DISEASES OF PLANTS 91 



Exanthema in Citrus is of widespread occurrence throughout 

 the world. The symptoms of it in Citrus trees in Western Aus- 

 tralia have been described by Pittman (1936) as follows. In the 

 orange strong water shoots tend to bear abnormally large leaves 

 while the shoots themselves, instead of growing straight, form 

 an S-shaped curve. Small blister-like swellings containing gum 

 develop on the young shoots. Later these swellings develop into 

 longitudinal ruptures bordered with brown or reddish brown 

 ridges from within which the yellow or reddish gum exudes in 

 wet weather. Shoots so affected lose their leaves and die back, 

 and lateral shoots developing at the base of affected twigs pro- 

 duce a typically bunchy appearance. A condition of 'multiple- 

 bud' development, resulting from the development of a cluster 

 of buds instead of two, is frequent, thus emphasizing the bunchy 

 habit of the tree. Die -back is typical of badly affected trees. 

 The fruit is small and frequently marked with irregular-shaped 

 brown spots or blotches where finally the skin, after becoming 

 dry, splits open. 



In the lemon, gum formation on the branches is rare, but gum 

 pockets may develop on the skin of the fruit. 



Haas and Quayle (1935) state that leaves of Citrus in the 

 early stages of exanthema may be unusually green, but may 

 become mottled or chlorotic in later stages of the disease. 



According to Smith and Thomas (1928), in French prune trees 

 (Prunus domestica) affected with exanthema there is vigorous 

 growth of new shoots each spring, but in June the terminal buds 

 wither and the terminal leaves develop a chlorosis. There is a 

 similar development of, lateral shoots and of multiple buds as 

 in Citrus as well as the production of eruptions of the bark. 

 Apples, pears and Japanese plums can develop similar symptoms. 

 Pittman describes exanthema of Japanese plums in Western 

 Australia as involving the development of cracks in the bark 

 which in some varieties may reach the cambium. Later, exuda- 

 tion of gum takes place through the ruptures. Die-back is here 

 also very characteristic of the condition. 



As long ago as 1917 Floyd reported the beneficial effect of 

 copper sulphate on Citrus trees in Florida affected with this 

 disease, and confirmation of this was forthcoming from Wickens 

 (1925) in the treatment of affected orange trees in Western 



