The Incidence of Xyloporosis (Cachexia) in Certain Florida Citrus Varieties

CHILDS ET AL: XYLOPOROSIS 77 to several dozen. These efforts that are con species is as great, or greater, than that paid tinuing over the years must be fruitful as they for organic miticide to control another mite. have in the past. In fact, after the adjustment And sulfur also brings on other biological has been made to the Miller Pesticide Residue complications because of its plant and soil Amendment, I think it veryprobable that the residue problems. > production of new chemicals as miticides From a study of the new publications on will again appear as frequently, or even more the biological and ecological factors surround frequently, than before. ing mite control, and through discussions with We know that pesticides sometimes fail with the specialists in this area, it seems that the out evident reasons. Sometimes it takes a effectiveness of miticides often has been al long time to discover these reasons. That may tered considerably by changing environmental be the case in Florida with miticides. It may factors to the point where failure has follow be a long, step-wise study over several years, ed. A great deal of study is going on in this to discover how to use present miticides alone, area and as more knowledge becomes available, or in combination. On the other hand, because it seems certain that the present miticides of favorable weather and other ecological con can be used alone, or in combinations, more ditions for mites in Florida, it may be this effectively, and the known ones in process same difficult situation may develop later on of development will find their places in the and more slowly in other areas. scheme of mite control more readily. As you can understand from the foregoing We all recognize'also, I think, that the new in the developing of a new miticide, the organic miticides are actually a great improve cooperation of independent companies, experi ment overthe inorganic ones, even though ment stations, growers, and many other they are not as yet perfect, nor have we agencies is necessary. We can, therefore, ex learned how to use them with the greatest pect the same continued progress that has efficiency. On a basis of cost per £bund prevailed during the last decade, with only the new miticides are expensive. But com a temporary slow-up in order to become ad paring their cost with that of sulfur for the justed to the new laws and regulations fol control of the citrus rust mite, the actual in lowing the enactment of the Miller Pesticide vestment in sulfur for the control of this one Residue Amendment. THE INCIDENCE OF XYLOPOROSIS (CACHEXIA) IN CERTAIN FLORIDA CITRUS VARIETIES

J. F. L. Childs1, G. R. Grimm2, T. J. Grant3, certification nursery, and from observations on 6 L. C. Knorr4, and G. Norman5 parent trees in the Certification Program, all Introduction of which attest to the high incidence of xylo porosis infection in Florida citrus. The re Evidence of the widespread occurrence of sults are discussed in relation to budwood the causal agent of xyloporosis (cachexia) in certification. Florida citrus trees is presented herein. In order that the disease, its cause, and range The causal agent of xyloporosis, a disease of in symptoms be understood, a brief review citrus trees on sweet lime (Citrus aurantifolia) is given of significant facts previously estab rootstock first described in 1934 in Palestine lished. This is followed by presentation of (12), is believed to be identical with the data obtained from general field observations causal agent of cachexia, a disease of Orlando on bud union disorders, from observations on tangelo first described in Florida in 1950 (I). Orlando tangelo test plants in the budwood The identity is indicated by the following considerations: Xyloporosis is characterized by V, a/» and V—Pathologists, Horticultural Crops Re search Branch, Agricultural Research Service, U. S. degeneration of the phloemand xylem of sweet Department of Agriculture, Orlando, Florida. _ V—Pathologist, Florida Citrus Experiment Station, lime rootstock and subsequent deterioration Lake Alfred. Fla. of the citrus scion top. On the basis of these «iB/7TIno char*e °* Budwood Certification Program, Florida State Plant Board, Winter Haven, Fla. symptoms xyloporosis was recognized in Brazil 78 FLORIDA STATE HORTICULTURAL SOCIETY, 1955

