30 Control of the Qreenhorme White Fly 



very severely burnt while tliat of the flaccid one was undamaged but its buds and 

 flowers were injured. 



(2) Two young plants with first truss in flower, growing in 8-inch pots, one 

 lurgid and one flaccid, were fumigated in a chamber (3/50 eft.) at the rate of \ oz. 

 cyanide per 1000 eft., the equivalent of at least double this charge in a green- 

 house; duration 2| hours, temperature 90-72°, relative humidity 80 per cent. 

 Even at this excessively high temperature the flaccid plant was undamaged 

 while the top of the turgid one was killed and the whole plant very severely 

 scorched. 



Nothing in this experimental work afforded any evidence that the 

 humidity of the atmosphere of the greenhouse was a factor which needed 

 to be considered in the fumigation and when plants were sprayed with 

 water immediately before the operation no damage could ever be asso- 

 ciated with this. Previous workers have disagreed about the importance 

 of this factor (8) and it is probable that some have failed to dissociate 

 the moisture of the air from that of the soil, the latter being all-important. 

 It is fortunate for the tomato grower that air humidity is not a factor 

 as the tomato house has necessarily a very moist atmosphere when it is 

 closed down. 



It is not, of course, practicable to allow tomato plants on which the 

 fruit is setting to flag as the crop would be thereby damaged, but the 

 principle has a practical apphcation in that the grower may be advised 

 to get the roots of the plants as dry as possible without harming them. 

 It is a common practice in the Lea Valley to drench the borders heavily 

 before planting (in one rather exceptional case to the equivalent of a 

 9-inch rainfall) so that the plants can grow for two or three months 

 without heavy watering. (This makes the plants throw deep roots.) 

 When the houses are in this condition there is clearly no control over the 

 turgidity of the plants. Those who grow in pots usually keep the soil 

 very wet while the first four trusses are setting. Under these circumstances 

 it is advisable to use only half the quantities of cyanide recommended 

 above which will give a very fair check to the pest and prevent it from 

 getting out of control. Later the plants in borders are watered periodi- 

 cally according to the character of the soil, generally once a week, and 

 older plants in pots do not suffer if they are allowed to dry until the 

 "pot rings hollow." The fumigation with the full amount of cyanide 

 may be given the night before the periodical watering is due or when the 

 soil in the pots is dry. The day after the fumigation the plants may be 

 watered freely. 



It is not possible to give any guarantee that with these doses no 

 damage to the foliage will occur, but an assurance may be given that 



