into them. Moderate-N was nearly as effective in these respects, though nitrogen uptake was not so high. Autumn-N did not increase growth, but concentrations tended, to be higher than in the controls and spring-N trees because of a somewhat, greater nitrogen uptake.

All additions of N increased the number of axillary blossom clusters per 100 cm. of shoot. The total number of axillary clusters on trees with autumn-N was the same as with spring-N, but autumn-N produced strong, fertile-blossom comparable in percentage set to that with summer-N and moderate-N trees, while spring-N blossom was weak and no more fertile than the low-N. flowers (controls).

Blossoming and leaf development were a few days earlier with autumn-N or moderate-N. Summer-N had this effect in one year.     

Effects of clonal rootstocks on ‘Hass’ avocado yield components,alternate bearing,and nutrition

Summary

Growth, yield, and leaf nutrient concentrations were measured in ‘Hass’ avocado (Persea americana Mill.) trees grown on one of ten clonally-propagated rootstocks (‘Borchard’, ‘D9’, ‘Duke 7’, G1033, G755A, G755B, G755C, ‘Thomas’, ‘Topa Topa’, or ‘Toro Canyon’) over a 10-year period in southern California. After 10 years, trees on ‘Borchard’ were larger than trees on all other rootstocks. Trees on all rootstocks displayed an alternate-bearing pattern, typical of avocado. Alternate-bearing was most pronounced in trees grafted onto ‘Topa Topa’ and ‘Toro Canyon’. Rootstocks in the G755 series had the lowest alternate-bearing index, but also had the lowest yields.Trees on ‘Duke 7’ and ‘Borchard’ had the highest cumulative yields, and trees on G755A, G755B, and G755C had the lowest yields. Differences in yield were due to differences in the number of fruit per tree, not individual fruit weight. When yield was evaluated in terms of canopy efficiency (kg fruit m^–3), no rootstock outperformed ‘Duke 7’, the industry standard rootstock. Leaf concentrations of all nutrients examined (N, P, K, Ca, Mg, S, Zn, Cl, Mn, B, Fe, and Cu) were within, or close to the recommended ranges. P, Ca, and S were higher, and Fe was lower in high-yielding years in all rootstocks.     

Soil and Leaf Sampling in Apple Orchards

In an investigation to determine the best ways in which to take soil and leaf samples characteristic of an orchard, 40 commercial orchards of Cox’s Orange Pippin, selected at random from the major fruit growing areas of England, were examined over a period of three years. It was found that:

1. 24 cores should be taken at random throughout an orchard in such a way that rows and alleyways contribute equally. The cores should be bulked, thoroughly mixed, and a single sub-sample taken for analysis. In any given season 80 per cent, confidence limits on the resulting determinations will be ±0.24 pH units, ±4·7 p.p.m. of P, ±24·1 p.p.m. of K, and ±13·3 p.p.m. of Mg.

2. For leaves taken from the middle third of current year’s extension growths in late August, a total of 100 leaves should be picked from at least five randomly selected trees. The leaves should be dried, ground, thoroughly mixed, and a single sub-sample taken for analysis. In any given season the 80% confidence limits on the resulting determinations will be ±0·22% N, ±0–01% P, ±0·17% K, ±0·18% Ca, and ±0.03% Mg, all in terms of dry matter.

It was noted that according to soil and leaf analyses the nutrient levels in the majority of the orchards were at least adequate for fruit growing. There was no relationship between any single nutrient concentration in soil or leaf and tree performance.     

