DISEASES AND PESTS 75 



grown in greenhouses with other plants. Azaleas, espe- 

 cially, are apt to become badly infected with thrips, but 

 it is only when the plants are not kept moist enough by 

 the free use of the syringe and watering-can. 



The fungi known to trouble Rhododendrons are, accord- 

 ing to Massee, rust (Chrysomxya rhododendri), which forms 

 small pustules on the foliage, but is not considered to be 

 very injurious ; blight (Pestalozzia Guepini), which causes 

 large, irregular blotches on the upper surface of the leaves ; 

 and gall fungus (Exobasidium rhododendri), which forms 

 fleshy, irregular-shaped galls on the young leaves, but does 

 not appear to affect the health of the plant. Another 

 species, Exobasidium japonicum, forms similar galls on 

 Indian Azaleas ; they grow to a considerable size, and are 

 like big, fleshy, green warts. Although these galls sometimes 

 occur in great abundance, especially when the season is 

 wet and the temperature unusually irregular, they do not 

 occasion any alarm among the growers. 



Rhododendrons of all kinds, including Indian and 

 Ghent Azaleas, are subject to a mysterious disease known 

 to cultivators as canker. It shows its presence by the 

 death of whole branches, and sometimes the entire plant 

 perishes. In Azaleas the stem turns brown at the base, and 

 this is followed by the yellowing and falling of the leaves. 

 Some cultivators attribute the trouble to over-cultivation, 

 some to sunstroke, and others to grafting. The disease is, 

 however, common to Ericaceae, and Cape Heaths sometimes 

 die by hundreds from it. Whatever the cause and it is 

 probably beyond control nurserymen know only too well 

 that it is very destructive, and the healthiest-looking plants 

 are often the first to perish. 



