80 DISEASES OF GLASSHOUSE PLANTS 
similar plants transferred to an unshaded house at the 
same average temperature. 
Plants in different stages of wilt were used, from a 
series where the wilt was just appearing to a series in 
an advanced stage after thirty days’ wilting. All the 
plants recovered in the shaded house, but only a per- 
centage recovered in that which was not shaded. The 
plants which did not recover in the unshaded house, 
being badly wilted ones, were probably desiccated before 
they had a chance to recover. 
These observations appear to justify the conclusion 
that temperature is a most important factor, while 
shading is valuable because it assists the plant by re- 
ducing transpiration. The minimum, optimum, and 
maximum temperature for growth in pure culture of the 
strains of Verticillium albo-atrum utilized for the inocula- 
tions were 4:4° C., 23-3° C., and 30° C. respectively. It 
will be seen, therefore, that the optimum temperature 
for infection coincides approximately with the optimum 
temperature for growth in pure culture. Verticillium 
wilt is distinctly a disease of low temperature, and is most 
severe in spring and autumn. 
Shade.—Shade, as we have seen, has a_ beneficial 
effect upon the resistance of the host to the disease. 
Probably this is due to retarded transpiration and conse- 
quently to the decreased rate of conduction of the water 
in the vessels, so that the toxic products excreted by the 
fungus are not carried up the plant in such large amounts. 
Sow Factors.—Experiments carried out with soils of 
different types show that Verticilliwm wilt is not restricted 
to any particular soil. 
Generally speaking, plants on soils which contain a 
large amount of humus yield a greater amount of disease 
than those growing on soils of a poorer nature. 
Clay soils, in virtue of their greater water-holding 
capacity, are cooler than sandy soils, and plants grown 
upon them are more prone to wilt than those grown on 
the latter. 
