34 DISEASES OF GLASSHOUSE PLANTS 
houses which are badly ventilated and too hot. It is 
thus apparent that the physiological condition of a plant 
may be adversely affected by light conditions. 
As a general rule, insufficient light produces immature 
tissues, which are more susceptible to disease than those 
developed under well lighted conditions. Thus the 
mildews of the grape, cucumber, and strawberry develop 
most rapidly in shaded positions or in dull weather, 
and may entirely disappear when exposed to strong 
sunlight. ‘‘ Damping off” of seedlings also has been 
found to increase most rapidly in the shade. De Bary 
was able to induce susceptibility to attack by Botrytis 
in petunias by placing them in the dark for some days, 
and it is a matter of common observation that those 
parts of the tomato which are shaded are the most 
easily attacked by this fungus. 
On the other hand, numerous observations have been 
made which indicate that half shade has a beneficial 
effect upon certain plant diseases. Thus, shaded celery 
plants are generally free from early blight caused by 
Cercospora apii, and asparagus rust caused by Puccinia 
asparagi is reported to react in a similar fashion. Again, 
observations at Cheshunt Experimental Station have 
shown that shade has a beneficial effect upon tomato 
plants attacked by Verticillium albo-atrum. 
Sunlight which is too intense may have an effect upon 
plants by burning them. Thus “tip burn” of lettuce 
and tomato occurs under glass when brilliant sunshine 
follows periods of dull weather. In lettuce plants the 
outer leaves wither at the tips and the affected areas 
curl backwards and turn brown. “Tip burn” of the 
tomato consists of a withering, curling, and browning of 
the tender tissues at the growing point. During the dull 
days which precede the burning the plants have been 
transpiring very little because of the high percentage of 
humidity in the glasshouse atmosphere, and the tissues 
become gorged with water. The sudden appearance of 
brilliant sunshine produces a rise in temperature and a 
