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11111 C.O.882/12

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Leaf Scald was only distinguished definitely from Gumming Disease in Mauri- tius in 1928, having been confused with the latter previously. In order to judge the degree of resistance of White Tanna to Leaf Scald it is desirable to form an estimate of the length of time the disease has been present in the Island. Although Leaf Scald has been present in Java for many years it was most probably introduced into Mauritius from Australia. North has recorded that it was apparently already in a number of localities in Queensland and Fiji during the early years of the present Introductions century, although not recognized as a definite disease until 1911. were made from Queensland into Mauritius in 1908 and 1909, from Fiji in 1914, from Queensland in 1915, from New South Wales in 1916, and again from Queens- land in 1919. In the Annual Report of the Department of Agriculture for 1915 (page 14), the Director referred to a disease present during the first half of that year in fields of virgin White and Striped Tanna, probably about 12 to 18 months old, on a number of estates, with symptoms which are now known to be character- istic of the chronic phase of Leaf Scald-production of side shoots from every node, dying off of the top leaves, and red fibres in the stems. An organism could not be extracted but may have escaped observation as B. albilineans, the cause of Leaf Scald, grows slowly and tends to develop only on special substrata. There had been a drought from December to February, some rain at the end of that month, and a dry period again during April and May, causing two checks to the growth of the cane, conditions which would favour the passage of that organism from relative dormancy into active growth. Leaf Scald has been present in Mauritius, therefore, for at least ten years and perhaps as long as twenty-two years. It is characteristic that the disease may remain dormant in apparently healthy cane for many months, at times for a year or more, and as the introduced varieties were in several instances distributed as early as possible to most of the estates, its wide occurrence in the Island at the present time on the Tanna canes can be understood. Leaf Scald has been, consequently, long enough on the estate culti- vations to judge that the Tanna canes show considerable resistance to it, sufficient to class them as, at least moderately resistant, while the other varieties The more serious acute or in general cultivation appear to be still more resistant. wilting phase of the disease has not been definitely detected in Mauritius.

A few instances of gum in the stems of White and Striped Tanna were seen, due to Gumming Disease (Bacterium vascularum (Cobb) G. Smith), from which the crop of 1929 has been very free generally. This disease caused heavy losses on the White and Striped Bamboo varieties (Batavian canes of the Preanger series) in 1893 following the great cyclone of 1892 and resulted in their replacement by other kinds. It has been in part responsible for the abandonment of a number of seedlings in the course of the last thirty years. The Tanna cane has certainly been exposed to this disease throughout the period of its cultivation since its reintroduction in 1891, and has shown itself to be markedly resistant. As a result of the elimination of the susceptible kinds, the other varieties now in general cultivation show strong resistance to Gumming Disease so that it cannot be considered now as a cause of very appreciable loss of crop.

Cases of Red Rot (due to the fungus Colletotrichum falcatum Went) were seen at a number of the estates. With the exception of D.K. 74 all the cultivated kinds are resistant to this disease, infection as a rule occurring only through borer injuries.

Smut (due to the fungus Ustilago scitaminea (Rab.) Syd.) was seen on young ratoons of D.K. 74 at one lowland estate in the north and on some seedlings in experimental plots in the west. The disease may cause appreciable loss of shoots of young cane during dry weather in dry districts, especially during a dry December, but is practically restricted to D.K. 74 and, in a less degree, to M.P. 131.

In each of the three years 1925-27 the Department carried on field surveys of the incidence of Gumming Disease, Red Rot, Root Disease, Smut and Pineapple Disease (infection of planted setts) on the four varieties White Tanna, D.K.” 74, M.P. 131, and M.P. 55 at a number of estates in all districts, and these were supple- mented in 1927 and 1928 by estimates of the proportion of Gumming Disease and Red Rot in large samples taken from the cane carriers at the factories. In 1928 an attempt was made to record Leaf Scald as distinct from Gumming Disease both in the field and carrier samples (the incidence of Root Disease, Smut and Pine- apple disease was omitted). These surveys were not undertaken for the crop of 1929. Examinations made at the carriers tend to give the most significant indications of the relative proportion of stalks infected by the major diseases-Gumming, Leaf

