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 {Exohasidium rhododendri), which forms 
fleshy, irregular-shaped galls on the young leaves, but does 
not appear to affect the health of the plant. Another 
species, Exohasidium 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. 
