52 DISEASES OF GLASSHOUSE PLANTS 
logically weak and highly susceptible to disease. It is 
of the greatest importance in the prevention and control 
of plant diseases that the physiological strength be 
maintained. In order to resist disease it is necessary 
to pay careful attention to all environmental conditions 
and feeding, for every check to which plants are submitted 
reduce; their physiological strength. 
Chlorosis 7 
An excess of lime in the soil has been shown to 
produce a chlorosis of many plants, and as these are 
more susceptible to disease than normal plants this soil 
factor is one to be avoided. Lack of iron in the soil is 
said to be productive of chlorosis, which in this case may 
be remedied by the application of small quantities of iron 
sulphate. 
Stigmonose 
Aphides and other sucking insects puncture the 
tender tissues of young leaves and produce minute lesions 
which have frequently been attributed to bacterial 
causes. Such spotting is called stigmonose. A suitable 
example is furnished by the carnation. The spots are 
at first pale yellow in colour and later turn a reddish- 
purple. The surface tissue dries, the spots enlarge and 
become sunken, the leaves turn yellow and wither, and 
the whole plant may be stunted. The changes have 
been traced to the injection of an irritant by the insect 
into the plant cells and their consequent reaction. 
Tomato fruits are attacked in a similar fashion, but the 
spots are circular and resemble small white blisters 
with minute dark purple centres. As the fruit colours, the 
spots become indistinct and are practically unnoticeable. 
Gas Injury | 
The presence of poisonous gases in glasshouses 
produces marked effects upon the plants. Thus fumiga- 
