FUNGI AFFECTING THE PLUM 



The Black Knot 



Plowrightia morbosa 



No fungous disease of fruits is easier to recognize 

 than this ; the black, wart-like excrescences upon the 

 twigs and branches of plum and cherry trees are too 

 well known to the majority of fruit growers. In many 

 of the older settled portions of the country the disease 

 prevails to such an extent, that it has led to the practi- 

 cal abandonment of the culture of these fruits ; and a 

 similar condition is threatened in other localities unless 

 preventive measures are vigorously applied. The fun- 

 gus attacks nearly all varieties of wild and cultivated 

 plums, and most varieties of cherries. 



During the earlier years of the present century there 

 was much discussion concerning the cause of black knot. 

 Some horticulturists contended that it was due to in- 

 sects ; some that it was a constitutional affection of the 

 trees ; and some that it was due to fungi. The latter 

 proved the correct supposition, the life-history of the 

 fungus having been first worked out in 1875 by Dr. 

 W. G. Farlow. Like other fungi, this one reproduces 

 by means of spores. The knots first appear as swollen 

 places on the twigs ; as the swelling increases the bark 

 cracks open longitudinally, and the fungus produces 

 quantities of spores within these cracks, making them 

 appear as if covered with a velvety olive coating. These 

 are the summer spores ; they are blown through the air 

 by wind, and washed from twig to twig by rain ; those 



53 



