CHAPTER XXX 



DISEASES OF PLANTS NOT CLASSIFIED. 



Agave. 



Anthracnose. 



Various species of Agave, including sisal, are subject to a 

 leaf disease of wide distribution caused by Colletotrichum Agaves 

 Cav. There are records of its occurrence in the West Indies in 

 Antigua, Trinidad, and British Guiana. 



Dark spots are caused on the leaves which may remain local 

 or result in a general sickly appearance and, according to reports, 

 the eventual death of the plant. 



The pustules, which produce masses of pink or orange-coloured 

 conidia, are usually arranged in concentric rings. 



Removal of infested leaves and spraying with Bordeaux 

 mixture may be expected to control the disease. 



Rose. 



Black Spot. 



The commonest disease of roses in the West Indies as else- 

 where is the black spot of leaves caused by infestation with the 

 fungus Diplocarpon Rosce Wolf {Actinonema Rosce Fr.). 



The disease reveals itself usually in mature or old leaves, 

 which show first brownish blotches with fringed edges and set 

 with black dots. Later the blotches turn black and examination 

 with a lens shows upon them a raised pattern of radiating lines. 

 The roundish dots are at first covered by the cuticle, which later 

 bursts irregularly and reveals beds of two-celled conidia. The 

 radiating lines mark the course of sub-cuticular hyphae, while 

 another set of hyphae penetrate the tissues of the leaf. 



The result of the infestation is that the affected leaves turn 

 yellow and fall. When this is confined to old leaves not much 

 harm is done, but under weather conditions favouring the disease 

 repeated defoliations may occur, which weaken the plants and 

 result in some dieback of the twigs. 



The disease may be controlled by timely applications of 

 Bordeaux mixture or lime-sulphur, but, as deposits left by these 

 sprays are objectionable in appearance, a preferable treatment, 

 which is simple and at least as effective, is dusting with a mixture 

 of fine sulphur 9 parts and powdered arsenate of lead, i part. 

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