The influence of tree training and plant density on growth,yield components and yield of Arabica coffee cv. Ruiru 11

Assessment of tree growth and yield components of Coffea arabica L. cv. Ruiru 11, as influenced by tree training on single or two-stem systems at densities between 1600-4800 trees per ha during the first production cycle was carried out at Ruiru, Kenya over three years. Trees trained on the two-stem system tended to be taller than those on singlestems. Although the results were not clear, trees at low plant densities had thicker stems irrespective of the training system. The total number of primary branches was significantly higher on trees trained on the two-stem system. However, bearing primaries and productive wood were higher on the single-stem system during the first year of production. The number of bearing primaries and productive wood increased in subsequent production years on the two-stem system. Training trees on the two-stem system significantly depressed yields in the first production year by 245 to 842%, irrespective of plant density. Yields of clean coffee increased significantly with tree density irrespective of the tree training method. Yields over the production period increased by 33, 45,27 and 9% with increases in planting densities from 1600 to 2400, 2400 to 3200, 3200 to 4000 and 4000 to 4800 trees per ha, respectively. However, yields increased at a decreasing rate at densities above 3200 trees per ha. The proportions of the large grade ‘A’ sized coffee beans were not significantly affected by the treatments. It was concluded that it is possible to raise two stems during the first production cycle of ‘Ruiru 11’, although this might not be economical. Single stems would be preferred and a high plant density of between 3200-4000 trees per ha would be optimum during the first coffee production cycle.     

Effect of hail nets on the microclimate,irrigation requirements,tree growth,and fruit yield of peach orchards in Catalonia (Spain)

Summary

Hail nets protect orchards from storm damage, but also offer potential water savings for fruit trees grown in arid areas. We investigated the effects of black nets on water use, growth, yield, and fruit quality in peach trees growing in Catalonia, Spain, over 2 years (2006 and 2007). Water savings were quantified by irrigating the netted trees to a similar midday stem water potential (Ψ_stem) as the control trees. This method was only partially successful in 2006, with the netted trees being over-watered early in the season (i.e., higher Ψ_stem than the controls), and under-watered (i.e., lower Ψ_stem than the controls) later in the season. A combination of water stress and a hail net resulted in lower yields because of a smaller fruit size. Skin colour was more enhanced in control fruit. In 2007, Ψ_stem values were similar between treatments. Any difference in tree performance was therefore due to the hail net per se. Fruit size was similar between treatments, and the lower yields of netted trees were due to decreased fruit number. There were no treatment differences in fruit skin colour or flesh firmness. Soluble solids concentrations were higher in nonnetted fruit. In both years, water savings amounted to approx. 25%, if irrigation replaced the potential evapotranspiration that was measured. Hail nets are therefore recommended for their effects on water saving in arid areas, despite lower fruit yields.     

An Experiment in Pruning Cox’s Orange Pippin Apple Trees in Alternate Years

Five pruning treatments were compared on Cox’s Orange Pippin on M.II rootstock, viz.

1. Open-centre tree, established-spur pruned

2. Open-centre tree, renewal-pruned

3. Delayed open-centre tree, established-spur pruned

4. Delayed open-centre tree, renewal-pruned

5. Regulated-pruned tree.

At 15 years, secondary pruning treatments were begun and carried on for a further 7-year period, the trees being pruned by the same methods but either annually or in alternate years. Differences in growth and cropping between trees pruned to open-centre and delayed open-centre shapes were small, but open-centre trees cropped more heavily than did delayed open-centre trees when renewal-pruned, probably on account of their greater branch spread. The method used in pruning the laterals had an effect upon cropping during the first 10 years. At the end of the trial there was no effect of either primary or secondary treatments upon trunk girth, neither was there a significant difference in girth increment between the secondary treatments. Almost the same amount of wood was removed from trees pruned in alternate years as from those pruned annually, but a greater proportion of old to new wood was removed from trees pruned in alternate years. Pruning in alternate years increased the cropping of established-spur pruned trees but not with regulated or renewal-pruned trees. There was no consistently adverse effect upon fruit size but, when a year of heavy setting followed a winter when the trees were left unpruned, fruit size was reduced and early fruit thinning would have been necessary to lessen the proportion of fruit in the two smallest size grades. With this variety fruit colour was unaffected by pruning in alternate years.     

