New Disease-Resistant Rootstocks Urgently Neededby Citrus Growers
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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 tree 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 trees 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 Plant 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 State Hort. 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 NEEDED BY CITRUS 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 orange has for many getting more citrus rootstocks? We already years been given up completely as a rootstock. use sour orange, sweet orange, rough lemon, Jn 1930 a destructive disease (now called Tris- trifoliate orange and Cleopatra mandarin, and teza*) of oranges and mandarins broke out more recently hybrid stocks such as citranges 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 grapefruit). 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 strain of sour stock. In 272. Oct. 1942. 202 FLORIDA STATE HORTICULTURAL SOCIETY veloped again on rough lemon 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 tangerine 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 fruit 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 kumquats. 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 <GENERA OF1 CITRUS FRUIT TREES AND WILD RELATIVES OF THE TRIBE AURANTIOIDEAE Date No. of No. of G g H Names of genera estab sp. in sp. in % in lished world U.S.A. U.S.A. G H Citrus L. 1753 16 11 68.7 G Clausena Burm. 1768 23 1 4.3 G Murraya Koen. 1771 11 2 19.2 Triphasia Lour. 1790 3 1 33.3 g Aegle Corr. 1800 1 1 100 G Feronia Corr. 1800 1 1 100 G Atalantia Corr. 1805 11 3 27.3 Glycosmis Corr. 1805 35 1 2.3 Micromelum Blume 1825 9 0 0 Luvunga Buch.-Ham. 1831 12 1 8.3 G H Poncirus Raf. 1838 1 1 100 G(? Paramignya Wight 1840 15 1 6.6 G Severinia Ten. 1840 6 2 33.3 G Hesperethusa Roem. 1846 1 1 100 G Merope Roem. 1846 1 1 100 Oxanthera Montr. 1860 4 0 0 g Balsamocitrus Stapf. 1906 1 1 100 g Aeglopsis Swing. 1912 2 1 50 G Feroniella Swing. 1913 3 2 66.7 G Citropsis Swing. & M. Kell. 1914 11 2 18.2 G H Eremocitrus Swing. 1914 1 1 100 g Afraegle 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 Pamburus Swing. 1916 1 1 100 Pleiospermium Swing. 1916 5 0 0 Merrillia Swing. 1918 1 1 100 G Swinglea Merr.