226 DISEASES OF CROP-PLANTS 



ViRUELA, American Leaf Disease. 



A fungus disease of coffee, second only in the attention it 

 has attracted to the Hemileia disease of the Old World, and 

 often known, to distinguish it from that affection, as the American 

 leaf disease, is widely distributed through the American tropics, 

 to which so far it appears to be confined. It is reported from 

 all the coffee-growing countries of Central and South America, 

 and in the Antilles from Jamaica, Porto Rico, and Dominica. 

 The writer has so far seen it only once in Trinidad. It has been 

 found on numerous herbs and shrubs of various natural orders, 

 and some trees, including orange and mango, and even occurs 

 on ferns growing in the neighbourhood of coffee. As is not 

 unusual with fungi attacking introduced plants, it has assumed 

 on coffee an especial virulence. 



The presence and dissemination of the disease are closely 

 dependent on humid conditions and in these islands it accordingly 

 occurs most abundantly in the wetter hill districts. 



Symptoms. 



The disease consists in the production by the fungus of spots 

 dark at first, ultimately white or light grey, on leaves, young 

 stems, and fruits. On the leaves the spots are numerous and 

 conspicuous. They are approximately round, equally visible 

 on both sides of the leaf, and range from 6 to 12 mm. {\ to \ inch) 

 in diameter. As they become old the central tissue drops out 

 leaving a perforation. On the shoots the spots are more or less 

 elongated and from them the cortex eventually flakes away. 

 The spots on the fruit are similar to those on the leaves ; they 

 are rarely more than one or two in number on the same berry. 

 The beans obtained from the affected berries are to some extent 

 discoloured. 



The effect of an infestation of the leaves is to cause premature 

 shedding, and entire defoliation may ensue from a heavy attack, 

 so that the ground is carpeted with leaves partly green. This 

 stops the development of any crop that may be on the trees, and 

 may lead, especially if soon repeated, to dieback of shoots and 

 to a condition of debility in the trees. 



According to G. L. Fawcett, in Porto Rico the older planta- 

 tions in districts liable to the disease are well infested, while the 

 younger ones are as a rule free, and the spread of the disease is 

 quite slow, having been estimated in an observed instance to 

 take place at the rate of about 200 yards in a year. 



Causative Fungus. 



The nature of the disease was first detected by M. C. Cooke 

 in 1880. He described the fungus found on the spots as Stilhum 

 flavidum, a name later changed by Lindau in a taxonomic re- 

 arrangement to Stilbella flavida. G. Massee in 1909 found peri- 



