DISEASES OF CULTIVATED PLANTS 445 



upon tho young fruits. Herein as elsewhere the philosophy of fun- 

 gicides comes to our relief. A good foliage fungicide is a relatively 

 insoluble compound which will not greatly injure the leaves with 

 which it is in contact. The remedies for foliage troubles are applied 

 in anticipation* of attack and for the purpose of checking the fungus 

 when it may appear. The relative efficiencies of various fungicides 

 in early summer will possibly depend upon the sticking qualities 

 of the sprays. 



Foliage diseases, moreover, are liable to recur each year and 

 this is an added reason for anticipatory treatments to ward them off. 

 Foliage diseases may not be neglected with impunity since the leaf 

 is the plant's vital working organ and the plant must suffer from its 

 impairment. 



BITING AND SUCKING INSECTS AND LEAF DISEASES. 



The part played by insects which wound the leaf epidermis, in 

 the spread of leaf diseases, is often very important. Such wounding 

 of the leaf or green stem whether by insects such as flea beetles, 

 foliage eating worms, or by sucking insects such as mites, leaf hop- 

 pers and plant lice, opens the way for the spores of parasitic fungi 

 or of bacteria or mere molds, any one of which may be injurious to 

 the leaf. The early blight disease of potatoes is a good example. In 

 seasons when there are many of these little black flea-beetles to punc- 

 ture the leaves, the thorough control of both these insects and the 

 early blight, Alternaria fungus, is called for. Many fungi of doubt- 

 ful penetrating powers are truly injurious when they follow insect 

 punctures of the leaves. Fortunately both these are secured by Bor- 

 deaux sprays. The reasons for such applications are of double char- 

 acter since they are to combat both the insect and the fungus to 

 follow it. 



With shade trees the leaf hoppers and mites may be so numer- 

 ous that tip-burn and various leaf dying results from the injuries 

 or punctures they inflict. A more startling relation is that of the 

 blade blight of oats, a recently investigated bacterial trouble. This 

 bacterium is distributed and inoculated very obviously by the aphids 

 or green flies (plant lice), and other sucking insects. 



WOUNDS AND WOUND INFECTION. 



With woody growths, especially in trees which attain consid- 

 erable size, we have the various phenomena of disease infection 

 through wounds ; this infection later becoming evident by reason of 

 decays set up in the woody tissues. Of course, in instances such as the 

 bark disease of the chestnut, Diaporthe parasitica Murr., the disease 

 may penetrate the living tissues. Not so, generally, in wounds of 

 woody plants. Any large wood growth as in forest or shade trees 

 and in larger fruit trees, shows the combination of an external or 

 living sapwood layer and an internal dead or heart-wood cylinder. 

 The commoner forms of wound infections are attributable to tho^e 

 species of fungi which cause decay of this dead heart wood. Among 

 these are the long list of saprophytic, agarics, polypores and stereums. 

 Because of the fact that this heartwood cylinder is dead, the.^e sapro- 

 phytic species of fungi, once they gain entrance into it, flourish there 



