14 DISEASES OF GLASSHOUSE PLANTS 
of the plants to be grown, and careful attention to this 
point when choosing a site will save the grower much 
worry and perhaps serious loss in ensuing years. 
(a) Slope-—An important point is the slope of the 
land. Wherever possible a slope with a southern aspect 
should be chosen; but the degree of slope is also im- 
portant, for it has a considerable bearing upon two 
factors of the plant’s environment: (1) the temperature 
of the air within the glasshouses, and (2) the efficiency 
of the drainage. In houses built in the block formation 
without dividing partitions there is a tendency for the 
heated air to accumulate at the highest part of the block. 
This uneven distribution of temperature has a detri- 
mental effect upon the total yield per acre and increases 
susceptibility to diseases. Several instances have been 
observed in big blocks of tomato houses built on severe 
slopes, where ‘“‘mildew” (Cladosporium fulvum) has 
completely destroyed the plants at the highest parts 
of the block. This is because the rapid spread and 
vigour of attack is intensified by reason of the accumula- 
tion of moist hot air at these parts. Reducing the 
number of houses in a block by building dividing parti- 
tions at every seventh or tenth house has invariably 
resulted in preventing a recurrence of severe attacks of 
‘“‘ mildew.” To maintain an even temperature, therefore, 
the slope must be gentle. The presence of depressions 
in the surface of the ground should be avoided by careful 
levelling, as at these places the atmosphere tends to 
stagnate. By taking careful records it has been 
demonstrated that the percentage humidity of the air 
over a depression is appreciably higher than that above 
the general level of the ground, and observations have 
shown that certain diseases generally begin at these places. 
(6b) Drainage.—Upon the nature of the soil and the 
slope of the land depends the all-important factor of 
drainage, and consequently the former must be taken 
into account when the drainage of any particular site 
about to be chosen is considered. 
