THE BREEDING PROBLEM

Transcription

1 SUGAR CANE BREEDING STATION, BARBADOS G. C. STEVENSON, B.A., A.I.C.T.A. Assistant Botanist Britislz West Indies Central Sugarcane Breeding Station, Barbados INTRODUCTORY Mosaic disease occurs throughout the area served by the British West Indies Central Sugar Cane Breeding Station, though its effects are more serious commercially in some islands than in others. It is controlled in Barbados, where susceptible varieties are almost exclusively grown, by an elaborate system of inspections and the roguing ol all fields each year. This system is unsatisfactory for many reasons, not the least of which are the labor and expense iavolved. This system of control is impracticable in certain regions-particularly in parts of Jamaica, where the disease causes heavy losses in the susceptible noble canes such as B.H. 10 (12), which are otherwise most suitable on account of their excellent field and factory qualities. The production of high yielding mosaic resistant seedlings of good quality, suited to West Indian growing coilditioils is, on account of these considerations, one of the inajor projects of this Station. THE BREEDING PROBLEM Sugarcane varieties belonging to the species Saccharunz oficinarz~m, the socalled noble canes, are all susceptible to mosaic disease, though minor differences occur in the ease with which they become infected, and the extent of the losses caused by the disease. Immunity, or high coinmercial resistance, to the disease can be obtained in certain nobilizations of wild Saccharum species, particularly those of glagah (the Javan form ol S. spontanez~nz). It is hoped, too, that nobilizations of the iinmune Uba (S. silzense), and certain seedlings of the Chunnee (S. barberi) nobilization will prove resistant. It is felt that the solution of the mosaic disease problem in the British West Indies will be by way of such 71

2 mosaic resistant nobilizations, back-crossed to noble canes in a special breeding scheme and selected after good performances in trials laid down under the long growing conditions of the West Indies. Until recently, in Barbados, the policy was to test for yield and sucrose quality those seedlings whose parentage would indicate a reasonable chance of their possessing mosaic disease resistance. Selections from such trials were then sent, through the Plant Quarantine Station into Trinidad, and planted in appropriate areas where 'the mosaic disease infestation was known. It was appreciated that this method was unsatisfactory on account of the labor and expense of testing seedlings which might prove susceptible to mosaic disease,. and recently facilities have been obtained for mosaic disease testing in Barbados. In the future only mosaic resistant selections will be tested for yield and sucrose quality in Barbados, and selections from these will be sent for trial to areas where the disease is of considerable economic importance. At present, a preliminary testing of seedlings of known high quality is being carried out, in order to work out a satisfactory testing scheme and to bring up to date the information on the resistance of seedlings from certain parentages in the nobilization program. Eventually, as the records accumulate, data will be available on the inheritance of mosaic disease resistance, which will be used to guide the breeding program. METHODS OF CONDUCTING MOSAIC DISEASE RESISTANCE TESTS The scheme for mosaic disease testing consists of noting the behavior of seedlings in a special trial laid out in the field under conditions favorable for,natural infection. In addition, all seedlings under test will be artificially inoculated with the mosaic virus when the most suitable technique has been worked out. Observation p'lots of seedlings under test consist of six-stool rows, each of which is adjacent to a row of diseased stools planted from infected cuttings. Rows of corn are included in the layout to attract the insect vector of mosaic disease, Aphis maidis. During dry weather the plots are irrigated. Observations are made at weekly intervals, ancl any new cases of infection in the experimental seedlings noted. The seedlings which become diseased are uprooted and discarded and their places are taken by new seedlings about which information is desired. The experiment is run' as a continuous one. When the stools become too large to be observed conveniently, both the infected canes and the experimental seedlings are cut down, and observations continued on the ratoon shoots. Replantings of corn are made at necessary intervals. At first, rows of a healthy susceptible noble cane variety were included to note the rate of spread of mosaic under the prevailing conditions. Three months after planting, almost fifty per cent of these stools were infected, showing that a considerable amount of natural infection was to be expected in such plots. These noble cane observation rows were subsequently removed, and replaced by other seedlings for mosaic disease testing. In the original planting in September, 1937, eleven seedlings were included for test, all of which were fourth or fifth nobilizations of glagah (S. spontaneum). 72 '

3 Uba (S. sinense), one seedling showed mosaic in February, The servati~ns are being continued. As stated above, it is intended to use a method of 'artificial inoculation in addition to the above testing scheme: Preliminary experiments have been started evaluate the known methods bf mosaic transmission under Barbados conditions, d to discover the most reliable one: The four methods which have been used 1. The Sein leaf-slip method. A strip of mosaic diseased leaf, about six inches g by one inch wide, is wrapped tightly round the unrolled central leaf spindle the sho~ot to be inoculated, and a bunch of very fine insect pins thrust reatedly through the diseased leaf tissue into the tissues of the spindle. Using bunch of fine pins, from seventy-five to one hundred punctures are rapidly ade, the object being to introduce infective juice into the healthy tissues with little exposure to air as possible. 2. The Matz method. A drop of juice extracted from diseased material is troduced into the cavity subtended between the last unfolded leaf and the ntral spindle of unrolled leaves. Through this juice a fine needle is thrust were used separately..' The hypodermic needle method. Infective juice obtained by crushing ed material is introduced by means of a hypodermic needle into the shoot.the growing point. Here again juice extracts from diseased canes and. The plug method with cuttings. A single eye cutting of a healthy cane ed, and a plug of tissue is removed with a sterile 3/s inch cork borer as as possible to the, bud without damaging it. Into the cavity thus made, a cut from the leaf spindle of a diseased shoot with the same cork borer troduced, and crushed with a sterile glass rod. The original plug is then ote from any,diseased canes, so that the possibility of their becoming naturally nfected was negligible. At the time of inoculation, the primary shoots were rom one to two feet in height, and just beginning to tiller. Inoculatidns were made using all four methods on January 20, Disease ptoms were first noted on February 23 in two plants inoculated by the Sein hod-approximately five weeks after inoculation. Further cases of infection

