Within the Rutaceae, subfamily Aurantioideae, Citrus grows poorly on the Tribe Clauseneae, subtribes Micromelinae, Clauseninae, and Merrilliinae. It does somewhat better with the Tribe Citreae, subtribe Triphasiinae, but there are no promising combinations as yet. As might be expected, Citrus grafts best within the subtribe Citrinae, but again, there are many incompatibilities. In this subtribe, Citrus can grow successfully on such genera as Poncirus, Severinia, Microcitrus, Citropsis, Eremocitrus, and Clymenia, either directly, or through or with the use of certain inter stocks. Within the Balsamocitrinae, Citrus has been grafted successfully on Swinglea, Aegle, and Feronia. Conversely, Hesperethusa grows very well on sweet orange, and Pleiospermium grows well on C. taiwanica. A more detailed discussion of the role of citrus relatives as citrus root stocks may be found in this text. This section will deal primarily with incompatibility problems with Citrus on Citrus, and the closely allied genera Poncirus and Fortunella and their hybrids with Citrus. A great deal of reference material is available on graft incompatibilities of tree crops in general. Among the best of these are Hartmann and Kester , Mosse , Herrero , and Scaramuzzi . There are no comprehensive publications dealing exclusively with incompatibilities with Citrus and therefore an attempt will be made to emphasize this area of discussion. Herrero suggested four categories of incompatibility which included the following: 1) graft combinations where the buds failed to grow out; 2) graft combinations where incompatibility was due to virus infection; 3) graft combinations with mechanically weak unions, in which the cause of death was usually breakage, and ill health,vertical grow rack if shown it was due to mechanical obstruction at the union; and, 4) graft combinations where ill health was not directly due to abnormal union structure but was usually associated with abnormal starch distribution. Whether this is the cause of ill health or the effect is not known.
Regardless of whose system of incompatibility categories are followed, it is a recognized fact that Citrus has incompatibilities which may fall into any of these designated groupings. It is essential, therefore, that the types of incompatibility be defined for clarification purposes. According to Hartmann and Kester , translocated incompatibility includes those cases in which the incompatible condition is not overcome by the insertion of a mutually compatible inter stock because some labile influence can apparently move through the inter stock. This type of incompatibility involves phloem degeneration, and can be recognized by the development of a brown line or necrotic area in the bark, and often designated as bud union crease. Consequently, restriction of movement of carbohydrates occurs at the graft union, with an accumulation of starch above the bud union and a reduction below. Usually a swelling of the scion immediately above the bud union results in a bulge, but not always. Reciprocal combinations may be compatible. In the various combinations in this category, the range of bark tissue breakdown can extend from virtually no union at all, to a mechanically weak union with distorted tissues, to a strong union with normally connected tissues. One component of the combination may carry the virus and be symptomless, but the other component may be susceptible to it. Such is the case with tristeza. The sweet orange component alone is not affected by it, and the sour orange component alone is also not affected by the disease. When sweet orange is grafted on sour orange and the virus is present, the adverse reaction occurs. Some lethal metabolic substance is produced in the sweet orange scion which, when translocated to and below the bud union, causes, according to Schneider , hyperplasia and hypertrophy, callusing of the sieve plates, necrotic sieve tubes, and other anatomical abnormalities of the vascular system in the sour orange stock, or in any other tristeza susceptible stock. The reciprocal is not affected, so the toxic substance is not produced in the sour orange leaves, only in certain scions, and only affecting certain root stocks. The use of a resistant intermediate does not prevent the toxic material from being translocated through it . A translocated incompatibility can be introduced into a compatible combination by the introduction into the system of a third component. Any time a tree is top worked, the compatibility of the two previously existing compatible components may change. Eureka lemon trees on Rough lemon root may be considered as compatible and tolerant to tristeza. In a 1976 visit to Japan, the author observed similar difficulties. The most popular tree combination is Satsuma mandarin on trifoliate orange root stock. Due to over planting and overproduction and resulting low prices, growers are indiscriminately top working to miscellaneous new varieties, and numerous incompatibilities are now appearing. Whenever the insertion of a third component is made, the performance of the resultant combination is much in doubt unless previous experience indicates otherwise.
