There are many similar top working examples too numerous to mention. Eliminating the lemon trunk, which was usually so susceptible to shellbark, appeared to have the same effect as high budding and eliminated the shellbark. Trees affected by known diseases probably under no circumstances should be top worked. In some instances top working may result in an incompatibility. For example, in 1976 while visiting Japan, the author was shown some satsuma trees on trifoliate orange root stock which had been top worked to Uwa pummelo. An incompatibility occurred at the juncture of the Uwa pummelo and the satsuma. In other instances, the top working may result in the susceptibility to a disease which would not have affected the original combination prior to the top working. To illustrate the point, lemon trees on sweet orange root stock are not susceptible to tristeza. However, if the combination was to be top worked to sweet orange the new combination would be susceptible to tristeza . Trees showing symptoms of tristeza should not be top worked to lemons; but susceptible trees can be top worked successfully if done before tristeza occurs . Growers in Spain, until the occurrence of tristeza, probably did more top working than in any other citrus area of the world. Citrus diseases are easily spread through top working, either by the mechanical means of contaminated tools, or the use of scion varieties in which symptoms of the presence of a pathogen were not expressed. In 1983, Dr. Luis Navaro, Director of INIA at Moncada , Spain,vertical hydroponics told a group of participants at the First International Society of Citrus Nurserymen that of all the Spanish citrus cultivars entered into the new budwood certification program established in Spain, over 50 per cent of the cultivars were carrying three or more viruses.

To make this information more available, Platt prepared a handout on citrus top working compatibility. It is reproduced intact in Table 12. While Platt prepared this information specifically for California growers, citrus growers in other areas will find that many of the recommendations can be appropriately applied to their orchard conditions as well. The largest related incidence of top working in the California citrus industry occurred in the early 1960’s. The incident had nothing to do with tristeza directly, but was associated with the rapid increase in citrus plantings in the 1950’s, particularly in the San Joaquin Valley, following the destruction of orchards in the Los Angeles basin by tristeza. The causal factor was a citrus bud variation referred to as the “acid-navel.” . This was a mutant of the ‘Frost’ navel which was so extensively propagated at that time. The variant was a fruit mutation characterized by smaller fruit, coarser rind, and other undesirable external characters. However, the interior of the fruit had a more watery-appearing flesh with juice of a higher than normal citrus acid content; hence the term “acid-navel.” The fruit characteristics were completely unacceptable for commercial purposes. Since the trees were disease free and no incompatibilities were involved, most of these trees were top worked, generally to another navel orange of the grower’s choice. Hundreds of acres, literally thousands of trees, were top worked. The episode stresses the importance of nurserymen cutting buds for propagation from fruiting trees so that the fruit characteristics of the scion variety can be observed. Inter stocking of citrus trees has not always proved useful. Diseased trees should not be top worked, and when and if top working is necessary, it should be with full knowledge of the consequences involved.

The fact that citrus inter stocks, except for preventing certain incompatibilities, have had so little effect on the physiological response of the scion should not imply that they may never be successful. Most citrus inter stocks have been genetically close and oftentimes commercial varieties. Maybe there are non-commercial citrus cultivars or citrus relatives which may produce desirable effects. The success of inter stocks with a tree crop like apples cannot be overlooked. Coincident with any discussion of inter stocks are other horticultural procedures, which may not be so difficult to perform, but perhaps could produce effects similar to inter stocks, and actually be considered as modified inter stocks. The first of these procedures is the ring graft. Considerable interest was generated in the 1930’s through the 1950’s in the technique . Much of the work was done with apples and other tree crops. The procedure is accomplished by using knives such as linoleum knives or grape knives with a curved point at the top of the blade. The knives may have spacers between the blades or handles so that parallel cuts can be made around the trunk or branches of the tree. Considerable pressure is put on the cutting implement so the blades penetrate through the phloem . It is impossible to control the pressure and, as a result, the knife blades cut not only through the phloem but also through the cambium and into the xylem elements. Penetration into the xylem may easily range from 1/32 to 1/8 inches . Thus, not only are the phloem elements severed, but xylem elements also, resulting in a xylem block as well as a phloem block. A single cut through the bark of the tree trunk is referred to as girdling or ringing . The technique is used almost exclusively to increase fruit set in navel oranges, although it is occasionally practiced with some mandarin varieties. The cut is not protected in any manner from desiccation. The resulting severance of both the phloem, cambium, and xylem tissues apparently heals quickly. Furr, Reece, and Hrnciar found there is a return to normal nutrient absorption two weeks after girdling, which suggests that the break in the cut tissues may be at least partially healed over in two weeks.

