7% DISEASES OF CROP-PLANTS 



This mixture may be sprinkled on to the trees with a brush, 

 or the twigs bent down and dipped into it, or it may be appHed 

 with a syringe or other type of sprayer after being strained 

 through a wire sieve. Cloth strainers keep back the spores. 

 Brass or copper sprayers may be used if they are clean and the 

 material is not allowed to stand long in the tank. 



Comparative trials of these methods have not been made 

 in the case of the West Indian species. Morrill and Back favour 

 the spore-spraying method for distributing the Aschersonias 

 concerned with white-fly control in Florida. 



The condition of the material used would appear to have 

 more influence on successful introduction than the method of its 

 application. It should be used as fresh as possible, and, in the 

 active spore-producing stage, recognizable on examination 

 with a pocket lens. In the case of the shield-scale fungus, a 

 powdery glistening appearance denotes the presence of the spores. 

 In the red-headed and white-headed fungi, the loose tufts of ripe 

 spores can be easily made out. 



When a branch or twig is tied into a tree, the immersion of 

 the cut end in water contained in a narrow-necked bottle is an 

 obvious benefit. The occurrence of the fungi on insects attack- 

 ing potted plants suggests that these might in some cases be 

 used with advantage. 



The caution must be repeated that all these measures depend 

 entirely for their success on the conditions being right for the 

 development of the fungus. In dry weather they are useless. 

 Moreover, experiments have repeatedly shown that, once a fungus 

 has been given a general start, attempts to increase its efficiency 

 by further spore-spraying have little visible effect. 



Caution should also be observed in making use of infested 

 material lest scale insects not already present on the plants be 

 introduced. Leaves of grape-fruit examined by the writer, on 

 which the scales were weU infested with three species of fungi, 

 nevertheless had numbers of the young of the mussel scale 

 crawling over them more than a week after they were picked 

 and dried. 



In the writer's judgment much more hope of effective inter- 

 ference lies in the direction of increasing humidity by the pro- 

 vision of windbreaks and other forms of shelter. With cacao, 

 and more easily with cotton, this may be carried too far, since 

 diseases of pods and bolls will also be encouraged, but with 

 citrus trees, which are the most liable of common crops to suffer 

 from scale infestation, shelter can scarcely be overdone save in 

 districts so wet that it would not be needed. With orchard 

 crops shelter takes two forms : a temporary crop in which the 

 young trees are nursed, and permanent windbreaks and hedges 

 enclosing or dividing up the fields. 



Young coconut trees in dry situations may be sheltered with 

 great advantage in the reduction of scale attacks. 



