572 SPECIAL PLANT PATHOLOGY 



account for the alcoholic flavor, if not lead to the decrease in acid 

 and the sweeter taste. 



Die-back or Exanthema of Citrus Fruits.^ — Exanthema is a disease 

 of the orange groves of the United States occurring in California and 

 Florida. It afifects all varieties of the genus Citrus, both young and 

 old trees being susceptible. The malady is worse in trees which grow in 

 poorly drained soils underlaid by an impermeable ferruginous sandstone 

 but it occurs in hammocks as well. Exanthema attacks the small 

 branches and shoots, though the fruit shows symptoms of diagnostic 

 value. The disease is diagnosed more surely when the shoots become 

 more or less stained sub-epidermally by a yellowish-brown material 

 and begin to die back. The fruit may become similarly stained and 

 its epidermis so dry that it cracks and splits by the pressure of the 

 developing pulp cells. The disease may be held in abeyance for a 

 number of years, but if it progresses, the shoots swell at the nodes, 

 infrequently along the internodes and as they mature, linear, erumpent 

 pustules break out on the internodes. On the older branches the 

 pustules may be extremely numerous and a small amount of gum may 

 be observed in them. Proliferation of young buds takes place and these 

 may develop into short branches with chlorotic foliage producing a 

 pseudo witches' broom. 



Exanthema is induced, like gummosis, by the concurrence of active 

 growth and active tissues. "The soils in which exanthema occur are 

 t)T>ically dry soils, which when saturated by irrigation water or rains, 

 promptly become dry once more when the weather clears or irrigation 

 is discontinued. The rings of growth, which, as we have seen, are very 

 marked in diseased shoots and branches of trees afi'ected by exanthema, 

 could not be caused except by a more or less rapid succession of maxima 

 and minima of growth." Obviously as climatic conditions cannot be 

 said to be causative, we must look to changes in the water relations of 

 the plants which causes a marked development of the rings of growth. 

 Webber and Swingle have observed that cultivation increases the sus- 

 ceptibility of the Citrus trees to exanthema, and even causes a 

 more virulent outbreak of the disease in the affected trees. Any 

 method of cultivation which tends to promote regular instead of fluctu- 



' Butler, Ormanu; \ Study on Gummosis of Prunus and Citrus with Obser- 

 vations on Squamosis and Exanthema of Citrus. Annals of Botany 2s: 107-153, 

 1911. 