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Scald, and Red Rot-because the infection shows most intensely and definitely on the mature cane. The recorded data indicated that the White Tanna was appreciably more infected by Gumming Disease in each of the four years than the three other varieties, and somewhat more infected than they were by Leaf Scald in 1928. Only D.K. 74 showed appreciable infection by Red Rot. In 1927 the crop was markedly below normal and much below the estimates, while in 1928 it was considerably in excess of normal and much above the early estimate, which was based in part on the field survey of disease incidence. The proportion of stalks of the four main varieties infected by the major diseases both in the field and at the carriers was higher in 1927 than in 1928, but not sufficiently so as to account for the large difference between the crops of the two years (about 35,000 tons of sugar or 15 per cent. of a normal crop of 235,000 tons). Walter (The Sugar Industry of Mauritius, 1910) has shown that, apart from cyclones, the amount and distribution of rainfall and temperature appear to be the most important factors determining the large fluctuations in crop returns which characterize sugar production in the Island. It is an indication of the marked resistance to the major diseases of the varieties in general cultivation (the susceptibility of D.K. 74 to Red Rot is an exception) that the incidence of these diseases has not hitherto been found to be notably higher in years when weather has been unfavourable to yields than in years of good weather and large crops. [The latest estimate indicates that the crop of 1929 has been about 238,000 tons, but as some 78 per cent. of it was in the form of raw sugar as against only 23 per cent. in 1928 the yield expressed as white sugar would make it rather below a normal crop because the return in the factories is higher for raw than for white sugar and will, in fact, be greater for 1929 than in any previous year. Dry weather in February and March, normally months of high rainfall, was the dominating unfavourable weather factor during the year.]

Some remarks may now be made about the individual varieties in general cultivation.

White Tanna (syn. Yellow Caledonia). Mention has been made of the very large proportion of the cultivated area planted to this variety on many estates- about 58 per cent. of the total area in cane according to the survey of 1925-but on many estates then and now from 75 to over 90 per cent. of the area in cane. It is believed that the great bulk of the crop now grown is descended from an unstriped sport which appeared in the Striped Tanna soon after the latter was introduced from Queensland in 1891. White Tanna has been widely grown for over 30 years, replanting being done on an average every seven years, so that its good and bad qualities are well known. It has shown itself adaptable to cultivation both in the uplands and lowland, giving good returns up to five ratoons and even up to seven and eight ratoons on the deeper, well-manured lands; it has shown marked resistance to diseases and was mainly responsible for the large crop reaped in 1928. It is a late maturing cane attaining a high sugar content only when fully mature, a condition which is promoted by the porous character of the soil and the normal cool dry weather preceding and during the crop season. It has the defect of springing with difficulty if cut flat, especially if reaped before it is fully mature, and this has necessitated the leaving of suckers and immature stalks (babas) at reaping, a practice which is now general over large areas. In the drier northern districts it has to be cut flat as the young growth already present is liable to be killed out by dry weather; as a result, many holes fail to spring and other varieties more resistant to drought have, to a considerable extent, replaced it except in situations where the rainfall shows better distribution. "hollow pith" in White Tanna during my tour, the tissues being otherwise of I saw many cases of normal colour; it is known to vary in amount from year to year without having any perceptible effect on the health of the cane or quality of the juice; it has been noted on other varieties and cannot be regarded as a mark of degeneration. I saw, however, not infrequently, a discolouration of the ground tissue adjacent to the pith of mature Tanna canes at first yellow and later reddish brown, from which Mr. Shepherd has isolated a bacterium which has not hitherto reproduced the symptoms following inoculation. The condition calls for further investigation as it may be due to an infection; an experiment is in progress to determine if it is transmitted by means of the sett. The opinions of planters differ widely in regard to the present day merits of White Tanna, some holding that it is "played out " while others claim that it is the best available kind and not likely to be surpassed by any new seedling. These extreme views are not shared by a growing number of planters.

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