Symptoms of the disease.
Although the effects of the disease on the fruit are the most con-spicuous, and of the greatest importance economically, numerous infections also occur on stems and leaves. The latter do not greatly affect the general health of the tree, but they serve to carry over the infection from one season to the next.
The first signs of infection noticeable on the leaves are a number of small, angular, water-soaked areas bounded by some of the veins of the leaf. These spots do not increase much in size and rarely exceed 2-3 mm. in diameter; but if they are very numerous they coalesce and larger spots are formed. The infected areas soon begin to discolour and become dark brown, the surface is SOJilewhat raised and shining and frequently there is a slight exudation of gum (Plate II). In very old leaves these discoloured spots become white and dry, and crack away. If infe~tion takes place in the petioles-as is often the case-longitudinal cracks resqlt which attain a length of 1 em. or less.
Diseased spots are also found on all parts of the stem, although it is evident that the majority of the infections take place in young and rapidly growing tissues; they are also common in the scars from which leaves have newly fallen. The first appearance of disease in the stem is a discolouration of the tissues. This ia accompanied by gummoaia and the formation of deep longitudinal cracks. The diacolouration is not merely superficial but penetrates some distance into the stem (Plate III).
The disease is especially evident on the peduncles and pedicels; frequently, by the time the fruit is half grown, the whole inflorescence is afiected. The stalks to which the mangoes are attached become black and dead, and consequently the fruit all falls to the ground (Plate VI):
AB has been mentioned above, although the spots on the leaves and stems are often very numerous, they do not noticeably afiect the general health of the tree. Their chief importance is as a source of infection for the fruit which is seriously injured by the disease. Large numbers of mangoes fall to the ground, and the small percentage which remains on the trees is so disfigured as to be practically useless (Plate IV). The mangoes on the windward side of the tree sufier most. The diseased spots first appear on the most exposed side of the fruit or else at the spot where two mangoes on the same bunch are in contact and where a drop of water would lodge after rain. The first sign of infection is a small water-soaked area round the white spot which indicates the presence of a stoma, or near a slight wound; this spreads considerably and then begins to discolour. Cracking takes place in s~veral directions and the surface of the diseased area becomes very much roughened. If infection takes place during a period of rapid growth, deep longi-tudinal cracks are formed, running almost the whole length of the fruit in bad cases (Plate V).
The discoloured spots vary much in size; they “re 1 mm. to 15 mm. in diameter and are irregular in shape, and the discolouration penetrates to a depth of 8-15 mm.
Especially noticeable in the case of infections near the point of attachment of the fruit is the exudation of gum which runs over the surface of the mango. This substance is highly infectious and diseased spots are developed wherever it touches the surface of the fruit.
When a mango has once become diseased the slightest air movement detaches it from the tree, and the ground becomes strewn with decaying fruit.
Hosts and Varieties affected.
The disease has been observed on all the varieties of mango commonly grown in this country, but all are not equally susceptible.
The variety known locally as the Peach Mango or Figette is the most resistant. This is a very fine-looking mango and keeps well, but is said to be of inferior flavour.
The most susceptible is the “Baissac” variety, known locally as the “Long Green”; the other three varieties which are commonly grown, namely “Maison Rouge,” “Corde” and “Dauphine,” are almost as badly affected. These are known locally as the “Long Red,” or “Red Kidney,” the “Common Yellow” and the “Round Green,” respectively.
In view of the fact that so far as is known at present this disease is confined to South Africa, search was made among nearly related indigenous plants for a source of infection. In particular, the “Maroola” tree (Sclerocarya caffra) was examined, as it is very common in districts where the mango is grown. The only specimen of this plant which showed any spots on the leaves proved to be infected with Cercospora sp.; and I have been unable to produce with the bacillus causing the mango disease, any infections on the leaves or fruit of the tree.
A more detailed study of the progress of the disease was made during the season 1911-1912 in connection with spraying experiments which were being carried out at Barberton in an orchard kindly placed at our disposal by Messrs Winter Brothers. The season was an exceptionally dry one, and consequently unfavourable to the spread of the disease. The following table compiled from statistics furnished by the Meteoro-logical Department shows that the rainfall has been much lower than in the previous seasons. The temperature has been exceptionally high.
Bacterial .Disease of the Mango
The orchard in which the spraying experiments were carried out is situated on a slope to the east of the town, and contains 28 trees arranged in four rows of seven trees each. The trees are ten or twelve years old and comparatively small-about 12 feet high and 35 feet in circum-ference-so that the spraying could be thoroughly done.
There is a range of hills to the south of the orchard and the pre-vailing winds are from the south-east. The disease had started in the south-east corner of the orchard, and from there had spread right through. Before the experiments were commenced the trees were all diseased: row (1) being bad right through while in the other rows trees and (7) were the worst.
Diseased spots were first observed on the fruit about three weeks after the first rain of the season.
The final estimate of the percentage of diseased fruit refers only to the mangoes still on the trees; large numbers had fallen to the ground and rotted.
The result of the experiment was far from encouraging. On the 20th January an estimate of the number of mangoes still on the trees showed 3″0·7 % free from disease on the trees sprayed with iron sulphide solution; 54 % on those sprayed with Bordeaux mixture and 67 % on the control trees.
A second spraying experiment was carried out during the season 1912–13, but unfortunately weather conditions were again unfavour-able.
The spraying was done in the orchard described above, trees Nos. 3, 5 and 7 in each row being left as controls and the remainder being sprayed with Hycol in the proportion of half a pint to 40 gallons of water (1 in 600). The trees were sprayed four times, and at the time for the first and second sprayings the ground was disinfected with Hycol in the proportion of half a pint of Hycol to 20 gallons of water (I in 300).
The season was an exceptionally dry one, the fruit developing slowly, and some trees failing to set any fruit at all.
Owing to the drought, the disease spread very slowly early in the season; the first infections on the fruit being observed early in Decem-ber. The trees were sprayed for the fourth time on the 4th December and on this occasion it was computed that ·45 % of the fruit on the sprayed trees was infected and 1·2% of that on the unsprayed.
In January the experiment was discontinued owing to the fact that the fruit had been severely cut up by a hailstorm on the 20th of December. On the 7th of January it was calculated that the unsprayed trees had dropped 36·1 % of their fruit and the sprayed trees 38·8 % since the previous examination. Of the fruit which remained on the tree there was 16·7 % of the fruit on the unsprayed trees diseased as compared with 7·3% on the sprayed trees. The latter seemed to have made more growth than the former, and the Hycol had not damaged the fruit or foliage in any way. The trees which were sprayed with iron sulphide solution during the previous season had received a severe check.
The results of this experiment as far as they went were more en-couraging than those of the previous one, but owing to their having been brought to such a premature end they were not sufficiently conclusive, and a further test was necessary. A table is appended showing the rainfall, temperature, etc., during the season.
The spraying with Hyco} was continued during the season 1913-14. The first spraying was done on the 29th August, when the trees and the soil underneath them were thoroughly drenched with Hycol 1·600. The treeB were then in flower.
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