Effects of tree age at planting,root manipulation and trickle irrigation on growth and cropping of apple (Malus pumila) cultivar Queen Cox on M.9 rootstock

Summary

The apple cultivar Queen Cox on M.9 rootstock cropped more precociously when planted as two year old trees than when planted as one year old trees, even though there were no significant differences in the sizes (leader height and branch length) of the trees at the time of planting. However, the two year old trees had larger root systems at planting. As the trees aged, those planted as one year olds grew more vigorously and bore higher cumulative yields than those planted as two year olds. Annual root pruning of the trees, commencing 15 months after planting, reduced extension shoot growth, crown volume and grubbing weights (final fresh weights of scions) severely. In some seasons root pruning increased the number of spur and terminal floral buds produced and also the final sets and yield efficiencies on the treated trees. Planting trees within semi-permeable fabric membranes also reduced extension shoot growth and tree size, but less severely than the root-pruning treatment. Root restriction increased the efficiency of fruit set and yields and also improved the grades of fruits produced. Trickle irrigation treatments increased shoot growth and tree fresh weight at the time of grubbing, but had inconsistent and small effects on fruit set and yields. Interactions between tree age at the time of planting and the root manipulative treatments were significant.     

Performance of ‘Discovery’ apple on M.9, M.26 and MM.106 rootstocks at two densities

Summary

The apple rootstocks M.9, M.26, and MM.106 were evaluated for their efficiency in bringing ‘Discovery’ apple trees into production. The experiment, carried out over a ten- year period compared two planting densities at 1666 and 3333 trees per ha. Tree vigour differences between rootstocks were measured in term of trunk growth, tree volume, weight of branches pruned off and final weight of the above-ground parts of the trees. Fruit production is presented both as total yield and as the weight of first class fruit. Fruit colour development is also shown. Cropping efficiency is calculated and presented in relation to the different vigour measurements. The results confirm that ‘Discovery’ is slow to come into production. M.9 was the most productive rootstock, but due to vigour differences MM.106 gave the same yields per tree, although the latter had the lower yield efficiency. M.26 performed poorly; its vigour was similar to M.9 but it produced the lowest yields.     

The Response to Epsom Salt Sprays of Mature Apple Trees of Three Varieties on Two Contrasting Rootstocks

Mature trees of Beauty of Bath, Tydeman’s Late Orange and Laxton’s Superb on M.VII rootstock, growing under commercial-type conditions, were severely deficient in magnesium as shown by leaf analysis and typical symptoms. Five sprays yearly of 2% Epsom salt (MgSO₄.7H₂ O) raised the concentration of magnesium in the leaves and largely eliminated symptoms: the concentrations of calcium and, to a lesser extent, of potassium in leaves were reduced. Over six years, there was little effect of Epsom salt on growth as measured, but crop weight was increased by 19–39% according to variety: this was largely, but not entirely, due to increase in fruit set. The proportion of fruit that dropped was decreased for Beauty of Bath and for Tydeman’s Late Orange sprayed with Epsom salt and the weight per 100 fruits of the late varieties Tydeman’s Late Orange and Laxton’s Superb was increased. Untreated trees on M.II rootstock, in contrast to those on M.VII, showed almost no foliar symptoms of magnesium deficiency, made more growth and produced larger crops; the concentrations of magnesium and calcium in the leaves were higher whereas potassium was usually lower. Epsom salt sprays did not increase growth or cropping; indeed, there was a tendency for sprays to reduce the crop weight of Beauty of Bath on M.II, indicating that they should not be used indiscriminately.

Epsom salt foliar sprays increased the “available” magnesium in the soil to a level considered to be medium—high; this effect was mainly shown in the top 6 in. of soil and gradually diminished to the 18–24 in. zone. Nevertheless, in the year following the termination of treatments, the proportion of foliage affected in Laxton’s Superb on M.VII, previously sprayed with Epsom salt, was almost as great as that of untreated trees and the concentration of magnesium in the leaves was as low. It appeared that magnesium in the soil did not reach the leaves.

