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New Disease-Resistant Rootstocks Urgently Neededby Citrus Growers

New Disease-Resistant Rootstocks Urgently Neededby Citrus Growers

FLORIDA STATE HORTICULTURAL SOCIETY 201

20 years of age before they bloomed in North recent plantings of this Rhodesian mahogany ern Rhodesia, Africa. in southern and south central parts of Florida The mother mahogany in Miami, which show that young seedlings of this species have is approximately 20 years old, probably came survived frosts, hurricanes, and dry weather from one of Dr. W. L. Thompson's numerous when planted in hammocks, but have not been introductions of Khaya nyasica seed from Mt. able to fare for themselves under exposed Silinda, Southern Rhodesia, the earliest intro conditions in the open. If these can sur duction being received in 1902 (1). This tree vive and reproduce themselves under South in the Miami City Cemetery most likely came Florida conditions, as at least one tree has from Introduction No. 59293 of Dr. done to date, there may well be possibilities Thompson's which arrived in Washington, D. for this new tree crop in this hemisphere. C, April 19, 1924. 1. Lynch, S. J., and H. S. Wolfe. Khaya The Khaya nyasica plantings among Carib nyasica, a new mahogany for South Florida. bean pine at the Sub-Tropical Experiment Sta Proc. Florida StateHort. Soc. 55:113-116, 1942. tion (2) continue to show promise, as well as 2. Lynch, S. J., and H. S. Wolfe. Future plantings at the Dade County nursery on Red may see mahogany forests in Florida. Florida Road, Coconut Grove, Miami, Florida. More Grower, August, 1942.

NEW DISEASE-RESISTANT ROOTSTOCKS URGENTLY NEEDEDBY GROWERS

WALTER T. SWINGLE Collaborator, U. S. Department of Agriculture, Consultant in Tropical Botany, University of Miami

