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STATE HORTICULTUKAL SOCIETY. 



tlie leaf are attacked and it is not uncommon to find yonng shoots also 

 badly affected, the nltimate result heing a check to assimilation and a con- 

 sequent weakening of the tree. 



THE CAUSE OF SCAl!. 



Apple scab is caused by a minute parasitic fungus which, like the tree it 

 lives iipon, is a true ])lant, having a A-egetative and reproductive system — 

 very simple, it is true, but ne\ertheless sufficiently perfect for rapid growth 

 and propagation. This little plant has not the power of obtaining its food 

 from the air and soil but must depend on material already prepared by its 

 host, the apple, which readily gives it up, the result being scabby fruit 

 and shriveled leaves and shoots. 



Fl(4. 1. 



Fig. 2. 



" Fig. 1. Section througli a sciib spot. (/, spore (conidinm); b, hypha or suitporting thread; c, mycelituu 

 or plant body of fungus; d, epidermis of apple; e, cells of apple. Fig. 2. /, spores, greatly magnified; 

 fir, h, spores germinating. 



A thin transverse section through a scab spot on the fruit or leaf (Fig. 1 ) 

 shows under the microscope clusters of short brownish threads arising 

 from a darker mass of roundish cells which are seated directly upon the 

 healthy tissue of the fruit or leaf as the case may be. The free ends of the 

 threads often bear pear-shaped bodies of nearly the same color as the sup- 

 porting threads; these are as a rule one-celled but occasionally they are 

 divided near the middle by a transverse partition. The pear-shaped bodies 

 ( Fig. 2 ) are the spores of the fungus and it is through their agency that the 

 parasite is propagated. The brownish threads serve merely as supports 

 for the spores, while the dark mass of tissue constitutes the body of the 

 fungus, or, if I may so express it, its roots, branches, and leaves. AVhen full 

 grown the spores separate readily from their supporting stalks, and being 

 exceedingly small and light are easily wafted from place to place by 

 currents of air. In this way they reach healthy fruit and leaves, and if 

 the proper conditions of moisture and heat are present they quickly 

 germinate by sending out slender tubes which bore their way into the 

 leaves or fruit and ultimately give rise, just beneath the cuticle or skni, to 

 dark masses of cells like those already described. At first this mass of 



