Chapter X. 



Fungi. Kinds of Fungi. Sac Fungi, 



Black knot and allies {Dothideacece in part). One of the 

 most conspicuous fungi of Minnesota is the so-called black- 

 knot fungus of cultivated and wild cherries and plums. In 

 the mature stage of the fungus, its host-plant branches carry 

 black knot-like swellings and distortions, which 

 are very conspicuous. These knots so interfere 

 with the nutrition of the branch beyond that 

 the latter usually dies off in a year or two. The 

 fungus then gradually works its way downward 

 to the intersection of another branch, when this 

 is in turn killed off. The mycelium, which lives 

 inside of the bark, causes an increase in the 

 thickness of the latter, followed by a splitting 

 lengthwise. There is also a swelling of the un- 

 derlying wood and the fungus feeds upon this 

 swollen, soft mass. It builds up a dense mass of 

 the mycelium on the outside of the branch. 

 This mass when it first appears is a light-yellow- 

 brown and forms on its surface numerous sum- 

 mer spores which rapidly spread the disease. 

 These spores are pinched off of the ends of short 

 upright threads and are produced during the 

 summer. In the fall the mycelial mass becomes 

 darker until it is jet black. It looks not unlike 

 charcoal. In this mass are formed numerous 

 pear-shaped sac-capsules over the entire surface. 

 These capsules open by minute pores to the ex- 

 terior so that in the spring the surface of the 

 knot appears to be covered with tiny warts, each wart indicat- 

 ing a spore-sac capsule. 



Fio. 59.— Black knot 

 of plum (Plow- 

 rightia morbosa). 

 The tiny warts on 

 the surface mark 

 the ends of the sac- 

 capsules. After 

 Clinton. 