(6) in 1937 and in Florida (5) in 1951. This budded with many different varieties of citrus*. disease was at first attributed to physiological In recent tests, xyloporosis was transmitted to factors although a virus relationship was con Orlando tangelo test trees from 25 previously sidered possible (12). Incompatibility between unreported varieties (6 species) of citrus. stock and scion was later suggested (9). Thus the extent to which xyloporosis virus is Examinations made in experimental root- distributed in commercial sweet orange and stock plantings in Florida (2) and in Texas grapefruit varieties is becoming apparent. (10) disclosed that 25 varieties including Surveys of citrus orchards in Argentina mandarins, (C. reticulata), mandarin hybrids, (4, 8) and Florida (7) carried out independ kumquats (Fortunelh sp.), and kumquat ently by some of the authors revealed that hybrids, when used as rootstocks, were sus certain bud-union abnormalities were frequent ceptible to cachexia. Cachexia has been dem ly observed on sweet lime, Everbearing lemon, onstrated to be bud transmissible (2, 3, 11). and Rough lemon rootstocks. The similarity In a previous report a possible indirect rela of these abnormalities to the symptoms of tionship to psorosis virus was discussed (2), xyloporosis in Orlando tangelo prompted an but further work indicated that none existed. examination of the orchard trees that were That the causal agent of cachexia is a separate candidates for registration as sources of bud- entity is also shown by Olson's report that it wood in the Budwood Certification Program, was transmitted with buds from trees tested in relation to the Orlando test trees in the and certified as free of psorosis (11). nursery where xyloporosis symptoms had been Similarity of the phloem and xylem symp noted. toms of cachexia on Orlando tangelo to those Phloem discoloration in Rough lemon root of xyloporosis on sweet lime rootstock has been stocks suggesting xyloporosis infection was first pointed out (2). Transmission of xyloporosis noted in Florida in 1953. In addition to show virus from sweet lime to Orlando tangelo and ing phloem discoloration, Rough lemon usually of cachexia virus from Orlando tangelo to exhibits other symptoms consisting of a nar sweet lime indicates that in Florida both row constriction or indentation of the wood diseases are probably caused by the same beneath the bark at the bud union and mild agent (3). The name xyloporosis has priority pitting of the Rough lemon wood (Fig. 1). and consequently will be used hereafter in There is a corresponding ridge in the cambial this paper. It is known that the xyloporosis * Suit, R. P., E. P. DuCharme, and I*. C. Knorr. virus causes severe and extensive phloem Citrus Experiment Station, Lake Alfred, Florida, un symptoms in Orlando tangelo, but it may be published data. present in sweet orange (C. sinensis), grape fruit (C. paradisi), and certain other species without causing any recognized symptoms (2). Citrus varieties differ in the time re quired to express disease symptoms. Satsuma mandarin (C. reticulata) rootstocks, for ex ample, required 2 years longer than Orlando tangelo to show symptoms (10). Although there is considerable variation in the severity of symptom expression in different varieties, this was not emphasized in previous publica tions. Distribution of Xyloporosis It has been suspected for some time that xyloporosis virus is widely distributed without causing symptoms in the commercial varieties of sweet orange and grapefruit in Florida and Texas (5, 11). In fact, xyloporosis symptoms have been observed in sweet lime and Ever Fig. 1. Constricted bud union of sweet orange (C. sinensis) on Rough lemon (C. limon.) infected with bearing lemon (C. limon) rootstock when xyloporosis. CHILDS ET AL: XYLOPOROSIS 79 face of the bark at which point phloem dis union symptoms with the evidence of xyloporo coloration is most pronounced and occasional sis infection as demonstrated in the nursery ly slight separation of the outer bark tissues test. Of the total trees, 262 were sweet orange occurs at the line of the bud union. To date, these symptoms have been confined almost or grapefruit trees on Rough lemon or sour entirely to Rough lemon rootstock budded orange (C. aurantium) rootstocks. The results with sweet orange scions (7). of these examinations are as follows: Sweet orange on Rough lemon rootstock: Of Examination of Nursery Test Trees 97 candidate sweet orange trees (5 varieties) Each candidate tree in the Budwood Pro on Rough lemon rootstock, 53 percent ex gram is budded on 3 Orlando tangelo seed hibited bud union symptoms in the parent trees lings to test for xyloporosis virus which may and also induced typical symptoms of xyloporo be present without causing symptoms in the sis in the Orlando tangelo test trees in the candidate tree. Many of these seedlings have nursery. Seven percent of the parent trees been budded 2 to 2& years, long enough for expressed no bud union symptoms but induced some to express symptoms; and it was round, symptoms on the Orlando tangelo test root upon examination, that 62 percent of the can stocks. Twenty-six percent of the parent trees didate trees had transmitted xyloporosis to the that exhibited bud union symptoms are nega Orlando tangelo rootstocks. In a few cases, tive to date by the Orlando tangelo test in the only one of the 3 Orlando tangelo replicates nursery. Failure of some Orlando test trees (budded from each candidate tree) had to show symptoms when budded from parent developed typical symptoms but, in the light trees that are undoubtedly affected by xylo of previous experience, that is considered suf porosis is probably due to the fact that not all ficient evidence that the parent tree is in Orlando tangelo test trees will show symptoms fected. within 2& years. The percentages of sweet orange trees on Rough lemon stock free of bud Examination of Parent Trees union symptoms in the parent trees.and free There were 339 candidate trees that; had of xyloporosis in the Orlando tangelo test are been in the Budwood Program for 2 years or as follows: Valencia, 2 percent; Pineapple, more, and these were carefully examined in 14 percent; and Hamlin, 53 percent (table 1). order to compare the incidence of their bud This group provides evidence that trees of

