SCIENCE AND INDUSTRY. 
be profitable from the stand-point of the health of the trees and of 
future production of fruit. Orchardists recognise what is known as the 
cumulative effect of continuous spraying. ‘This is very evident in the 
rejuvenating of neglected orchards. The first year’s pruning and spray- 
ing may not show very much reduction of disease, because there are too 
many sources of infection to be exterminated or controlled in one 
season. With the continuation of the second and third year the results 
of the cumulative treatment will be obvious. The Black Spot disease 
(Phoma citricarpa) of oranges, and the Brown spot disease (Colleto- 
trichum gle@osporioides) of the Mandarin show striking examples of 
this cumulative result. We have to beware, however, of an opposite 
injurious effect, also cumulative, on the vitality of the tree, if the 
treatment after two or three years be not reduced to the minimum 
consistent with efficient disease control. Excessive spraying may inflict 
a serious check on the vigorous growth of the tree. 
For many legitimate reasons, therefore, the practice of reducing 
the number of sprayings or other similar treatments to as few as 
possible, while increasing the number of ingredients in the spray so as 
to counteract both fungi and insects in the one operation, has greatly 
increased, and many growers have adopted certain mixtures with suc- 
cess. There are many pitfalls, however, as some of the most widely 
used spraying materials are complex chemical substances, varying in 
composition according to the method of manufacture, purity of materials 
used, and the ideas of various manufacturers of the commercial pro- 
ducts. Some substances are insecticides only, or their fungicidal pro- 
perties are of little value, ¢.g., oil emulsions and lead arsenate; others 
are fungicides only, e.g., Bordeaux; while others act as both insecticide 
and fungicide, e.g., limesulphur solution. Although many of these sub- 
stances can be mixed and used as combined sprays, indiscriminate mix- 
ing will result in disaster—injury to the trees, loss of time, materials 
and crop, and possibly loss of faith in such remedies. The mixing of 
such complex substances demands a good chemical knowledge, and when 
the grower is in doubt as to the results of mixing two or more insecti- 
cides or fungicides, the best advice that can be given is “Do not mix 
them.” The following reasons may be given in favour of not adopting 
the practice of mixing up “one specific to be given in one dose, to 
exterminate all pests” :— 
(1) Each type of remedy generally requires application to the 
plant in a special manner, and in particular with a varying adjustment 
of the spraying nozzle, e.g., nicotine, a coarse, driving’ spray; lead 
arsenate or Bordeaux, a fine, misty spray. 
(2) Each pest has its own particular period when it is most sus- 
ceptible to attack. The best success will be obtained by using the most 
efficient remedy at the most advantageous time. 
(3) Many of the substances used for spraying, when mixed, react 
chemically with one another, whereby the efficiency of one or both may 
be reduced, or new products are formed, which may act injuriously on 
’ the leaves or fruit. 
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