362 DISEASES OF CROP-PLANTS 



The first application should be made on the appearance of the 

 first signs of the disease or at the beginning of seasons in which 

 experience has shown it to be prevalent. 



Mildew. 



Certain varieties of roses, particularly of the rambler class, 

 are subject to severe attacks of powdery mildew on developing 

 shoots and young fohage. Only the Oidium stage has been 

 observed in these islands, consisting of a white surface mycelium, 

 on and about which a powdery deposit of conidia occurs. The 

 fungus is assumed to be the conidial stage of the common rose 

 mildew Sphcerotheca pannosa Lev. 



Dusting with sulphur, or with sulphur 9 parts and arsenate of 

 lead powder i part, forms the most effective means of control. 



Other Diseases. 



A disease referred to by J. B. Rorer as canker occurs in 

 Trinidad, and an affection probably identical is somewhat 

 common in Barbados. Small purplish spots appear on the stems 

 and gradually increase in size, sometimes even girdling the branch. 

 The bark becomes shrivelled and sunken and in the later stages 

 is covered with the bright red stalks and red globular heads of a 

 species of Stilbella (Stilbum). Affected branches should be 

 pruned out, or the whole crown is liable to become infested. 



Rorer records a leaf-spot due to Cercospora rosicola Pass., 

 which causes rounded spots which are yellowish with a distinct 

 brown or purplish margin, and also reports very severe effects 

 from infestation of the stems with the algal disease, described 

 in Chapter XVL 