Curtis and Clark reported that regeneration of new phloem within 10 to 15 days resulted in translocation so nearly normal that no difference could be measured between control and treated shoots in analysis for sugar, starch, nitrogen, and ash. Murneek showed that apple fruit set was increased an equal amount whether girdling affected carbohydrate and nitrogen levels or not. Thus, carbohydrate and nitrogen levels appear to be of little importance in single, knife-cut girdling . Sax reports that phloem blocks induced by killing a small section of the stem with heat prevented the downward movement of vitamins, organic carbohydrates, and grow-regulators . Bonner , in girdling stems of tomato plants, found that in a few days there was an accumulation of thiamin, pyridoxine, pantothenic acid, riboflavin, sucrose, and nitrogen above the girdle. Lewis and McCarty state that it appears possible that the severance of phloem tissues causes a temporary increase or decrease in a growth regulator influencing fruit set. Much of the effects of girdling have thus been done with a single cut. However, other studies of girdling physiology have involved more severe cuts , including the removal of a band of bark from the tree trunk. This is what happens with a ring graft. With a ring graft the strip of bark which is removed from around the tree trunk is immediately replaced with a comparable sized strip of donor bark. The donor strip in this instance is replaced in the proper polar orientation. The ring graft can consist of a bark strip from sweet orange, sour orange, Rough lemon, sweet lime, trifoliate orange, Morton citrange, whatever the researchers’ choice. In unpublished data by the author, plastic electrical tape, or duct tape, was used to hold the donor bark strip in place and prevent desiccation. Since the procedure of a ring graft is somewhat more severe than a simple girdle, the healing process may be a little longer, perhaps weeks, or even months. When the ring of bark is removed from the parent tree, the bark separates at the cambial tissue. Enough cambial initials remain to form new xylem elements to the inside and new phloem tissues to the outside. The strip of donor bark inserted into the removed bark area also has cambial initials on its inner surface. Again,hydroponic vertical farming systems xylem elements are found to the inside, phloem elements to the outside . Roberts was one of the pioneers to explore the use of ring grafts in apples. The author was a graduate student of Roberts in the early 1940’s and recalls in several discussions that Roberts was quick to point out that the effects did not last since the donor bark strip soon became nonfunctional and sloughed off as with a cork cambium. Sax extensively investigated the aspect of bark inversion of the donor bark strip so that the polarity of the conducting cells is at least temporarily reversed and transport of nutrients, metabolites, etc. reduced. Sax considered inverted bark strips as phloem blocks. Reversed polarity is only a temporary problem in bark regeneration . Sax , referring to the work of Roberts , states that the inverted rings checked the growth of the trees, but without the deleterious effect resulting from girdling. He further emphasizes that apparently this work was not continued by Roberts. The author had the pleasure of a long discussion with Sax in 1957, and Sax admitted that the effects of bark inversion were temporary and had very little overall effect on tree growth. In 1956 , the author had conducted some bark inversion experiments with a grower cooperator. The trees were young lemon trees of uncertain destiny since the cooperator indicated the orchard would be removed in one to three years. Replicated treatments were made and the size of the tree trunks above the girdled area measured. The treatments consisted of different widths of inverted bark from 1/4 inch to 2 inches . In some treatments, height of the ring insertion varied. In other treatments, a double ring of bark was inverted, approximately 10-15 cm apart.

Another treatment consisted of an incomplete inverted ring with a 2.5 cm wide strip of normal bark at 180°. See Figures 11 – 21 [Image for Figure 21 could not be located]. Observations were limited because the orchard was pulled before final measurements could be made. In no instance was there any observable difference in tree growth with any treatment as compared to the checks. The width of the inverted ring did not seem to make any difference, the 5 mm was as effective as the 5 cm. Perhaps the use of two inverted rings had more effect than a single inverted ring. The most notable effect was with the incomplete rings where small islands of normal bark were left at 180°. The effectiveness of the blocks was clearly demonstrated since the nutrients and metabolites were translocated to the two normal bark strips resulting in accelerated growth increase in that area. The use of ring grafts using other varieties or species of citrus genetically different from the parent tree produces no permanent effect on the parent tree. Imposing the additional effect of bark inversion to alter polarity is also a temporary effect. Since nearly all commercial citrus trees are budded, it is not surprising that, like other budded or grafted fruit tree combinations, they may be subject to incompatibilities. The terms compatible and congenial refer to healthy long-lived combinations, and incompatible and uncongenial refer to combinations which are not healthy and long-lived. Sometimes the generalization is made that compatibility is a function of the closeness of genetic relationship between the stock and the scion, but there are certainly many exceptions to this hypothesis. If the generally accepted systems of citrus classification are used, there is a wide diversity in the germplasm available within the conservative classification of Swingle , who lists 16 species, and that of Tanaka , who lists 159 species. However, within the Family Rutaceae, subfamily Aurantioideae, there are a total of 33 genera available with considerable interspecific and intraspecific crosses and bigeneric and even trigeneric hybrids available. It is possible to graft any Citrus with Citrus, and apparently many genera with many other genera, but the degree of success may vary greatly. While the author is unaware of Citrus being successfully grafted on any genus outside the subfamily Aurantioideae, he has kept orange buds alive for one year when grafted on Casimiroa edulis, but the buds did not push. In a 1977 conversation with Professor Giovanni Fatta Del Bosco of Palermo, he indicated that mandarin scions will grow, but poorly, on this cultivar.