512 INSECTS AFFECTING VEGETATION 



trees. As a preventive measure, use resistant stocks as already indi- 

 cated, and where trees have died out re-set with others budded on 

 sour orange, pomelo or rough lemon, as the soil conditions render 

 necessary. (Fla E. S. B. 53.) 



Scab. This citrus disease has been known in Florida for the 

 past fifteen years or more, having been observed first on the sour 

 orange, C bigaradia. The bitter-sweet and lemon are also very much 

 subject to the disease. This year it has been collected on young, 

 tender leaves of the pomelo, C decumana, and on the foliage of the 

 Satsuma, C nobilis (?), and Kumquat, C. Japonica. A large num- 

 ber of young Satsuma trees badly affected by the disease were found, 

 and the leaves showed that considerable damage had been worked 

 by the fungus. It is not so common on the Kumquat; only a few 

 diseased leaves were found, though several hundred trees have been 

 examined. It is very probable that on one of the last-named varie- 

 ties it was introduced into Florida from Japan, and, here finding 

 host plants and climatic conditions adapted to its development, has 

 become a serious inconvenience in the successful growing of certain 

 citrus fruits. 



Scabby leaves, twigs and fruit are very characteristically marked. 

 Warty, corky elevations cover the surface, giving to it an unsightly, 

 roughened appearance. Often the leaves are twisted or drawn out 

 of shape, and they are in a considerable degree deprived of the 

 power to fulfill their natural functions, namely, respiration, trans- 

 piration and the assimilation of food. Beneath the warts on the 

 opposite side of the leaf, there is often a well marked conical depres- 

 sion, corresponding to the elevation on which the excrescence is situ- 

 ated. Under the warts on the fruit there is an abnormal thickening 

 of the tissue resulting in the formation of somewhat conical eleva- 

 tions. Thus the corky portions are lifted above the normal level 

 of the rind. The warts are at first yellowish then grayish, becoming 

 dusky in appearance as the disease advances, until they become 

 almost black, and eventually crack and open. 



The specific cause of the disease is a minute parasitic fungus, 

 a species of Cladosporium described by Prof. F. Lamson-Scribner in 

 1886. The spores are very small, smoky in color, and usually one- 

 or two-, though sometimes three-celled. They are borne on brown- 

 ish colored sporophores (spore-bearing filaments). When mature 

 they become detached, and through me agency of the wind they 

 are carried about from one tree to another. They fall upon the 

 leaves of their host, and, under favorable conditions, germinate by 

 sending out a delicate, slender tube, which enters the leaves and gives 

 rise, in due time, to the well-marked diseased condition. 



The experiments carried out by Webber and Swingle have gone 

 to show that the disease can be successfully controlled by using one 

 of the copper spraying solutions, Bordeaux mixture or ammoniacal 

 solution of copper carbonate. The latter must receive the higher 

 recommendation, as it is less likely to injure the tender leaves and 

 blossoms of the lemon but a weak solution of Bordeaux mixture is 

 likely to prove quite as efficacious without any harmful results. 



