385 



or germ tube from the particle of copper compound nearest to it ; 

 while the fungicidal action of the particle only took place when this 

 distance was very small. Another point determined was that there 

 was no secretion from spores, provided with a special, thick protec- 

 tive wall, and that these could only be killed after the formation of 

 a germ tube. This makes it still more improbable that the epidermal 

 cells of the sprayed leaves can give rise to the necessary secretion, 

 as these also are furnished with a special thick cuticle in almost all 

 cases. 



These results have an important practical bearing on the applica- 

 tion of Bordeaux mixture. In the first place, they indicate that the 

 mixture is of more service as a preventive than as a remedy ; this is 

 supported by actual results. In cases where fungi have already 

 gained a hold on the leaves before spraying, the liyphae in the plant 

 tissues will not come into intimate contact with the particles of copper 

 compound on the sprayed surfaces, and will, therefore, not be affected. 



In the second place, the film of coi:)per compounds deposited 

 must be even and universal, for, if small untreated areas are left, 

 fungus spores will be able to germinate on them unharmed, and the 

 resulting hyphae will penetrate the internal tissues underlying the 

 fungicidal film, without coming into close contact with this, and, 

 therefore, without receiving any check. It is because of the advan- 

 tage gaind by uniformity in the film deposited that a second spraying 

 is particularly beneficial. Excess of lime in the mixture would appear 

 to have but little effect on its action, except in so far as it increases 

 the distances between individual particles of the copper compounds 

 in the film. 



A final point not considered by these workers is of importance in 

 the tropics. Frequently, extra adhesives must be added to Bordeaux 

 mixture when it is used in places where the rainfall is high. The 

 choice of such adhesives requires care, since they might conceivably 

 form a film over the particles of copper compounds, such as would 

 prevent the secretions of the fungi from exerting Iheir solvent action 

 on those particles. 



(West Indian Agricultural News Vol. X, No. 243. 



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