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Reference
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The raising of seedlings.
In addition to many that have been introduced, seedlings have been produced in large numbers in Mauritius for nearly 40 years, first by the Station Agronomique under Boname for 20 years and then by the Department during the last 17 years. In his address in July, 1927, the then Director was able to say: "There is no country in the world where cane seed is more fertile than in Mauritius and con- ditions here are ideal for seedling work." The fine collection of varieties at the Experiment Station Pamplemousse has provided and still offers an abundant and varied material for such work. In 1928 a Sugar-Cane Breeder was appointed and was sent to study the methods now in use at the well-known Coimbatore Station in South India. More recently a Research Division with a Botanist and Geneticist, a Chemist and a Plant Physiologist has been established with quarters at the College of Agriculture, Reduit.
Quarantine.
Following the introduction of Mosaic Disease on some seedlings from South India in 1923 (the disease was detected promptly, the infected plants destroyed, and no further cases have occurred) the then Director abandoned further intro- ductions and focussed his attention on the home production of seedlings. In 1928, however, the Committee of the Technical Conference issued a report recognising that it would be inexpedient to exclude foreign seedlings which had gained a high reputation elsewhere, but recommended that they should be introduced under most rigid precautions such as would be provided by the erection of a quarantine greenhouse of proper design. A small glasshouse was accordingly put up at Reduit, but as this was inadequate two large glasshouses have been built adjacent to it, designed on the basis of the descriptions and plans of the quarantine houses of the United States Department of Agriculture at Washington, D.Ç., which were com- municated by E. W. Brandes to the Conference of the International Society of Sugar Cane Technologists at Havana in 1927 and published in its proceedings (the Director of Agriculture has kindly presented the Bureau with a copy of the plans). These houses were soon occupied early in 1929 by introductions direct from Java- mainly P.O.J. 2878 with a few stools of P.O.J. 2725 and as an additional control a few plants of a variety susceptible to Mosaic Disease. The canes have grown well, are apparently free from disease, and will be released from quarantine early in the present year (1930). The early requirements of the Cane Breeder have been met by raising in quarantine in the small house, introduced Kassoer and strains of Saccharum spontaneum. At the instance of an Estates Company with large interests in the Island, setts of P.O.J. 2878 obtained from Java were, by arrangement with the Department of Agriculture in Ceylon, grown to maturity at Peradeniya subject to inspection by the mycologists and entomologists of the Department there, and being found free from visible diseases (especially Mosaic), are being sent to Mauritius and planted in the open in field plots at an estate near Reduit where they can be inspected with frequency by the Mycologist of the Department. I saw a plot of young plants of this introduction which were just coming up, and also a consign- ment of cuttings which arrived during my visit, and were being inspected by the Mycologist for the presence of borers from which they proved to be free. It can hardly be claimed that these introductions are being made with the same rigid precautions as those which are being passed through the quarantine glasshouses of special design.
A standard method might be adopted for the passage of introductions through the isolation glasshouses. One house should be used exclusively for raising plants direct from the imported setts. Not more than three setts of any one variety might be planted (preferably " one-eye setts), and not more than three varieties at the same time from any one country. If varieties are introduced about the same time from more than one country, they ought to be kept apart in partitioned divisions of the house. Not more than a limited number (say about six) of shoots (primary and secondary) ought to be allowed to grow on from any one sett to facilitate inspection. After being allowed to grow for at least six months in the first house subject to close and frequent inspection, top and first cuttings of each variety (a maximum of 18 cuttings of each would be available but not necessarily used) should be planted in the second house and allowed to grow on for at least six months. If the plants can be passed as healthy, top and first cuttings from the six stalks allowed to grow on from each sett could be released for planting in the open, subject to periodic inspection up to maturity. After cuttings have been taken from
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the plants in the first house and have begun to grow satisfactorily in the second house, the stools in the first house ought to be taken out and destroyed by fire. After cuttings are taken for planting in the open the plants in the second house might be allowed to ratoon for not less than three months. If a serious disease new to the Island appears on the ratoons, those planted in the open should be taken up and destroyed.
Both Fiji Disease and Leaf Stripe (Sclerospora sacchari Miy.) are liable to be carried in apparently healthy stalks in a dormant condition for long periods (North: Control of Sugar Cane Diseases, page 21) as well as Gumming Disease and Leaf Scald and probably other, as yet unknown, serious diseases. A long period in iso- lation and a total inspection period of not less than two years are therefore very desirable.
Sett selection.
It
The Department and some estates are attempting to raise a supply of seed-cane especially of White Tanna, free from Gumning Disease and Leaf Scald, by selecting cuttings from mature, apparently healthy stools of field cane and planting them in nurseries. The cuttings are carefully inspected and any showing internal dis- coloration or red fibres are rejected. It has not been possible to make the selec- tions from fields of White Tanna entirely free from the two diseases. is hoped, however, to raise a healthy stock by roguing out young plants showing the leaf stripe symptoms. The canes will be grown to maturity and selections again made, planted out in nurseries and the young plants again kept under inspection. These attempts with a relatively resistant variety like White Tanna may result in the raising of a stock nearly if not quite free from the two diseases. North's experiments in Australia have shown that it is very difficult to free cane from Gum- ming Disease and Leaf Scald by means of sett selection unless the selections are made in fields free from visibly diseased stalks. free fields unsafe which are within a quarter of a mile of diseased cane.
He considers apparently disease- He has found that Gumming Disease is primarily an infectious leaf stripe, the bacterium being carried by wind and driving rain from the wounded leaves of diseased stalks to wounded leaves of healthy stalks on which the characteristic leaf stripes develop in two to three weeks. The occasional long-distance transmissions appear to be effected by flies which suck up the bacterial ooze on wounded diseased leaves. If the variety is resistant or the external conditions favourable to growth, the disease may remain restricted to the infected leaves and be thrown off when those are shed. If growth is checked by harmful external conditions such
as cyclone injury, deficient drainage, dry weather, or abnormal low temperature or the normal ripening of the cane, the bacteria may pass down the vascular strands in the leaf sheaths into those of the stems and produce gum. If the variety is resistant to the discase the stem infection is scanty and the production of gum light. Cuttings taken from such scantily infected stalks transmit the disease to the young shoots which, if of a resistant kind, may develop normally and show no symptoms of the disease; they may grow away from the disease or harbour it in a dormant state up to maturity or, if they suffer a check during growth, develop young leaves showing an occasional leaf stripe from which the infection can again be carried by wind and rain to the leaves of healthy plants. Such stalks, showing few or no signs of the disease, transmit the disease again through the cuttings and to the ratoons.
years
The discase, therefore, may persist for a more or less indefinite number of on resistant varieties with seasonal fluctuations in amount, but never severe enough in the stems as to appreciably affect growth or to cause trouble in the factory. This appears to be the situation in regard to the resistant varieties now in general cultivation in Mauritius. More stalks may carry the infection (dormant) than those showing symtoms of the disease. In any case, setts for the nursery should not be cut from stalks showing any trace of leaf stripes (leaf stripes caused by Eyespot Disease should not be confused with those due to Gumming Disease). There is evidence that virgin canes may carry more stem infection than ratoons and the upper joints be more often free from infection than the lower. Sett selections for the nursery might therefore, with advantage, be first or second ratoons, and tops only should be planted.
The cycle of Leaf Scald is not so definitely known as that of Gumming Disease. It is still more difficult to free cane from Leaf Scald because the disease tends to remain more frequently dormant in apparently healthy stalks and no symptoms