Just why should we be concerned about the island of Java sour has for many getting more citrus rootstocks? We already years been given up completely as a rootstock. use sour orange, sweet orange, rough , Jn 1930 a destructive disease (now called Tris- and Cleopatra mandarin, and teza*) of oranges and mandarins broke out more recently hybrid stocks such as in Argentina. It spread with great rapidity (trifoliate orange x sweet orange) and tan- and in a few years reached the great navel gelos (orange x ). These stocks orange groves of the State of Sao Paulo, in have all been sufficiently studied so that southern Brazil, killing all trees grafted on growers know what each type of stock is fit sour orange. These groves are now being de- ted for. Commercially grown nursery stock is practically limited to these varieties for *Webber, H. J., The "Tristeza" Disease of rootstocks. Sour-Orange Rootstocks, in Proceedings Amer. The chief reason for the sudden interest in Soc. Hort. Sci., 43: 160-168. Nov. 1943. (Dis new rootstocks is the complete eclipse of sour tributed Jan. 1944.) First description in Eng stock in some regions. This has been the case lish. Named Tristeza by Moreira, Sylvio. Ob- since the beginning of this century in South servacoes sobre a ''Tristeza" dos Citrus, ou Africa where it was found impossible to grow "Podridao das radicelas". O Biologico 8:269- sweet orange on any strainof sour stock. In 272. Oct. 1942. 202 FLORIDA STATE HORTICULTURAL SOCIETY veloped again on rootstocks. ing danger that rootstock troubles may arise Very recently a new disease of citrus has in regions that have previously been free from come to light in California. This disease has them. In South Africa and South America spread rather slowly for the past five or six orange, grapefruit and grown on years but within the last twelve months has sour orange died but the plantings have suddenly attracted attention because, like the been replaced promptly by the same varieties Tristeza of South America, it kills only trees grown on rough lemon and sweet orange. In on sour stock. This trouble, now called Quick some soils this latter stock suffers from gum- Decline, does not so far as known attack lemon mosis. In California all three stocks—sour on sour stock, in this respect resembling Tris orange, sweet orange and rough lemon—are teza. However, Quick Decline does not seem commonly used and if the trees suffer on one to be identical with Tristeza; it certainly they can be grafted on another of the known spreads more slowly, kills the trees more slow stocks. In some localities, however, the sour ly and has somewhat different symptoms. The orange stock seems to be far the best known. root of the sour orange is apparently starved In the Lower Rio Grande Valley in Texas it in both Tristeza and Quick Decline by the is practically the only rootstock used. The failure of food material to pass from scion to rough lemon rootstock came into disrepute in stock. The stopping point is apparently near many parts of Florida during the boom of the the graft union. Temple orange since this variety often does Very recently a somewhat analagous root- very poorly on this stock. If it should sud scion disharmony has come to light in Western denly become necessary to substitute rough China. Dr. Wen-Tsai Chang, Professor of lemon for sour orange rootstock because of Pomology at the University of Nanking at Quick Decline or some other rootstock trouble, Chengtu, has been making field investigations many Florida orange and grapefruit growers in citriculture in Szechuan Province. He found would probably refuse to use rough lemon sweet oranges were there propagated almost root stock because of their unfortunate ex entirely from seed. Some of the seedlings perience with the Temple orange. In the Gulf produced very high quality and he at Coast region it is well known to all growers tempted to propagate these superior strains by that sour orange rootstock does not support grafting them on pummelo rootstocks (like the Satsuma orange or the . our shaddock). The pummelo is a very vigor It is important for citrus growers to be ous tree and considered an excellent rootstock given as promptly as possible information as in eastern China. In West China, much to his to stocks adapted to all localities so that if surprise, Dr. Chang found that these seedling any new rootstock trouble does appear they strains grafted on pummelo began to decline will know promptly what stocks are best for and none of them grew into healthy fruiting replacement. It is quite possible that State trees. As yet little is known as to this strange and Federal Experiment Stations might be behaviour. The same strains of sweet orange able to work out techniques for replacement grafted on sweet orange roots grew very well. of rootstocks that are beginning to fail. With such insidious new citrus diseases ap One sometimes sees in Florida old sweet pearing in many parts of the world it becomes orange trees, formerly on sweet orange stock, necessary to initiate and carry out carefully that had been so severely attacked by gum- actual tests of as many rootstocks as can pos mosis as to be in danger of dying, but were sibly be obtained in order to be prepared to saved by approach grafting them with several rebuild citrus plantations on new rootstocks vigorous young sour orange seedlings. In which have proved to be adapted to the va some cases the old sweet orange root has died riety of citrus fruit grown and to the local out completely, leaving the tree supported soil and climate conditions. only by the sour orange approach grafts. This This is not a discussion of plant pathology technique in reverse is likely to be tested im but I do want to call attention to the increas mediately in California where sweet orange on FLORIDA STATE HORTICULTURAL SOCIETY 203 sour stock in the early stages of Quick Decline so they could be planted some distance from will be approach-grafted with sweet orange the sick tree, to avoid competition from the or rough lemon seedlings, both immune to still living roots of the sour orange rootstock. Quick Decline. Such special provisional interplants need not Citrus growers and Experiment Stations be particularly well adapted to nourish bearing

TABLE 1. CHRONOLOGIC LIST OF THE

G H Citrus L. 1753 16 11 68.7 G Burm. 1768 23 1 4.3 G Koen. 1771 11 2 19.2 Lour. 1790 3 1 33.3 g Aegle Corr. 1800 1 1 100 G Feronia Corr. 1800 1 1 100 G Corr. 1805 11 3 27.3 Corr. 1805 35 1 2.3 Blume 1825 9 0 0 Buch.-Ham. 1831 12 1 8.3 G H Poncirus Raf. 1838 1 1 100 G(? Wight 1840 15 1 6.6 G Ten. 1840 6 2 33.3 G Roem. 1846 1 1 100 G Merope Roem. 1846 1 1 100 Montr. 1860 4 0 0 g Stapf. 1906 1 1 100 g Swing. 1912 2 1 50 G Feroniella Swing. 1913 3 2 66.7 G Swing. & M. Kell. 1914 11 2 18.2 G H Eremocitrus Swing. 1914 1 1 100 g Engl. 1915 4 2 50 G H Fortunella Swing. 1915 4 3 75 G H Microcitrus Swing. 1915 6 4 66.7 Wenzelia Merr. 1915 9 0 0 Swing. 1916 1 1 100 Swing. 1916 5 0 0 Swing. 1918 1 1 100 G Merr. 1927 1 1 100 Tan. 1928 1 0 0 Swing. 1939 1 0 0 Swing. 1939 1 0 0 Swing. 1940 1 0 0

