DISEASES OF COTTON 285 



determined with certainty, but it is very similar to Alternaria 

 tenuis, Nees. 



" Artificial inoculations show the fungus to be a weak parasite 

 and able to infect healthy tissue only under most favourable 

 conditions." 



Leaf Mildew. 



Under the name of West Indian leaf mildew a leaf affection 

 of Sea Island cotton has been referred to in all accounts of the 

 diseases met with on Sea Island cotton in the Lesser Antilles 

 since the revival of cotton-growing. Perennial cottons are also 

 attacked. The disease is universally distributed through the 

 islands and occurs in abundance in most fields towards the end 

 of the crop. The fungus was described in 1920 from material 

 collected by the writer as Ovulariopsis Gossypii E. M. Wakefield. 



Symptoms. 



The affection is caused by the growth of the fungus, in patches 

 or with a general distribution, on the backs of the leaves. The 

 dark patches produced are covered with a more or less dense 

 white web on which abundant large conidia are produced. On 

 the upper surface of the leaves the infested areas are marked by 

 irregular reddish-purple or purplish-black blotches or by a more 

 general suffusion of the same colours. 



The Fungus. 



The mycelium is almost entirely superficial, hyaline, and 

 freely septate. A few hyphae penetrate by the stomata into the 

 mesophyll. The conidiophores are upright and simple, septate, 

 bearing terminally in succession the hyaline, reticulate conidia, 

 which are oblong or broadly elliptical, with one end rounded or 

 slightly produced, the other flattened at the point of attachment, 

 and measure 50-60 x 16-22 microns. Not more than two, one 

 immature, have been seen attached to the conidiophore at the 

 same time. 



Incidence. 



The disease has been usually reported from the various 

 islands as attracting attention only after periods of wet weather. 

 Its occurrence on the foliage borne by the plants towards the end 

 of the crop season is not limited in this way, for it can be general 

 even in very dry weather, but it appears that excessive moisture 

 induces an earlier prevalence, and in such an event it is probably 

 responsible for some indirect loss of crop. That it is not neces- 

 sarily confined to old or even to mature plants has been shown by 

 its occurrence on the fresh leaves of quite young plants grown in 

 pots, but under normal conditions the plant is resistant up to a 



