142 



THE AGRICULTURAL NEWS. 



April 22, 1916. 



PLANT DISEASES. 



LATER EXPERIENCE WITH THE CITRUS 

 CANKER IN FLORIDA. 

 Articles have already appeared in the Agricultural News 

 dealing with the fiisc recognition of citrus canker in Florida, 

 and w?th the drastic measures, culminating in the burning 

 up of infested groves with flaming sprays, taken in the 

 attemp to stamp out the disease. The discovery of its cause, 

 fl, bacterium, by Miss Clara Hasse, of the Bureau of Plant 

 Industry, has also been recorded. 



Bulletin 128 of the Florida E.xperiment Station, pub- 

 lished November 1915, entitled 'Citrus Canker III,' by H. E. 

 Stevens, gives some very interesting further information as 

 to the nature of the disease and its present position in 

 Florida. 



The author remark.s that there are certain peculiar 

 features of the disease that are not easily explained. Canker 

 seems to be capable of remaining dormant on a tree, or quies- 

 cent in its vicinity, for long periods without any symptoms 

 of the disease appearing: but eventually such trees may 

 develop typical cases of canker. This has been illustrated 

 on a number of occasions when trees have been removed 

 to new localities from nurseries infected with the disease. 

 These trees had been defoliated, and they showed no signs 

 of the disease at the time of removal. To all appearance 

 they were healthy and free from canker, although they had 

 been exposed to the disease. Shipments of such trees were 

 made to diflerent parts of the State. A year later some of 

 them had developed virulent cases of canker. Thus any 

 tree that has been exposed to the disease may possibly 

 develop canker at some future date. The same holds true 

 where the disease appears in the groves. All healthy trees 

 surrounding the centre of infection must be kept under 

 constant observation for future outbreaks. 



A map illustrating the bulletin shows that the places in 

 Tvhich infection has occurred are scattered wdely over the 

 whole State. The author states that no canker-infected trees 

 really exist in any of the localities indicated, for all infected 

 trees are burned on discovery. In most cases the entire 

 grove is burned if the trees are small. All properties where 

 infection has been found are quarantined and kept under 

 observation until they are considered free from disease. The 

 quarantine area is included by a circle with a radius of 

 a mile from the infected area. 



The disease remains most serious in its eft'ect on grape- 

 fruit, but almost any of the varieties of citrus may be severely 

 attacked when the conditions are favourable. Young growth 

 is most readily attacked, but tissue of any age may become 

 infected. Canker has been observed to develop in the bark 

 of grapefruit branches two or three years old. On the leaves 

 infections first appear as small watery dots with raised 

 convex surfaces. These dots are usually of a darker green 

 than the surrounding tissues. Sometimes, however, the 

 surface of the spots is broken as soon as they appear. Spots 

 may appear on either surface of the leaf, but they do not 

 at first penetrate through the leaf tissue. They gradually 

 increase in si/.c, chxnge to a light brown and become visible 

 on both sides of the leaf. In the older spots one or both 

 surfaces may be bulged or raised, and such spots are com- 

 monly surrounded by a narrow yellowish band or zone. In 

 the more advanced stages the surface of the spots becomes 

 white or greyish and finally ruptures, exposing a light brown 

 spongy central mass. On the fruits the spots are very 

 similar. They project and retain a circular outline. They 

 may be scattered or may occur together in an irregular 

 znass. 



The spots on young twigs are like those on the leaves 

 and fruit; those on older twigs are more prominent and may 

 be irregular in shape. They show the same spongy tissue, 

 but assume a cankerous appearance and may persist for a 

 long time. 



The author adopts and confirms the conclusion of Miss 

 Hasse that the disease is of bacterial origin. Inoculations are 

 easily made by spraying uninjured foliage with suspensions 

 of bacteria in water, provided that the surrounding air is 

 maintained in a humid condition. 



Results which have a very important bearing on th& 

 possibility of exterminating or controlling the disease were 

 obtained by laboratory investigation of the organism. It was 

 found that the bacterium is capable of remaining alive for 

 a long time in sterilized soil, in which under favourable 

 conditions it can multiply rapidly, and penetrate to a con- 

 siderable depth. In cultures of this nature the bacteria were 

 alive and active at the end of a period of six months. 

 Even in air-dry soil they have Iseen found to survive and 

 retain their infective powers for three or four months. 



The bearing of these facts upon the possibility of trans- 

 porting infective material for long distances, and on the roots 

 of plants other than- citrus species, is obvious. 



With a view to testing the likelihood of infection being 

 carried on the clothing of workmen or others coming in 

 contact with diseased trees, pieces of cloth were wetted with 

 suspensions of bacteria from cultures. After being kept dry 

 for five weeks the organism was found to be still alive. Dried 

 leaves kept in the laboratory for a month were still infective. 



In the groves citrus canker is spread by some carrier. 

 Insects, birds, or animals may carry it from tree to tree. 

 Rains and dews assist its spread within the tree. The bacteria 

 ooze out in multitudes from the spots when they are wetted, 

 and when dry adhere firmly to any surface to which they 

 may be carried. 



The disease spreads rapidly during rainy weather and is- 

 more or less retarded in dry periods. High temperatures and 

 high humidity favour its rapid development. 



The vigorous attempt which is being made to eradicate 

 this very serious menace to a valuable industry will be 

 regarded with interest and sympathy by all to whom the 

 possibility of similar danger presents itself, but this further 

 information as to the nature of the problem makes clear the 

 very great difficulties involved. 



W.N. 



A Motor Plough for Light Work.— It i* 



interesting to note that a new motor plough for light 

 work on small and medium holdings, especially for shallow 

 ploughing, or for deep ploughing in light soils, is described in 

 the Monthly Bulhtin of Agricultural Intelligence and Plant 

 Diseases for October 1915. The machine is a comparatively 

 small and light arrangement, and is provided with two long 

 wooden bars which the operator uses like the handles of a 

 plough. The motor is situated in front inside the frame. 

 The driving wheel is a sheet iron drum fitted with a 

 certain number of cleats with which it grips the soil. To the 

 frame the ploughs are hitched as to the usual forecarriages of 

 ploughs. Two small ploughs hitched to this machine did good 

 work in a trial in Italy. The strip ploughed was 20 inches 

 wide and 6 to 7 inches deep, on meadow land with a shallow 

 but very hard turf and loose subsoil. The work done in ten 

 hours was about 3h acres, and the total cost of work did not 

 exceed 4s. per acre. Completely equipped, the machine costs 

 in normal times, about £120. The motor develops 3 to 4 h.p. 

 The total weight of the machine with ploughs and petrol is 

 about 840 lb. 