4 and the first appearance of symptoms would appear to range between four and seven weeks. By the plug method, cases of infkction were seen as soon as the first few green leaves unrolled. The ;es;lts of the experiment were as follows: Sein method Matz method Hypodermic rnethibd Plug method Inoculations Infections Percentage The results show that the Sein method was by far the most successful. With the plug method only moderate success was obtained, and no infection was obtained by the other two methods. Further experiments will be made, using certain modifications of the Sein and the plug methods in order to try to increase the percentage of infection. before inoculation is used as a routine procedure in the resistance trials. In view of the recent discovery of distinct strains of the mosaic virus in Louisiana, it is possible that mosaic strains differ in the regions served by the Station from those present in Barbados. Should this be so, it will be readily shown by the performance in these areas ol the seedlings proved resistant in Barbados, in which case certain modifications in the testing scheme will become necessary. DISCUSSION Concerning methods of artificial inoculation, Dr. McIntosh stated they had obtained no infections with the Matz method although the juice had been extracted with the same type of grinder demonstrated by Dr. Summers during the field tour. Dr. Matz stated that he obtained 100 per cent infections by this method, and suggested that Dr. McIntosh's lack of success might be due to overheating of the juice in extracting it from the cane tissues. Dr. Summers mentioned that he obtained higher percentages of infection when the plants to be inoculated were young and rapidly growing. To stimulate growth, the plants are given an application of sodium nitrate following inoculations. He stated, however, that some batches of juice give almost no infection, indicating that something occasionally happens to inactivate the juice. He had not observed any marked heating of the juice during the extraction. Mr. King inquired if the age of the plant at the time of inoculation was important. Dr. Summers replied that he obtained the best results when the plants were 18 to 24 inches tall, although, as pointed out by Dr. Forbes, infections could be obtained with plants 5 or 6 feet tall. Dr. Forbes inquired as to how late in the season infection could be obtained. Dr. Summers said he had made no special study of this. His field inoculations are made in the late spring, when infections are readily obtained. Dr. Forbes said he obtained very little infection in late July. The kind of needle used was then discussed. Dr. Matz emphasized that the size and quality of the needle was very important, pointing out that a large needle breaks down too many cells thus preventing the virus froin entering a living cell. He exhibited the very fine, steel 7 4

5 needles he uses. Dr. Summers said he found thcse needles too fragile for extensive field use, and preferred a small sewing machine needle which did not break so Another paper by G. C. Stevenson was also introduced by the chairman. PAPER BREEDING AND TESTING SUGARCANE SEEDLINGS FOR GUMMING DISEASE RESISTANCE AT THE BRITISH WEST INDIES CENTRAL SUGAR CANE BREEDING STATION, BARBADOS G. C. STEVENSON, B.A., A.I.C.T.A. Assistant Botanist British West Indies Central Sugar Cane Breeding Station, Barbados INTRODUCTORY Gumming disease (Bacterium vascularurn, [Cobb) Grieg Smith) was identified in Barbados in 1929, and by 1930 had spread over the greater part of the island. Since that date, it has appeared regularly whenever susceptible varieties have been grown. Consequently, and in view of the fact that the disease occurs in certain of the islands in the British West Indies supplied by the Station, it was apparent that a reliable method of testing seedlings for resistance to gumming disease was desirable. A scheme has been evolved which, in addition to its immediate application, yields cumulative results on the reaction of seedling popuiations to the disease from year to year. These results serve to give information on the inheritance of gumming disease resistance, and thereby to guide the breeding program. The early work on gumming disease in Barbados consisted in comparing, by observations of leaf symptom development, the resistance of newer varieties with that of Ba , which w$s at that time the standard commercial variety of the low rainfall regions of the island. This variety developed the leaf symptoms to a high degree and is, indeed, still used as a standard for comparison in testing seedlings. Later work showed, however, that the more serious aspect of the disease from the commercial standpoint-namely, systemic infection, involving as it does the production of gum in the canes, leading to difficulties in juice clarification and sometimes death of canes in the field-did not appear to occur to any extent in Ba Losses due to the disease in commercial plantings of this variety were, for this reason, inappreciable, and Ba inay be classed as susceptible, but tolerant. The presence of such a tolerant type in commercial plantings is, however, highly undesirable, for it represents a constant source of infection to other varieties, and makes it impossible to rid any particular area of the disease. Further, it is always possible that its tolerance might break down with a change in the virulence of the organism or under a particular set of environmental conditions, with disastrous results. It may be stated, parenthetically, that Ba has now been almost completely replaced in commercial plantings by the resistant B