Table 1. The incidence of xyloporosis and the expression of symptoms in sweet orange (C. sinensis) varieties on Rough lemon (C. limon) rootstock*

Symptoms present Symptoms ab Parent tree Nursery Parent sent, parent Total and nursery trees tree tree and No* Scion variety trees only nursery trees trees

Valencia 30 2 11 1 44 Pineapple 18 1 10 5 34 Hamlin 1 2 4 8 15 Jaffa 1 0 0 0 1 Temple 1 2 0 0 3

Percent of total 53 7 26 100

♦Trees under test in the Florida Budwood Certification Program 80 FLORIDA STATE HORTICULTURAL SOCIETY, 1955

Table 2. The incidence of xyloporosis and the expression of symptoms in sweet orange (C. SINENSlS) varieties on sour orange (C. AURANTIUM) rootstock* Symptoms present Symptoms ab Parent tree Nursery Parent sent, parent Total Root- Scion and nursery trees tree tree and No. stock variety trees onlv nursery trees trees

Rough Duncan 1 3 0 5 9 lemon Marsh 1 16 0 5 ■ 22 Thompson 0 13 0 3 16 Red 0 5 0 2 7

Percent of total 4 68 0 28 100

Sour Duncan 0 0 0 3 3 12 orange Marsh 0 12 0 0 Thompson 0 9 0 2 11 Red 1 6 0 9 16 McCarty 1 1 0 0 2

Percent of total 32 100

"Trees under test in the Florida Budwood Certification Program

Table 3. Incidence of xyloporosis and the expression of symptoms in grapefruit (C. PARADISI) on Rough lemon (C. LIMON) and on sour orange (C. AURANTIUM) rootstocks* Symptoms present Symptoms ab Parent tree Nursery Parent sent, parent Total and nursery trees tree tree and No* Scion variety tregs only JSS&ZL nursery trees

Valencia 0 20 0 9 29 Pineapple 0 3 0 7 10 Hamlin 0 2 0 3 5 8 Lue Gim Gong 0 *► 0 mm Parson Brown 0 5 0 2 7 Navel 1 2 0 0 3 Temple 0 h 0 1 5

Percent of total 1 60 0 39 100

♦Trees under test in the Florida Budwood Certification Program sweet orange varieties may be found that are absent from all trees, However, when scions free of xyloporosis. of these candidate trees were tested in the Sweet orange on sour orange rootstock: The nursery, 41 (61 percent) transmitted xyloporo 67 sweet orange candidate trees (7 varieties) sis to the Orlando tangelo test plants, indicat on sour orange rootstock, with one exception, ing that this virus is present in the same pro did not reveal symptoms that could be attribut portion of these sour-orange-rooted trees as in ed to infection by xyloporosis. Slight phloem the Rough-lemon-rooted sweet orange trees discoloration was exhibited by the one tree, (table 2), despite its failure to induce recogniz and other symptoms such as pitting of the wood able symptoms of xyloporosis in the sour orange or indentation of the wood at the union were stocks. CHILD3 ET AL: XYLOPOROSIS 81

