Pomona College Journal of Economic Botany 



31 



spots) may, however, be located on the leaf entirely removed from the margin. 

 The characteristic color noted above marks the commencement of the dis- 

 ease, the period during which the mycelium (fungal threads) is growing 

 most actively in the tissues of the leaf." 



"As the disease progresses, the affected area becomes more clearly defined. 

 The areas representing a number of separate spots of infection, close to each 

 other, becomes continuous. The color changes from yellowish to the gray 

 hue of dead tissue. The line of division, between the dead and the green, 

 living tissues, is slightly elevated and as already noted, clearly defined. 

 At this stage, a close examination of the discolored spots on the upper side 

 of the leaf reveals the presence of a number of minute dark dots. These 

 dots are arranged more or less completely in concentric circles or ovals. 



Figure 16. At left a characteristic leaf spot of lemon probably also due to wither-tip; 

 at right fully developed acervuli of wither-tip on orange twigs. 



These dark dots are in reality the spore-bearing parts of the fungus (Acervuli) 

 containing large numbers, the spores, by means of which the disease is spread 

 from one portion of the tree, from one tree or from one grove to another. 

 The affected, or diseased areas are usually located near the tip or side of the 

 leaf, though they may be situated near the central portion of the blade." 



Of this leaf-spot Prof. Rolfs says, "The first point of attack is in the leaf. 

 The development of the fungus takes various peculiar forms. At times 

 the acervuli are distributed in a more or less irregular way from the center, 

 resembling 'fairy rings.' At other times the infection takes place in the 

 tip of the leaf, which gradually withers back to the stem. Small trees 

 may be defoliated and the fungus continue to develop in the twigs." U. S. 

 Dept. Agrcl. Bull. No. 52, p. 15, 1904. 



