CONTROL OF VIRUS DISEASES QI 
consists of dealing with the plant itself in different ways. These 
methods are summarized under five heads. 
I. Measures against Insect Vectors 
As with the animal viruses, complete elimination of insect vectors 
is impracticable, though a certain amount of success in conjunction 
with other measures is possible. 
There are various methods of approaching the problem, such as 
spraying or dusting, screening, fumigation, systemic plant poisons, 
avoiding the insect vector, biological control, and trap crops. So 
far, spraying to control the insect vector of plant viruses has had only 
a limited application, but with the discovery of new insecticides 
and the development of aerial dusting this method may yet prove 
effective. Screening the plants to keep off the insects has been used to 
some extent, mostly in the U.S.A. It was found that upright, low 
wooden frames, covered with cheese cloth or similar material were 
effective in warding off the leaf-hopper vector of the aster-yellows 
virus from the aster beds. In those parts of N. America where the 
aphis-borne viruses of cruciferous crops are of serious economic 
importance, the seedlings are covered with light canvas frames, until 
the plants pass the most susceptible stage for infection. 
The attempt has also been made to grow seed potatoes under 
canvas frames to protect them from the several aphis-transmitted 
viruses. 
Fumigation is mainly applicable to horticultural crops under glass 
where routine fumigation with nicotine will keep down the aphis 
and thrips which are the insects which transmit the virus diseases of 
horticultural crops in Great Britain. Nicotine fumigation of straw- 
berry fields is also practised in this country and is carried out under 
a canvas sheet slowly towed over the plants, the nicotine vapour being 
pumped under the canvas by the towing vehicle. 
There are certain chemicals which are taken up systemically by 
plants when watered into the soil and this offers a possible method 
of control of insects of the sap-sucking type such as aphides and 
leaf-hoppers. 
By choosing the site or by timing the planting of crops it is some- 
times possible to avoid the insect vectors of virus disease. Perhaps the 
best example of this on a large scale is the production of seed potatoes 
in those areas of Scotland where the climate is unsuitable for the aphis 
vector of potato viruses. 
7—(T.502) 
