BLACK KNOT (PLOWRIGHTIA MORBOSA) 369 
produce conidiospores abundantly. The conidiospores 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 intoa 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 820. The mycelium attacks the 
cambium, phloem, and cortex, causing at first an abnormal growth 
and later the death of these tissues. Asa 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-like 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 hyphae forming the covering of 
the knot numerous erect hyphae arise which give the knot a 