The increase in “available” magnesium due to sprays was greater in soil carrying trees on M.II than M.VII, a result probably due to the extra quantity of spray required for these larger trees.     

Influence of rootstock on orchard productivity and fruit quality in peach cv. ‘Redhaven’

Summary

We have evaluated the influence of four different peach rootstocks (Prunus pumila,‘GF 655/2’ ,‘Missour’ and ‘GF 677’ as a standard) planted in medium-heavy to heavy soil on phenological events (flowering and harvest date), growth, yield, fruit quality and mortality of peach (P. persica L.) scions of cv. ‘Redhaven’. At the end of flowering, there were no significant differences between the different rootstocks in the years 1999, 2000, 2003 and 2004. In 2001, trees grafted on P. pumila reached the end of flowering 2 d before trees grafted on the other rootstocks. ‘Redhaven’ fruits ripened on the same day on trees grafted on the different rootstocks. The differences were only between years. The results showed that significantly less vigorous trees, according to their trunk cross-sectional area (TCSA) and canopy volume, with the lowest yield, were observed on P. pumila rootstock. The rootstock P. pumila had a significant negative influence on fruit dimensions and mortality was 50%. Trees grafted on ‘GF 655/2’ rootstock produced significantly lower yields than those grafted on ‘GF 677’ or ‘Missour’ rootstocks. Only the ‘GF 655/2’ rootstock had root suckers. Trees grafted on ‘GF 677’ rootstock were significantly more vigorous than on the other rootstocks, and produced the highest yield with good fruit quality. The final canopy volumes of peach trees grafted on the different rootstocks were very different. The most vigorous trees were those grafted on ‘GF 677’ rootstock, where the final tree volume was 12.8 m³. Trees grafted on P. pumila and ‘GF 655/2’ had significantly lower TCSA values than on the other rootstocks.     

Annual dry-matter production by three apple cultivars at four within-row spacings in New Zealand

Summary

Annual dry-matter production was estimated from eight year old apple trees of ‘Fuji’, ‘Braeburn’ and ‘Royal Gala’ on rootstock M.9, trained as slender spindles, in a spacing trial with trees spaced at 1.3, 1.5, 1.9 and 2.4 m within the row with tree rows 3.5 m apart. Light interception was recorded at intervals during the season. Both fresh weight yield and total dry-matter production were linear functions of light interception, with a higher slope recorded for the two late season cultivars compared with the early maturing ‘Royal Gala’. Comparison of the relationship between mid-season light interception and apple yields with published values from elsewhere, reveals that the slope of the relationship is enhanced under New Zealand conditions. The relationship suggests an upper limit of yield of 169 t ha^–1 at 90% light interception, slightly higher than the maximum yield recorded of 163 t ha^–1. Energy conversion of intercepted light over the growing season into dry-matter production was similar to that observed in the United Kingdom, suggesting that the major climatic advantage in New Zealand over the United Kingdom is the higher growing season radiation integral. Dry matter was allocated in the order fruit>woody tissue>leaves, with partitioning to the fruit of 74, 72 and 63% for ‘Braeburn’, ‘Fuji’ and ‘Royal Gala’, respectively. Under New Zealand conditions, sustainable 70% harvest index values may be achievable for mature apple trees on dwarfing rootstock with late maturing cultivars.     

Effects of ethephon and 2,4,5-T on fruit size,rind pigments and alternate bearing of ‘Imperial’ mandarin

Ethephon and 2,4,5-T were used on ‘Imperial’ mandarin to study their effects on fruit thinning, fruit size, rind pigments and alternate bearing.Ethephon at 250 p.p.m. produced the largest and heaviest fruits. Pigment levels were not affected by 2,4,5-T, but ethephon increased carotenoids and reduced chlorophyll a and b levels. The effectiveness of a colouring agent seems to be related to its ability to reduce both levels of chlorophyll a and b provided that carotenoid levels do not decrease.Treated trees exhibited less tendency towards alternate bearing than control trees. Ethephon had a greater thinning-effect than 2,4,5-T. It also evened cropping for 2 successive years. One spray of ethephon in an “on” year would be effective in reversing and leveling cropping of ‘Imperial’ mandarin for at least 4 years under Mildura conditions.     

