58 DISEASES OF GLASSHOUSE PLANTS 
a fact that is well known to nurserymen. The disease 
thrives upon rich composts, and such are therefore more 
favourable for it than loam alone. 
Causal Organisms.—Considerable experience of this 
disease has shown that it is caused mainly by three fungi 
—Rhizoctonia solani Kuhn, Phytophthora cryptogea Pethy- 
bridge and Lafferty, and Phytophthora parasitica Dastur. 
The first is less common than the latter two fungi, 
which are consequently the most important cause of the 
disease. Rhizoctonia solani is the 
sterile stage of a spore-bearing 
fungus, Corticium vagum B. & C., 
var. Solant Burt. The young 
hyphe produce branches which 
generally leave the mother fila- 
ment at an acute angle and. 
subsequently turn and lie parallel 
to it. The branch is constricted 
near the point of origin and a 
cross wall is formed a little 
beyond the constriction (Fig. 5). 
In age, the angle of branching 
becomes more nearly a_ right 
angle, and the hyphe turn first 
yellow and then a deep brown 
colour. Resting bodies or sclero- 
tia, capable of resisting abnormal 
conditions, are produced, being at 
first small, soft white masses, but later they increase in size 
and turn darkand hard. TheCorticiumstage, which forms 
later, shows as an ashy-grey tufted layer at the base of the 
stem, from which innumerable small spores are produced. 
These are easily carried by the wind, and so disseminate the 
disease. This stage is not always produced, and indeed 
requires special cool, moist conditions for development. 
Rhizoctoma solani is commonly found in rich soils, 
where it constitutes an important disease of many plants. 
Phytophthora parasitica Dastur (16) was first described by 
Fig. 5. Hyphe of Rhizoctonia 
solani. 
