Herrero made extensive studies of the size and areas occupied by fibers, vessels, parenchyma and rays in the xylem of hardy fruit trees. The width, distance between, and relative numbers of uniseriate and multiseriate rays in the cambium were also determined. He could not correlate any of these anatomical characteristics with incompatibility. Robitaille and Carlson , working with five apple species on six clonal root stocks, also report no significant correlation between anatomical characteristics and graft take. However, when histograms of the poorer groups were compared to those of the better groups, their results did contradict those of Herrero , since they found that when the xylem structure of two species is sufficiently different, those two species will not intergraft with much success. With regard to citrus, Mendel and Cohen in Israel studied the starch level in the trunk as a measure of compatibility between stock and scion. They found no clear indication of impaired carbohydrate movement through the bud union of Shamouti orange on seven root stocks inducing low growth vigor. The starch level in the bark and wood of the root stock portion of the budded trees was negatively correlated with tree size. They concluded that the starch level results from the growth vigor of the tree, and is not a factor which affects it. In California, Jensen, Wilcox, and Foote found greater amounts of starch above the bud union of lemons on sour orange stock as compared to those on grapefruit. This again is probably the effect of impaired translocation. Mendel and Cohen said that an early evaluation of orchard tree performance is possible if clear correlations can be established between the behavior of, or rate of,vertical hydroponic farming physical processes in a young root stock or budling and the later performance of the budded trees. This certainly will not work in the case of delayed incompatibility or virus-induced incompatibility.
They also found that the total phenolic compounds in Citrus buds and branches accumulate with scion growth, reach a peak with growth cessation, and decrease during the rest period between growth flushes. TPC, rather than phenols, exhibit highest correlation with growth. There was a significant inverse relationship found between the TPC level in the bark of young seedlings and with the size of five-year-old root stock seedlings, and with budded trees three years after budding on these root stocks. They concluded the level of TPC in the bark of young seedlings eight to ten months old is a fairly reliable indication of the performance of young trees until entering bearing age. In Australia, Bevington, Greenhalgh, and McWhirter used reciprocal ring grafts of trifoliate orange and sour orange with Eureka lemon to study the incompatibility problem. They found an initial disturbance of tissue in the outer phloem of affected trees as early as two months after grafting. Whether this technique is suitable to study incompatibility in other combinations, or in a delayed incompatibility, is unknown. However, better indices of ultimate root stock performance are needed. Compared to the amount of anatomical work done on incompatibilities of deciduous fruit trees, the number of anatomical studies on similar problems with citrus has been relatively little when one considers the size of the industry and the problems that confront it. Most of the anatomical work on mature trees pertinent to bud union disorders has been done at the Citrus Research Center in Riverside, California. Without going into detail, the most informative of these are: Schneider on the anatomy of tristezaaffected trees, Schneider on phloem studies of sour orange, Schneider on effect of trunk girdling on phloem, Schneider on phloem studies of sweet orange, Schneider on ontogeny of lemon bark, Schneider on sour orange necrosis, Schneider and Wallace relevant to anatomical studies of top worked trees, and Schneider on dealing with a bud union problem on C. macrophylla root stock.In addition, a better understanding of the anatomical problems involved with the incompatibilities associated with lemon tree decline may be found in Schneider et al. . Also, investigations of graft incompatibility among Persea species by Aaouine has an excellent discussion of tree crop incompatibilities and hundreds of references.
False bud unions occasionally occur in citrus, but their cause is not always definable. For example, in some areas of California subject to heavy surface flooding, bud unions may have been buried under a foot or more of soil and for obvious reasons never excavated. In looking for such bud unions in association with tristeza studies and proper root stock identification, sometimes constrictions in the scion variety under the soil line closely resembled a bud union. In another case, an orchard was sprayed with 2-4-D for weed control purposes and apparently over dosed. At the soil line, the root stock was severely constricted and appeared to be a second union. The author has grown most of his research trees and exercised close surveillance over their propagation, yet occasionally a tree has appeared to have an intermediate stem piece. In Spain during 1968, the author also observed several trees in an orchard which appeared to have an inter stock , but the grower assured him the trees had never been top worked. While such incidences are rare, they may occasionally occur. Usually the external location of the bud union agrees exactly with the internal location. The author has observed, however, some bud unions in which the external appearance of the bark of the stock is so distinct from that of the scion variety that the external union of the two tissues is 4.5 cm away from the internal union, and in some instances more . Also, it is not uncommon to find scion suckers arising on the root stock and as much as 2.5 to 5 cm below the union, particularly with trifoliate orange stock and its hybrids . One might attribute some of these occurrences to double budding and eventual sprouting of the latent buds, but the frequency and diversity of such observations makes one wonder if all such observations can be reconciled with such a procedure. Occasionally, graft hybrids arise at the bud union and result from an adventitious bud which initiates from the union of stock and scion tissues. Examples of this phenomenon are the Kinkoje-unshiu and Kobayashi-mikan of Japan. The famous “Arancio bizzarria” of Nati is another illustration and so is that of sweet lime plus sour orange described by Casella . Probably graft hybrids occur more frequently than suspected, but the common cultural practice of removing suckers from the bud union area most likely eliminates most of them before they are recognized as such. The possibility of using certain graft hybrids as root stocks should be considered.
