CITRUS DISEASES. 



97 



of the species. The other citrus types are also subject to it, but to 

 a less extent, heavy infections having been noted only in the vicinity 

 of lime- trees. In addition to citrus, a very large number of other 

 evergreen-leafed trees and shrubs serve as hosts, among them being 

 the breadfruit, camphor, hibiscus, guayaba, nispero. and many orna- 

 mentals. Very little damage can l)c attributed to this disease be- 

 yond the slight reduction in leaf surface; 



The nearly circular spots occur, for the most part, on the upper 

 surface of the leaves, vary in number from a few only to many, 



and in size range from a few millimeters 

 to nearly a centimeter. They are slightly 

 raised, at least after the initial stages, and 

 in color vary from dull red to brown, and 

 finallN' become deep dull bi'owii. On dead 

 leaves tliey take on a grayisb tinge. The 

 leaf tissue beneath each spot is killed, show- 

 ing on the under surface as a l)rown area, 

 otherwise unchanged, and more or less the 

 size of the spot above. At certain stages 

 of growth, the surface of the spot shows 

 a deep orange or red fuzzy appearance, 

 due to the presence of a large number of 

 short, erect hairlike processes, which bear 

 the I'e I ) reductive bodies at their tips. 



The parasitic organism involved is not 

 a fungus, as with various of the otiier maladies with which we have 

 been dealing, but an alga, a low type of plant related to the pond 

 scums and sea weeds. 



In addition to this form on the leaves, a type, verv similar if 

 not identical as far as the cause is concerned, occurs to a limited ex- 

 tent on the twigs and branches, particularly of sour orange and 

 grapefruit, and is common on the gandul {Cajanus indicus). In 

 this type the areas are irregular, several often coalescing and ex- 

 tending for considerable distances along the twig or branch. The 

 fruiting stage has much the same appearance as in the leaf form. 

 Some doubt is entertained as to the identity of the two forms, not 

 only because of the slight differences in appearance, but because 

 they have never been found in the same tree, or even in the same 



Fig. 16. — Algal leaf spot 

 (111 linio leaf. 



grove. 



■ Corrective measures are hardly necessary, attacks on orange and 

 grapefruit being so slight. It is probable that ordinai'y spra.ving 