G—Can serve as root stock for Citrus or closely related genera H—Hybridizes with Citrus or closely related genera g—Can serve as root stock for Bael-fruit ()

in citrus growing regions would' do well to citrus trees. Their function would be to main consider the possibility of making temporary tain the sick tree while the affected sour stock use of extremely vigorous rootstocks for in is dying. After the sour orange root has be arching trees that begin to show symptoms of gun to die it would be easy to plant and es Quick Decline, of Tristeza, or of other similar tablish root stocks known to be well adapted diseases. Seedlings for inarching would need to the soil and variety alongside the recuper to be two to three years old and tall enough ating tree and use them as permanent ap- 204 FLORIDA STATE HORTICULTURAL SOCIETY proach grafts. Although this system would relatives in order to determine their relation be expensive it might easily prove to be cheap ships. Some 250,000 serial microtome sections er than losing outright splendid trees bearing were made in the Bureau of Plant Industry of 10-25 boxes of fruit annually. Naturally the U. S. Department of Agriculture and 100,- such a recuperating tree would have to be 000 at the University of Miami. These are pruned back and not fruited while the double all under study and recent discoveries have approach graft was in process. thrown important light on the close relation In view of this situation let us consider the ship of many members of the orange subfami prospect of securing new rootstocks adapted ly which have not yet been introduced into to all the different types of soil now planted the New World. to citrus in our citrus-growing states. In 1897 I made among other hybrids a cross In May, 1943, I read a paper before the between the trifoliate orange and the sweet Krome Institute of the Florida Horticultural orange. The 11 sister hybrids (citranges) Society at the Winter Haven meeting and in grown from the seeds of a single fruit of this cluded a census of the citrus fruits of the cross showed considerable diversity in leaf world and their wild relatives. None of these character and very striking diversity in the are native in the New World but they fruit characters. One of these hybrids, the are widely distributed in the Old World from Morton, has large, round, smooth, orange-col western tropical Africa through southeastern ored fruits; another, the Coleman, has de Asia to the East Indian Archipelago, the pressed-globose, yellow, fuzzy-hairy fruits; Philippines, New Guinea, New Caledonia and the Willits has a large percentage of fingered Australia. This census included 33 genera and fruits; the Rustic often has double fruits, a 203 species. A check of these relatives shows smaller fruit growing on top of a larger one, (Table 1) the number of genera and species and the calyx is often enlarged; the Phelps that have been introduced into the United has large, subglobose, reddish orange-colored States, and also the results of test grafts and fruit with very bitter juice; while the Saun- hybridization. This table is arranged chrono ders has small, light-orange colored fruits with logically in the order of the discovery of the scarcely a trace of bitter in the juice; and the genera from 1755 up to 1940. Although less Sanford also has small fruits, ripening early. than 76% of the genera and only 21.7% of the The number of segments varies greatly in species have been grown so far in the New these sister hybrids; the Savage has often as World, citrus has been grafted successfully few as 8, the Morton from 9 to 13, and the on no fewer than 16 genera. The reverse large-fruited Coleman from 10 to 14. The graft has also been made. Phelps and the Sanford produce numerous sec Of the 16 genera established before 1900, ond generation citranges or "segentranges" all but 2 have been introduced into the U.S.A. which develop from fertilized true embryos Of the 17 genera named after 1900, 7 have not while some others, like the Morton, are sterile yet been introduced into the United States. unless cross pollinated with citrus pollen (es Of the 24 genera so far tested, 16 (2/3) will pecially seedy grapefruit) and then produce take Citrus grafts and 4 more will