A 
REFLECTIONS ON DISEASE TREATMENT 193 
resorted to when selection is impossible because the first 
slightly resistant plant is unobtainable. 
Wilt resistance has been shown by Orton (35) to be of 
a heritable character by his creation of a wilt resistant 
edible water melon. For this purpose an inedible form 
of melon, Citrullus vulgaris, known as citron or stock 
melon, which was resistant to wilt, was crossed with the 
Eden variety of melon. The first generation resulting 
from this cross proved to be extremely vigorous and 
productive. From the second generation ten fruits 
from 3,000 or 4,000 plants were selected for resistance 
and quality, and the seeds from them were planted in 
infected soil. Continued selection in this manner resulted 
in a resistant variety of good quality fruit. 
_ At Cheshunt Experimental Station a similar investiga- 
tion is being carried out in an endeavour to produce a 
tomato resistant to Verticillium wilt, and of good quality 
and productiveness. 
For breeding investigations large areas are necessary, 
but any expenditure is amply rewarded if a resistant 
variety is ultimately obtained. 
CONCLUSIONS 
THE ultimate end of investigations upon plant diseases 
must be an effective control, and in conclusion it is fitting 
that the general principles governing disease control 
should be recited briefly. 
General principles must come under three main 
headings : plant hygiene; spraying, dusting, and 
sterilizing ; resistant varieties. 
Plant Hygiene 
(a) The Elimination of Centres of Infection.—This 
process has been exploited extensively in the control of 
human diseases, and the resultant success may readily 
be seen when one compares the death-rate due to disease 
during the last war with that during previous wars. 
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