According to Hartmann and Kester , localized incompatibility includes combinations in which the incompatibility reactions apparently depend upon actual contact between stock and scions, and separation of the components by insertion of a mutually compatible inter stock overcomes the incompatibility symptoms. In the incompatible combination, the union structure is generally mechanically weak, with continuity of cambium and vascular tissues broken, although in some instances the union is strong and the tissues join normally. Oftentimes external symptoms develop slowly, appearing in proportion to the degree of anatomical disturbance at the bud union. Root starvation eventually results due to translocation difficulties across the defective graft union. In some cases actual breakage may occur due to the masses of parenchymatous tissue rather than normally differentiated tissues which are commonly found at the bud union. In Citrus, an example of this would be the decline of Eureka lemon on Troyer citrange root stock, first pointed out by Bitters, Brusca, and Dukeshire , and later confirmed by Weathers et al. . Weathers states that this incompatibility cannot be prevented by an intermediate stem piece such as sweet orange. However, this was successfully done by Nauriyal, Shannon and Frolich . The author in 1951 budded Cook Eureka on a Frost Valencia intermediate on Troyer citrange root stock involving numerous trees, and those trees, as of 1987, are still vigorous, healthy, and productive. Mechanically weak unions and breakage occurs readily; for example, Figures 22 [Image could not be located] and, Limoneria 8A Lisbon on Citrus taiwanica and also Frost Lisbon on Ponkan mandarin. There are also delayed symptoms of incompatibility. In these instances, the stock-scion combination grows in a normal fashion for perhaps 15 to 20 years or longer, and then the trees decline. One of the best examples of delayed incompatibility is the socalled “black-line” of walnuts in California, Oregon, and France. Serr reports that when varieties of Juglans regia are grafted onto seedling root stocks of J. hindsii, the northern California black walnut, or onto Paradox roots , affected trees grow normally, bearing good crops, until they reach an age of 15 to 20 or more years of age, then the difficulty begins. This disorder has recently been identified by Mircetich, Refsguard, and Matheron as being caused by a graft-transmitted virus, namely, the cherry ring spot virus. Comparable types of delayed incompatibility occur commonly in citrus. Examples are the decline of Satsuma mandarin on Troyer citrange at 16 to 18 years, Washington navel on trifoliate orange and Morton citrange at 15 to 20 years,vertical planting tower and a lemon bud union disorder of both Eureka and Lisbon cultivars on Sampson tangelo, Cleopatra mandarin, and many other stocks at 5 to 10 years of age.
The role that viruses may play in these disorders is unknown. Hoy, et al. discovered that prune brown line disease was caused by the tomato ring spot virus, and that the organism was spread by the dagger nematode. The disease affected French prune, Empress prune, and President plum cultivars on Myrobalan plum and peach root stocks, but Marianna #2624 was not affected. The above mentioned citrus maladies are accompanied by a swelling or bulge at the bud union and compression girdling results. The swelling and girdling occurs above the union with the lemons on Sampson tangelo, and below the union with the Satsumas on Troyer or the navels on trifoliate orange. While swellings at the bud union are often believed to be a positive indication of incompatibility , in Citrus there are many combinations in which the scion severely overgrows the stock, and others in which the stock severely overgrows the scion, and yet the combinations are healthy, vigorous, and productive at 30, 40, or even 50 years of age. Obviously, other factors may be involved, as with other tree crops. A more extensive discussion of individual incompatibilities will be detailed later in this section. While the actual time required for bud union continuity to take place may vary with the environment, time of budding, and other factors, it is clear from Mendel’s investigations that the process can be essentially completed in 45 days. His observations are in complete agreement with those of Randhawa and Bajwa , who found that the normal union of Malta Blood and Jaffa sweet oranges on Karna khatta root stock was complete in about six weeks. This time factor is in sharp contrast to the work of Malik , who states that with the Valencia sweet orange and Kinnow mandarin budded onto Rough lemon root stock, that practically no union takes place six months after budding and even after a year has lapsed, the bud union is not 100 per cent complete. Mendel also points out that normal wood and bark elements are laid down after the formation of a common cambium; a connection of the vessels and sieve tubes is thus established. Thereafter the exact border between the graft mates cannot be determined. Thus, nothing can be detected anatomically at the border zone between the scion and the root stock which would indicate a disturbance of the sap streams, be it in the wood or in the bark, proving the progress of the union was a normal one. The initial union of Malta Blood red sweet orange on Karna khatta described by Randhawa and Bajwa was normal, but they point out that Bajwa and Singh and Kirpal Singh and Singh indicate that this combination exhibits incompatibility in the orchard, which suggests a delayed type of incompatibility, or perhaps the entry of a pathogen. Whereas the work of Mendel was concerned only with the anatomy of the actual union process, the work of Goldschmidt-Blumenthal was conducted on trees 20 years old. She investigated the bud-union anatomy of Shamouti orange on Rough lemon, sour lemon, sweet lime, citron, Goliath shaddock, Duncan grapefruit, sour orange, Baladi sweet orange, and Shamouti sweet orange. The grafted root stocks did differ in their anatomical structures, especially in the number and width of vessel clusters, the amount of parenchymatous tissue, the structure of rays, and the number of idioblasts. The Shamouti scions also displayed differences in the number and cross-sectional area of the vessels, amount of parenchymatous tissue, size of rays, and the number of idioblasts. The relative water conductive area of the Shamouti scions was markedly affected by the root stock. Shamouti scions on Shamouti, Baladi, sour orange, Rough lemon, and grapefruit stocks showed a good correlation between bud union morphology and the anatomical structure. In the remaining combinations, there appeared to be no correlation. The predicting of compatible or incompatible combinations and ultimate root stock performance by either anatomical, chemical, or other rapid methods would be of great value to the root stock research worker in developing improved root stocks. However, none of these methods have proved satisfactory so far. Various laboratory methods have been developed for evaluating stock-scion incompatibility in young apple nursery trees without growing trees to maturity by Evans and Hilton . These include water conductivity measurements through the graft union, macroscopic evaluation of the external appearance of the graft union, microscopic evaluation of the graft union, and breaking-strength tests. A biochemical method used by Samish was based upon the presence of a glucoside associated with pear-quince incompatibility. There is a low prunasin content in one quince stock and a high prunasin content in another. Lapins was able to detect some symptoms of stock-scion incompatibility of apricot varieties on peach seedling root-stocks by macroscopic examination of discontinuity of inner bark tissue at the graft union of the young trees, but found no other correlation of anatomical structure to compatibility.