They would of course have to be vegetatively propagated. Lee first pointed out that Valencia oranges in the Philippines had poor bud unions on Calamondin. He also pointed out that Oneco mandarin had good bud unions on this stock. DeLeon indicated that Tahiti lime, pummelo, grapefruit, and lemon did poorly on this stock in the Philippines,vertical gardening systems but recommended it for limes other than Tahiti, as well as for mandarins. Temple orange and Marsh grapefruit were designated as incompatible on calamondin by Traub and Friend and Friend, Mortensen, and Stansel . Later, Friend and Mortensen classified the calamondin as an unreliable root stock in the Lower Rio Grande Valley and in the Winter Garden area of Texas. In California, Webber pointed out that Eureka and Lisbon lemons, Washington navels, Valencia orange and Marsh grapefruit did very poorly on calamondin and that many of the trees died at an early age. He makes no statement regarding the bud union, which usually shows some external symptom of the disorder, and failed to point out that Satsumas on this root stock performed satisfactorily. Marloth mentions that in South Africa, Marsh grapefruit trees on calamondin made such poor growth that they were removed after two years. Nagami kumquat trees on calamondin root stock were compatible according to Traub and Friend . Tanaka states that the calamondin was used as a root stock in the Changchou area of China, although it was not specified as with what scions, but Webber implied that it was probably with mandarin scions. Baker reported that Meyer lemon did well on calamondin. Olson reports that kumquat hybrids have not done well on sour orange root stock, and Robinson and Savage indicated that in Florida limequats succeeded on any common stock except sour orange. Traub and Friend stated that calamondin and Eustis limequat were uncongenial on sour orange root stocks. A bud union disorder of calamondin was called to the attention of Weathers and Calavan when it was noticed in a citrus variety collection at Santa Barbara, California. Affected trees were found on root stocks of grapefruit, sweet orange, and Troyer citrange. Subsequently, other affected trees were discovered on root stocks of trifoliate orange, sour orange, and lemons in other citrus areas of California. Almost all calamondin trees over four years of age, and on roots other than their own, show the abnormality. Calamondin trees growing as seedlings or cuttings show little or no evidence of the abnormality. They concluded it was a bud-perpetuated, nontransmissible disorder. The author observed bud union abnormalities with Valencia orange trees on calamondin in the tristeza plantings at Baldwin Park , and in root stock plantings made in the Coachella Valley of California , but the trees were not in distress after 13 years of age. However, Marsh grapefruit and Eureka lemons on calamondin in the desert area were in severe decline within three to four years, but Dancy tangerines on calamondin stock performed well. A similar bud union disorder of both old-line and nucellar-line Red Blush grapefruit on calamondin and other kumquat hybrid root stocks was described in Texas by Cooper and Olson , and Olson . Many other instances of failure of calamondin as a root stock were reviewed by Olson and Webber and will be discussed in more detail later in the text. A bud union disorder with Nippon orangequat on sweet orange was also noted by Weathers and Calavan in California. Similar bud union abnormalities were observed with Eustis limequat on Sunshine tangelo, grapefruit, and sour orange by Olson .
Weathers and Calavan in California obtained no evidence of virus transmission to any inoculated seedlings of sweet orange, sour orange, calamondin, West Indian lime, Sunshine tangelo, Rangpur lime, or Palestine sweet lime when they used buds from severely affected calamondin trees as a source of inoculum. They also grew nucellar seedlings of calamondin and propagated them on root stocks of sour orange, sweet orange, grapefruit, Troyer citrange, Morton citrange, trifoliate orange, Willow leaf mandarin, Wekiwa tangelo, and Rangpur lime. Some of these seedlings were inoculated with buds from affected calamondin trees, others left as checks. Scions from affected old-line calamondin were propagated on seedlings of sour orange, sweet orange, grapefruit, Troyer and Morton citranges, trifoliate orange, Willow leaf mandarin, Rangpur lime, Wekiwa and Sampson tangelos, Ponkan mandarin and Rough lemon. Wekiwa and Sunshine tangelos were also grafted with seedlings of calamondin, and others were inoculated from affected trees. With the nucellar lines on sweet orange, grapefruit, Troyer and Morton citrange, trifoliate orange, and Rangpur lime there were no symptoms of decline after three years. With the nucellar line, calamondin on Wekiwa tangelo, Willow leaf mandarin, and sour orange, both inoculated and check trees declined. In the buds from old-line calamondin, all those except on Rough lemon wilted and many died. With the Wekiwa and Sunshine tangelo on calamondin inoculated with old-line calamondin buds, there were no effects after three years. The time period for observations may have been too short for some symptoms to develop. All this is very confusing. Weathers and Calavan suggest that if a virus is involved, it must be seed transmitted , and either carried in the root stock seedlings of the calamondin seedlings. The causal agent could also be transmissible by a vector or by mechanical means. In Texas, Olson noted Shary Red grapefruit, Webb Red Blush grapefruit, and Valencia trees on calamondin had a bud union disorder. Shary Red grapefruit or Valencia orange on lemonquat showed symptoms similar to those on calamondin.