MOSAIC DISEASES 141 
hosts, and as these include many of the food plants of 
the world, the economic importance of the disease cannot 
be over-estimated. Growers of glasshouse crops are 
mainly interested in the fact that tomatoes and cucumbers 
are susceptible to this disease, but it must be very 
clearly realized that infection may be introduced from 
diseased plants of other species, such as certain common 
hedge and field weeds, and therefore that a wide know- 
ledge of the disease as a whole must be obtained if 
complete control is to be effected. 
General Symptoms of the Disease 
The chief symptoms of mosaic disease consist of a 
mottling and abnormal development of the leaves, 
flowers, fruit, etc. While diseased plants generally 
possess both symptoms, they may have one or the 
other separately, or indeed both may appear on the 
same plant at different times in accordance with the 
conditions to which it is exposed. 
Mottling—Mottled leaves develop irregular, pale 
green or yellow patches alternating with patches of dark 
green. The mottling may be so slight as to be almost 
indistinguishable, or it may be strikingly evident. Under 
certain conditions the light areas turn brown and wither, 
but frequently the tissues remain alive. Mottling of 
stems, flowers, and fruits also occurs. 
Abnormality—This may include curling, blistering, 
and distortion of the leaves, flowers, and fruits, and a 
dwarfing of the stem. Curling of the margin of the leaf 
is a common occurrence in the case of the clover, 
cucumber, lettuce, potato, raspberry, tobacco, and 
tomato. Where blistering occurs, small, raised islands 
or blisters of dark green show up in striking relief to 
the rest of the surface, which is flat and light green 
in colour. ‘this type of symptom is especially notice- 
able in the case of the cucumber, petunia, tobacco, and 
tomato. — 