Carbaryl as a Fruit Thinning Agent

The results are presented of fruit thinning studies carried out over several years with the apple varieties Laxton’s Superb and Worcester Pearmain.

The thinning activity of carbaryl was negligible at 0·0028% a.i. but increased rapidly over the range 0·0083% to 0·075%. Comparatively little additional thinning resulted from a further increase in concentration.

In Worcester Pearmain thinning was stimulated by applications of carbaryl over a 4–5 week period from petal fall, the cessation of thinning activity coinciding with the end of the main period of natural fruitlet shedding. The intensity of thinning was greatest at petal fall and declined with delay in application. This decline was not continuous but showed two intervals of 7 to 13 days duration during which the intensity of thinning remained relatively constant. Up to 12 days elapsed between application and the commencement of induced fruitlet shedding.

Carbaryl was found to thin within, rather than between, fruitlet clusters, and to stimulate the abscission of lateral fruitlets more than terminal ones.

The growth of young fruitlets was retarded for a short period following the application of carbaryl, but as a result of thinning the size of mature fruit was increased. In the year after treatment increased blossoming and cropping was most marked with biennial trees of Laxton’s Superb. With regularly cropping trees of Worcester Pearmain, although thinning induced a moderate increase in subsequent blossoming, no residual effects on cropping were detected.     

Apple Mosaic Viruses Host Reactions and Strain Interference

The annual growth of an apple tree in the vegetative condition is distributed in a definite pattern between leaves, stems and roots ; when a crop is borne, this constitutes an additional end-point for growth materials. The disturbances in the pattern of vegetative growth caused by this additional region of utilization are described. Two-year-old apple trees were deblossomed at flowering or defruited on 30th May, and their growth was compared with that of cropping trees. The increments in weight were estimated by samples taken at treatment times.

After deblossoming, the trees made extra growth in terms of dry weight, more and larger leaves, and longer stems. The periods of greatest and least growth were the same as for cropping trees. Defruiting also resulted in extra growth, but this came later, and so was out of phase with the growth of fruiting trees. This extra vegetative growth (including more trunk thickening and root growth as well as leaves and new shoots) weighed more than the crop on the fruiting trees. Defruiting had slightly less effect than deblossoming.

The diversion of metabolites to the crop changed the pattern of growth in the rest of the tree. Cropping trees had 50% more leaf proportionate to total vegetative increment, and 50% less root, than deblossomed ones. The intervening regions varied roughly according to their position. Despite this greater leafiness of cropping trees, they produced significantly more total dry matter (vegetative growth plus crop) per unit area of leaf.

These results are attributed to the greater demanding power of the growing crop in diverting photosynthates from the lower parts of the tree and in increasing the rate of removal of photosynthates from the leaves.     

