214 DISEASES OF CROP-PLANTS 



that some of the so-called epidemics of foot-rot have been due 

 to this stage being reached by the trees of a plantation or of a 

 district. 



The mandarin or tangerine varieties, and the sweet orange 

 are most susceptible, the lemon less so, while the sour orange 

 and the rough lemon are decidedly resistant. In these islands 

 the shaddock and grapefruit are regarded as resistant, but 

 according to H. S. Fawcett this is not the case with some strains 

 of the latter. 



Prevention and Control. 



The surest method of prevention is the use of sour orange 

 stocks. With seedling trees already established the ameliora- 

 tion of such conditions as are mentioned above should be carried 

 out so far as it is possible. Especially should the collar and 

 crown be kept free from soil. Treatment of existing cases is 

 possible on the following lines : permanently remove sufficient 

 soil to leave the cohar and crown roots exposed ; scrape and cut 

 away the dead bark until the patch presents a clean surface 

 surrounded by a margin of healthy bark, and paint the whole 

 place over with Bordeaux paste or a good wood preservative 

 such as is used for treating pruning cuts. Remove any low 

 branches which interfere with the free circulation of air around 

 the base of the tree. 



Scaly Bark. 



According to H. S. Fawcett, the term scaly bark is used for 

 two distinct affections in California and Florida respectively. 

 The first, psorosis, occurs in Cahfornia, Florida and Cuba. The 

 same or a closely similar affection occurs on lemon trees in the 

 Dominica Botanic Gardens. A patch of outer bark breaks up 

 into irregular scales, which are pushed up by new growth beneath. 

 The area expands slowly for years, and finally the bark may die 

 quite through and the wood begin to decay. There seems to be 

 little to distinguish the affection from a form of gummosis, and it 

 should be treated by the scraping and painting method described 

 under foot-rot. 



The Florida scaly bark, or nail-head rust, is described as a 

 specific disease, mainly of the sweet orange, not known outside 

 that State, and believed to originate from infection with a 

 variety, ciiricolum, of the fungus Cladosporium herbanim Lk. 



On the smooth bark of branches, of the age of 6 to 9 months 

 or more, raised rusty spots one-sixth to one-half inch broad, 

 with well-marked edges, are formed. The bark becomes glazed, 

 and then breaks into flakes and scales. On larger limbs the bark 

 may become rough and shaggy. On the fruit occur brown 

 sunken spots, some of them in the form of rings, one-fifth to one 

 half inch across. The spots are yellowish to reddish brown on 



