266 DISEASES OF CROP-PLANTS 



ficiently for the species to be recognized, but comparison of 

 cultures has led to the tentative conclusion that Bacterium 

 nialvacearmn is frequently, but by no means invariably, the 

 organism involved. 



Incidence. ' 



The accompanying curve illustrates the incidence of the 

 disease where cotton stainers become generally abundant about 

 December, as was the case in Montserrat and St. Vincent before 

 controlling measures were taken. Nevis has the same distribu- 

 tion. In St. Kitts, Antigua, and the Virgin Islands the insect 

 is not usually abundant before February, and the damage has 

 consequently been much less in amount. In Montserrat it was 

 possible to avoid the worst effects by early planting, but in St. 

 Vincent the heavy autumn rainfall made late planting a necessity, 

 and the precarious chances of the crop depended on whether 

 the first picking escaped the effects of this weather, since the 

 second picking, produced under more suitable climatic conditions, 

 had come to be regularly and completely destroyed by stainer 

 injury and internal boll disease. (See Fig. loo.) 



The effects of green bug outbreaks are less severe. Infection 

 as a rule is much less general and the production of stained 

 lint consequently small. (See Fig. loi.) The outbreaks are 

 usually controlled in the course of a few weeks by e^^ parasites. 



Control. 



Realization of the fact that the constant heavy losses in the 

 second half of the crop in St. Vincent were due to disease borne 

 by the cotton stainer led to more careful study of the habits of 

 that insect on the part of the resident agricultural officers. The 

 previous vagueness as to its connection with staining or other 

 injury had been reflected in the attitude taken towards it as 

 a pest. 



It was found that although stainers in the absence of cotton 

 could subsist for a considerable time on other foods they were 

 only capable of breeding to any extent on two trees, the silk 

 cotton {Eriodendrun anfractiiosum) and the seaside mahoe or 

 John Bull {Thespesia popidnea). This observation provided the 

 key to the situation. The trees were eradicated at the cost of 

 much labour but at a relatively small expense. In the following 

 year the appearance of stainers in significant numbers was so 

 far delayed that the picking of clean cotton continued into April, 

 and conditions in other respects being also favourable the crop 

 was double that of the previous year. In the untreated neigh- 

 bouring island of Bequia the crop was destroyed as usual, but 

 equal success was attained there in the following year by the same 

 method. It is now being tried in the Leeward Islands. 



Where Malvaceous weeds, such as Malachra and Sida species 