Grapefruit on Rough lemon rootstock: None previously reported to harbor the virus. Many of the 54 parent grapefruit trees (4 varieties) of these are not commercially important at on Rough lemon root exhibited wood indenta present but formerly were grown in Florida tion at the union or pitting of the Rough or elsewhere to some extent. Bud union lemon wood adjacent to the union, and only symptoms suggestive of xyloporosis have been two of them exhibited slight discoloration of found in a very high percentage of the sweet the phloem at the bud union.. Despite the orange trees on Rough lemon stock in Florida absence of xyloporosis symptoms in these can and Argentina, and it has been shown that didate trees, 72 percent of them transmitted 62 percent of 339 candidate trees induced xyloporosis virus to the Orlando tangelo stocks xyloporosis symptoms on Orlando tangelo in in the nursery test (table 3), indicating that 2 to 2% years. Bud union symptoms suggestive they are symptomless carriers of xyloporosis of xyloporosis were not found on grapefruit virus. Why Rough lemon rootstock should trees on Rough lemon nor in sweet orange or show symptoms when budded with xyloporosis- grapefruit trees on sour orange rootstock, infected sweet orange and not show symptoms though many of these trees are infected with when budded with similarly infected grapefruit the virus. is unknown at present. The high incidence of infection shown by Grapefruit on sour orange stock: Of the 44 these results indicates that xyloporosis virus grapefruit trees (5 varieties) on sour orange is present in a majority of the sweet orange stock, only two had phloem discoloration at trees in Florida. If the cause of the described the union suggestive of xyloporosis, although bud union disorder of sweet orange on Rough many revealed an irregular surface of the sour lemon rootstock has been correctly interpreted, orange bark adjacent to the union. When, these findings also indicate that certain symp however, scions of the parent trees were tested toms heretofore little understood are diagnostic on Orlando tangelo seedlings, 30 (68 percent) of xyloporosis in Rough lemon, a variety pre proved to be carriers of the xyloporosis virus viously considered symptomless. (table 3). From the transmission tests mentioned pre Other varieties of citrus in the nursery test: viously (2, 3) and from other experiments not Other citrus varieties occurring as parent trees discussed, it is believed highly improbable in small numbers or on other rootstocks were that the Orlando tangelo test trees were in infected with xyloporosis virus as shown by fected from any source other than the can the Orlando tangelo test: Pope Summer orange didate trees from which they received buds. (C. sinensis), 10 trees, all infected; sweet orange seedling (C. sinensis) and Murcott or The data presented show that among the ange (C. reticulata), 1 tree each, neither in commercial varieties of citrus currently and fected; Foster grapefruit, 2 trees, both infected; formerly grown xyloporosis virus is widespread. Orlando tangelo, 8 trees, none infected; That it has been present many years in Florida Minneola tangelo (C. paradisi x C. reticulata), even in apparently healthy citrus trees becomes 1 tree, infected; Dancy tangerine (C. reticulata) apparent when the age of infected candidate seedling, 7 trees, not infected; Satsuma man trees is considered. That the citrus industry darin (C. reticulata), 3 trees, 1 infected; in Florida has been able to produce citrus Calamondin (C. mitis), 1 tree, infected; and fruit successfully in spite of the presence of limequat (C. aurantifolia x Fortunella sp.)y xyloporosis virus is evident. The primary ob 1 tree, not infected with the virus. jective of a bud certification program is to It should be emphasized that the nursery assist nurserymen and growers to obtain virus- test figures are for 2 to 2& years only and are free trees, and the first step toward that ob not final, and the number of Orlando tangelo jective is the obtaining of psorosis virus-free stocks exhibiting symptoms of xyloporosis can budwood sources. This goal appears to be be expected to increase to a maximum at the within reach, but the objective of obtaining end of about 4 years. xyloporosis virus-free trees presents more prob Summary and Conclusions lems in the light of recent findings. Its at In the tests reported here, xyloporosis virus tainment may be delayed but eventually can has been found in 30 varieties of citrus not be reached. 82 FLORIDA STATE HORTICULTURAL SOCIETY, 1955

