BLACK KNOT (PLOWRIGHTIA MORBOSA) 369 



produce conidiosporcs abundantly. The conidiosporcs can live 

 over till the succeeding season and start the disease anew. The 

 disease is propagated chiefly by conidiospores. It was a long 

 time after the disease was known before ascospores were found 

 and of course it was not then classed as an Ascomycete but was 

 put into the class Fungi Imperfecti. Apparently ascospores are 

 often not formed at all, and, when they are, they occur in the 

 diseased fruits after they have dried up and usually fallen from 

 the tree. As the fruit decays it dries up into a mummy. In this 

 dried-up fruit, regardless of whether it is on the ground or on the 

 tree, the mycelium becomes changed into compact masses called 

 sclerotia. Later, probably the next spring, upon these sclerotia 

 are developed bell-shaped apothecia in which the ascospores occur 

 (Fig. 319). Thus in controlling the disease the destruction of 

 the mummied fruits as well as spraying to kill the conidiospores 

 that are sticking to the buds and bark are advised. 



The' Closed or Black Fungi (Pyrenomycetales). — These 

 Fungi, of which there are about 11,000 species, include both 

 parasites and saprophytes. They vary much in form and 

 manner of growth. They are chiefly characterized by a super- 

 ficial, compact, black mycelium looking as if it had been charred 

 by fire. The structure in which the asci are produced is a peri- 

 thecium, a small commonly flask-shaped cavity with a small 

 pore-like opening. Many of these Fungi produce destructive 

 plant diseases, of which the Black Knot, Ergot, and Chestnut 

 Disease are familiar ones. 



Black Knot (Plowrightia morbosa). — This Fungus occurs on 

 the twigs of Plum and Cherry trees, producing wart-like excres- 

 cences as shown at A in Figure 320. The mycelium attacks the 

 cambium, phloem, and cortex, causing at first an abnormal growth 

 and later the death of these tissues. As a result of the attack, the 

 twig is much injured or killed. The attack is often so general that 

 the entire tree is killed. The wart-Hke excrescences or knots con- 

 sist of the mycelium and the abnormally developed tissues of the 

 host. During the first summer the disease shows as slight swell- 

 ings, but with the renewed growth of the following spring, the 

 swellings enlarge rapidly, and during May or June the mycelium 

 breaks through the bark and forms a dense covering over the sur- 

 face of the swellings. From the hyi^hae forming the covering of 

 the knot numerous erect hyphae arise which give the knot a 