take Bael- only nucellar-bud embryos which reproduce fruit (Aegle) grafts. Of the 9 genera not yet the mother parent exactly. tested, probably 5 will take citrus grafts and These nucellar bud embryos are of great im one will take Bael-fruit graft. Intergeneric portance for use as rootstocks for two reasons: hybrids with Citrus have been made between (1) they are completely free from virus dis 4 cf the 24 genera now in the United States, eases (even if the mother tree is infected) and but probably will be made with at least 2 or also from minor virus ailments too mild to be 3 more genera not yet introduced. recognized as diseases, but which, neverthe A special study is now in progress under less, reduce the energy and fruitfulness of the my direction at the University of Miami on tree; (2) they reproduce exactly the original the anatomical characters of all the citrus hybrid as in case of the two outstanding, well- FLORIDA STATE HORTICULTURAL SOCIETY 205 tested hybrid stocks (Morton and In testing new or little-used stocks for citrus Sampson ), so that a whole grove can fruit trees it will at first be necessary to make a future-delivery contract between nursery be planted to trees grafted on identical stocks, man and grower, with an advance payment not propagated from cuttings but grown from from the grower. Such a contract would pro seed so they develop a rootsystem with a tap tect the nurseryman and encourage him to root and laterals, preferred by nurserymen. grow new rootstocks. Curiously enough it is very difficult to se Two years ago, at the fifty-sixth meeting of cure viable hybrids between kumquats (For- the Florida State Horticultural Society at Win tunella) and the trifoliate orange (Poncirus), ter Haven, I told the Krome Institute the while it is easy to secure trigeneric ones by thrilling experience I had had fifty years be crossing kumquats with citranges, hybrids of fore when at the sixth annual meeting of this sweet oranges and the trifoliate. These hy Society at Pensacola many of the leading brids are called citrangequats and are often orange growers (there were no grapefruit more vigorous and fruitful than hybrids be growers then) got up and warmly approved a tween citranges and , although the proposal I made to organize a non-profit com grapefruit is a much more vigorous grower pany to import citrus varieties and citrus wild than any true . relatives from their Asia&tic and East Indian Complex hybrid rootstocks can be expected homes. The great freeze of December, 1894, to produce vigorous rootstocks for citrus fruit and February, 1895, killed most of the orange trees. In my chapter, "The Botany of Citrus trees in Florida and made it impossible to and its wild relatives" (published in Webber carry this project through. and Batchelor, "", Univ. I hope that the orange and grapefruit grow of Calif. Press, 1943, page 355, fig. 55) there ers of today will again approve the program is a diagram which shows the large number of I have briefly sketched and urge their repre intergeneric hybrids already known. Many of sentatives, both local and federal, to request these hybrids show much promise and merit legislation to make possible the prompt intro immediate testing as rootstocks for our prin duction into this country and the speedy test cipal citrus fruit trees. In this same chapter as rootstocks of all the citrus species and their the numerous hybrids shown in the diagram wild relatives in order to protect the future are discussed under the parental species. prosperity of our most valuable tree crop.

PERSONAL RECOLLECTIONS OF GEORGE B- CELLON, HORTICULTURAL PIONEER OF SOUTH FLORIDA

DAVID FAIRCHILD

Whenever the drama of the avocado and the cal accuracy about the role which he played mango in America is written, as it should be in the early days of these two growing indus some day, the name of George B. Cellon will tries of South Florida. I was out of the coun take a prominent place in the beginning chap try much of the time when the things happened ters. I am not a historian and am incapable which give his life the spectacular character of writing with strict historical and chronologi which I have always associated with it. But

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