lo July, 191--] " E.xanthcma" of Orange Trees i)i Australia. 401 



ERUPTIVE DISEASE, OR "EXANTHEMA" OF ORANGE 

 TREES IN AUSTRALIA. 



By C. C. Brittlehank, Vegetable Pathologisf s OifLce. 



From time to time specimens of diseased oranges, together witli their 

 leaves and branches, have been forwarded to the office of the Vegetable 

 Pathologist, with a request that a determination of the disease be made, 

 and, if possible, a remedy suggested. When on a recent visit to the capital 

 of a sister State, a request was made by several fruit-growers that 

 T should visit a number of orange groves in which an unknown disease, 

 had appeared. As this was causing .serious trouble to the growers, so much 

 .so that they intended to grub out their trees if relief could not be obtained. 

 During this in'ierview I was informed that they could not obtain anv in- 

 formation as to the cause or control cf the disease. A visit was paid to 

 the district and an examination of the (lisea.sed trees made. This, together 

 with the knowledge gained from specimens forwarded to Melbourne, soon 

 convinced me that the trouble was physiological, and not due to anv special 

 fungi. 



Description of the Disease. 



A most striking feature of this disease is the general healthy appear- 

 ance of the trees, the leaves of which are of a beautiful dark-green, and 

 frequently nearly twice the normal si/e. This apparent symptom of health 

 is followed by a yellowing of the young tender shoots, from which the 

 leaves fall. Soon after, or concurrent with the fall of the leaves, the shoots 

 turn reddish-brown, owing to the middle layers of the bark becoming en- 

 gorged with a resin-like substance. Twigs affected as described die back 

 for a length of from 6 to 12 inches. Numerous bushy twigs arise from 

 the smaller branches, these however .••oon die away. gi\ing the characteristic 

 appearance to diseased trees. When a number of affected shoots are borne 

 on a larger branch the bark of the latter is often blistered, split, and 

 ruptured to such an extent that the injured portions become almost con- 

 fluent. (Plate I., Fig. i.) From the.se injuries a hard rusty-coloured resin- 

 bke gum exudes; this granulates on the edges of the injuries and does not 

 run or collect in tears, as in some other citrus diseases. (Plate I., Fig. n.) 



An examination nf the bark shows that the actual ruptures do not extend 

 into the cambium or sapwood, but generally only affect the middle laver 

 of the bark outwards. Howe\er, beneath the ruptures numerous minute 

 gum pockets are formed in the sapwood directlv beneath the ruptures 

 These appear as clear glass-like specks, if a shaving be removed and held 

 against the light. (Plate II.. Fig. IV.) Sometimes even before the tree 

 shows any sign of disease in leaf or shoot, it can be detected by the pale 

 unhealthy colour of the fruit, which often falls before becoming mature. 

 Oranges borne on affected branches are frequently stained by the reddish- 

 brown exudation which hardens the skin, causing them to split or crack. 

 (Plate II.. Fig. II Ia.) In some instances the diseased fruit becomes quite 

 hard, and in others it falls early. Eveai those which remain ripen pre- 

 maturelv, and are of a pale-greenish -lemon yellow, and quite insipid to 

 taste, as are also the green immature oranges. 



Nature of Soil in Affected Areas. 

 As nearly all soils ar- determined by the nature of the underlying 

 jreological formation, and as the main mass is composed almost entirely of 



