DISEASES OF CULTIVATED PLANTS 327 
ALTERNATION OF HOSTS IN FUNGUS SURVIVAL 
This relation of alternating forms in the life cycle of a given 
parasitic species, to its survival, has been mentioned in wheat scab 
wherein we have the Fusarium or pink mold and the Gibberedlla 
forms; in rot of stone fruit where we find Botrytis and Sclerotinia 
forms, and in apple anthracnose or bitter-rot where we discover the 
Gloeosporium followed by the Glomorella ascospores. In these 
instances there seems no real need for the advent of another host 
plant. In other groups of fungi, notably among the Uvredinee or 
rusts, we discover in certain species, that survival is accompanied 
by a necessary change of host plant. The apple rust is known in 
summer to attack the leaves and fruit of apple, thorn apple 
(Crataegus), june-berry and mountain ash. This is the aecidial or 
cluster-cup stage of the apple rust and has its counterpart in the 
aecidiospores or cluster-cups of the wheat rust upon barberry as 
well. With apple rust we climb far on the plant ladder and find the 
teleutospores of rust survive upon the cedar treesas branch enlarge- 
ments called cedar apples (Gymnosporangium). 'The dry looking 
apples upon the cedar trees take onanew form during spring 
showers when they become great, jelly-like masses which emit the 
teleutospores of the rust, to be carried to apple, juneberry and 
crataegus leaves by whatever agency is available. 
The relation of cedar trees to the prevalence of apple rust isa 
practical matter for each orchardist. It may be better to make 
firewood of the cedar trees than to combat the apple rust in his 
orchards. A similar problem as between the barberry hedges 
which adorn rural England, and the virulence of wheat rust in their 
grain fields, may also be raised. With us we have plenty of grain 
rust in the absence of barberry hedges. An adaptive form of 
survival apparently takes the place of the alternating hosts, and we 
still have the wheat rust. 
The instances given are simply illustrative and the student of 
plant pathology will discover many more in the course of his study. 
Likewise a careful perusal of the special part of this bulletin will 
show other instances of survival under many various and instructive 
conditions. 
HOW THESE PARASITES ROB THE HOSTS 
There is an old saying about the stable door and the stolen 
horse; similar application may be made for plants-and parasitic fungi 
in a manner which we shall presently perceive. To obtain food we 
must reach the source of supply; the manner of reaching it is less 
important than the result. Now it occurs that cultivated and wild 