Productivity of olive trees with different water status and crop load

SUMMARY

A field experiment was conducted over two growing seasons to determine the combined effect of crop load and irrigation on yield components of olive trees (Olea europaea L. ‘Leccino’) planted at 6 m 3.8 m in a sandy-clay soil. Different crop loads were established by manual thinning of fruits. Drip irrigation was managed to maintain pre-dawn leaf water-potentials (PLWP) within the following ranges: (i) higher than –1.1 MPa (FI; fully irrigated); (ii) between –1.0 and –3.3 MPa (DI; deficit irrigated); or (iii) below –1.2 MPa, but not lower than –4.2 MPa (SI; severe deficit irrigated). The irrigation period lasted from 6 – 16 weeks after full bloom (AFB) in 2003, and from 5 – 19 weeks AFB in 2004. In 2003, full bloom was on 26 May; in 2004, it was on 3 June. Neither irrigation regime nor crop load affected flowering or flower quality the following Spring. The combined fruit yields [on a fresh weight (FW) basis] over both years in SI and DI trees were 49.0% and 81.6% of FI trees, respectively. The oil yields of SI and DI trees were 52.5% and 81.2% of FI trees, respectively. Fruit FWs in FI trees were greater than those of DI or SI trees at 8 weeks AFB. At harvest, FI trees bore the largest fruits, and SI trees the smallest fruits. The FWs of individual fruits at harvest in the FI and DI treatments decreased as crop load increased, but no such relationship was apparent for SI trees. The oil content of the mesocarp increased as PLWP increased from approx. –3.5 MPa to –1.5 MPa. The oil content of FI trees at harvest decreased from 53.1% to 45.7% dry weight as fresh fruit yield increased from 5 – 25 kg dm^–2 trunk cross-sectional area. However, crop load did not have any effect on the oil content of the mesocarp in DI trees. Fruit maturation was delayed by irrigation. Maturation index also decreased (indicating delayed maturation) as the crop load on FI or DI trees increased, but did not vary with crop level in SI trees.     

Induction of roots on jujube softwood cuttings using Agrobacterium rhizogenes

Summary

Cuttings from trees of jujube (Ziziphus jujuba Mill.), particularly mature ones, root with great difficulty. A greenhouse experiment assessed the effects of two strains of Agrobacterium rhizogenes (A4 and TR105) on softwood cuttings from two sources: ten year old trees of cv. Li and a tree approximately 40 years old of the cv. Contorta. The cuttings were collected and trimmed to 7.5 cm. Ten weeks after inoculation, both strain and source of cutting influenced visible symptoms of infection. Jujube cuttings were more responsive to TR105 than to A4, and the ‘Li’ cuttings were more responsive than ‘Contorta’ cuttings. Strain TR105 was very effective in increasing rooting percentages and root number. ‘Li’ cuttings had better rooting percentages, greater root number, and greater root length than did the ‘Contorta’ cuttings. Inoculation with A. rhizogenes may also possess great potential for promoting adventitious root formation in other difficult woody ornamental or fruit tree species.     

The Residual Effects of Nitrogen,Calcium and Soil Management Treatments on Yield,Fruit Size and Composition of Citrus

A long-term field experiment testing the effects of 4 soil management treatments and 4 levels of ammonium sulphate on Washington Navel and Late Valencia oranges, budded on rough lemon or sweet orange rootstocks, was terminated in 1965. At this stage the difference in crop yield between the worst and best treatments was about 150%; average annual yields for the 4-year period 1962–65 varied from 18.3 to 46.2 t/ha according to treatment. After 1965 remedial N and Ca treatments were applied to restore the experimental area to a uniformly high level of productivity.

For the 2-year period 1968–69, average annual yields from plots previously receiving the 4 ammonium sulphate treatments varied from 39.1 to 44.6 t/ha; uniformity in terms of yield had thus largely been achieved. Slightly less success, however, was achieved in terms of fruit quality.

Lime, applied to counteract soil acidity induced by the previous ammonium sulphate treatments, produced greater responses on tilled plots than on non-tilled plots. The smaller response to lime on non-tilled plots was largely due to citrus roots being close to the surface and able to absorb applied fertilizer P before fixation could occur.

Some trees, receiving a bare-surface chemical weed control treatment and no N since 1947, were in severe decline in 1965 ; however, they were restored rapidly to full vigour by a midwinter application of 255 kg N/ha in 1966. Furthermore, these trees produced high yields in 1968 when the rest of the orchard was in an ‘off’ year, thereby suggesting that suitable annual adjustments to levels of applied N could be a means of limiting or controlling biennial bearing.

Soil management treatments had no significant effect on the size distribution of the fruit, but the nil and highest N treatments, applied until 1965, had an adverse effect on the size grade (63–75 mm diam.) preferred commercially.