286 DISEASES OF CROP-PLANTS 



time when the prevalence of the affection is of no practical 

 importance. 



Areolate Mildew. 

 The cotton leaf disease known in the U.S.A. as areolate 

 mildew, due to Ramularia areola, has recently been recognised 

 to be common in Trinidad. It gives rise to small angular 

 whitish spots on the undersides of the leaves. 



Crinkled Dwarf, Man Cotton, Mottled Hybrids. 

 Certain types of cotton plant abnormal in form owing to their 

 genetic constitution are met with in the cotton-growing islands 

 and are liable to be regarded as diseased. In the first two cases 

 to be described they occur as very occasional individuals in a 

 normal Sea Island population, in the third as a result of accidental 

 or deliberate crossing of Sea Island with local perennial cottons. 



Crinkled Dwarf. 



This type has been noticed for many years in Barbados, and 

 has been studied from a genetic point of view by S. C. Harland 

 in St. Vincent. 



The plant as a whole is not more than 2 feet 6 inches in height 

 and is generally less than 2 feet. There is a general reduction of 

 all the vegetative and reproductive parts. The leaves are small 

 and characteristically crinkled, with torn and ragged edges, and 

 are uneven in colour from the presence of patches of yellowish 

 green. Vegetative branches are seldom produced and flower 

 production is early and profuse. Shedding takes place to such 

 an extent, however, that only an occasional boll matures and 

 many plants are quite sterile. 



Crosses with normal Sea Island result in a first generation 

 indistinguishable from the normal parent. The second and 

 subsequent generations reveal segregation of the simple Mendelian 

 type into pure and heterozygous Sea Island and pure dwarf. 

 It would appear that the crinkled dwarf is a mutation resulting 

 from the loss of a single factor. 



Man Cotton. 



The definite morphological type known as " man cotton " 

 occurs in Barbados, St. Kitts and St. Vincent, and probably in 

 the other islands. Its genetics have been studied by S. C. 

 Harland. 



Man cotton differs from normal Sea Island in the reduction in 

 size of its parts and in a degree of sterility which is complete as 

 regards the absence of viable pollen from the anthers and almost 

 complete in respect of the ovules, even when pollinated from 

 normal plants. Many of the fruiting branches abort after pro- 

 ducing one or two nodes. The effect of these factors on habit 



