358 Minnesota Plant Diseases. 



of beads. This summer-spore was formerly known as a loose- 

 spored "imperfect" fungus. The winter spores, however, are 

 now known. Sclerotia or storage organs, formed from densely 

 woven fungus threads, are sometimes produced in the fruit. In 

 the following spring, these sclerotia send up a cup fungus fruit- 

 ing body with a long stalk, and on the inner surface of the cup 

 is formed the layer of spore-sacs. The latter each contain eight 

 spores. These spores probably cause infection in the spring. 

 Attacked fruit falls to the ground or may remain attached to the 

 tree and becomes mummified, producing then an enormous num- 

 ber of spores. These fruit mummies, moreover, may persist 

 through the winter and continue to* produce spores in the follow- 

 ing spring. It is therefore important that all decaying and rot- 

 ting fruit, whether on the tree or ground, be gathered and burned. 

 Affected twigs should also be pruned and burned. A winter 

 spray has been suggested and spring and summer sprays with 

 bordeaux have proved beneficial. In addition to the winter 

 spray the following applications have been recommended : with 

 bordeaux (i), just as the leaves begin to unfold; (2), just after 

 the petals fall; (3), after the fruit sets, and with potassium sul- 

 phide as the fruit begins to ripen. 



Black knot of plum and cherry [Plowrightia morbosa. (Schw.) 

 Sacc.]. This is a very common disease of our wild cherries and 

 is also common on wild and cultivated plums. The disease de- 

 rives its name from the black charcoal-like knots in the branches 

 of the tree. These knots are caused by the threads of the fungus 

 which inhabit the branches at the knotted points. The fungus 

 gains entrance, perhaps, through a crack or wound and imme- 

 diately causes a stimulation of the tissues, so that a large, soft 

 mass arises, which contains but a small amount of hard woody 

 tissue. This enlarged portion of the branch splits off its outer 

 cork layer and exposes a cushion of densely wefted fungus 

 threads. The cushion is at first yellowish-brown to yellowish- 

 green and turns finally to an olive or dark yellow-brown color. 

 This surface is at first covered with the summer spores which are 

 borne on short upright threads and are capable of causing infec- 

 tion during the same season in which they are formed. Later in 

 the year the knotted portion of the branch turns black and char- 

 coal-like and the surface is then covered with very fine, pimply 



