THE PEAR LEAF-BLIGHT 43 



injured they fall off, and whole orchards are some times 

 defoliated by the disease, the trees appearing as bare in 

 midsummer as they normally do in winter (Plate IV, 

 unsprayed). Of course, the effect upon the tree is very 

 injurious; it is unable to store up the materials of 

 growth properly, and it becomes weak and impoverished. 

 But this fungus does not confine its attention to 

 the foliage ; the stems and fruit are also attacked. The 

 former become blackish and dead, and the latter is at 

 first covered with reddish pimples, which finally become 

 blackened and roughened, and generally cause the pear 

 to crack open in such a manner as to ruin the fruit. 

 Several such pears, photographed by Dr. 11. H. Lamson, 

 are represented in Plate V. 



This disease is especially destructive to young pear 

 trees in the nursery, and the defoliation caused by it 

 quite generally prevents successful budding, unless pre- 

 ventive treatment is given. A good general account of 

 the fungus occurs in the 1888 report of the Department 

 of Agriculture. 



The little black dots in the centers of the spots con- 

 sist of the fruit of the fungus. If a vertical section be 

 made of one of these and it be placed under the micro- 

 scope, it will be seen that the pecu- 

 liar two- to six-celled spores shown in 

 Fig. 21 are borne upon short stalks 

 coming from the mycelium in the 

 leaf cells. These spores germinate 

 under favorable conditions, sending 

 out a tube which burrows its way 

 into the tissues of the leaf, and starts 

 the fungus again. These spores are 

 the conidia, or summer spores. Other 

 MAGNIFIED. spores the so-called winter spores 

 are said, by some botanists, to be found among the 

 fallen leaves. 