LITERATURE CITED 7. Grimm, G. R., T. J. Grant, and J. F. L. Childs. A bud union abnormality of Rough lemon rootstock 1. Childs, J. F. L. The cachexia disease of Or with sweet orange scions. Plant Dis. Reptr. 39: lando tangelo. Plant Dis. Reptr. 34: 295-298. 1950. 810-811. 1955. 8. Knorr, L. C. Arrugado del tronco (rugose trunk) 2. Childs, J. P. L. Cachexia disease, its transmis del limon rugoso. Argentine Min. Agr. y Gan. IDIA sion and relation to xyloporosis and tristeza. Phyto- No. 58: 22-25. 1952. path. 42: 265-268. 1952. 9. Moreira, S. Xyloporosis. Hadar 11: 234-237. 8. Transmission experiments and xyloporosis- 1938. cachexia relations in Florida. Plant Dis. Reptr. 40(2) : 10. Olson, E. O. Some bark and bud union disorders page numbers unknown 1956. of mandarin and mandarin-hybrid rootstocks in Texas 4. DuCharme, E. P., and L. C. Knorr, Vascular pits citrus plantings. Proc. Amer. Soc. Hort. Sci. 63: and pegs associated with diseases in citrus. Plant 131-136. 1954. Dis. Reptr. 38: 127-142. 1954. 11 and A. Shull, Red grapefruit strains 5 and R. F. Suit, Xyloporosis of as symptomless carriers of the causal agent of citrus in Florida. Plant Dis. Reptr. 35: 556-557. 1951. cachexia, a bud-transmitted disease. Proc. Rio Grande Val. Hort. Inst. 9: 46-50. 1955. 6. Fawcett. H. S., and A. A. Bitancourt. Relatorio sobre as doencas dos citrus nos Estados de Pernam- 12. Reichert, I., and J. Perlberger. Xyloporosis, buco, Bahia, S. Paulo e Rio Grande do Sul. Rodriguesia the new citrus disease. Bull. 12, The Jewish Agency anno III; 213-236. 1937 (not seen). for Palestine, Agr. Exp. Sta., Rehoboth, pp. 1-50. 1934. FACTORS INFLUENCING THE ABSORPTION OF ZINC BY CITRUS

Ivan Stewart, C. D. Leonard and compounds was applied to provide an ac tivity of approximately one microcurie to a George Edwards leaf. All of the treatments were replicated Florida Citrus Experiment Station at least three times; some as many as six times. Lake Alfred In one to six months following the applica tion of radioactive zinc, the leaves were re Zinc sprays are the most dependable method moved from the trees, pressed and dried. The of controlling frenching in citrus, but these leaves were then put into a light-tight box sprays are much more effective in some groves with no-screen X-ray film and allowed to ex than in others. Although zinc has been com pose the film for one month. Developing the monly used on citrus for over twenty years, films and photoprinting on positive paper there is relatively little information on the produced the radioautographs presented as factors that contribute to the uptake of this figures. element through the foliage. From the autographs, it was possible to A study of some of these factors is reported study various properties of zinc compounds here. This work was carried out by two on the leaves, The lighter areas indicate con methods—first, by making studies on absorp centrations of zinc85. The white, washed-out tion and translocation of radioactive zinc areas indicate the zinc was primarily on the through the foliage; second, by determining surface of the leaves. The veining pattern the zinc content of leaf samples collected from was attributed to translocation within the a large number of groves that were treated leaf. A lack of any exposure showed that the with a wide variety of zinc sprays. zinc had been washed off the leaves by rain Methods before any absorption occurred. The absorption studies were carried on Absorption of Zinc using radioactive zinc65 obtained through the When zinc compounds were applied to the Atomic Energy Commission at Oak Ridge, top and bottom surfaces of leaves and to the Tennessee. The radioactive zinc was con bark of twigs, the zinc was absorbed and verted into various compounds, and then each translocated from all three surfaces (Fig. 1). was diluted with water to a zinc concentration The zinc applied to the bark was readily taken equivalent to the amount of this element in up and translocated to most of the leaves on three pounds of zinc sulfate in 100 gallons of the twig. A comparison of several autographs water. showed that in most cases there was con Radioactive zinc treatments were applied siderable more absorption of zinc from the to orange trees growing in the field. In each under surface of the leaves than from the treatment two to three drops of one of the upper surface. At least two factors probably Florida Agricultural Experiment Station Journal contribute to the increased uptake from the Series, No. 486.