DISEASES OF CULTIVATED PLANTS 439 



Botrytis. Recently Norton has discovered the Sclerotinia or asciger- 

 ous stage developed from the mummy fruits in which the fungus lay 

 dormant for a time awaiting spring or summer conditions. 



The bitter-rot of apple and its cycle of development not long 

 since brought to light in Illinois, also shows the relation of the apple 

 mummies, decayed by attacks of this anthracnose, to its survival. 

 The fungus lives over in the old rotted fruits, acted upon by bitter- 

 rot alone, which hang upon the trees. The fungus may also survive 

 in branch cankers upon the tree adjacent to mummies of the bitter- 

 rot. In these branch cankers the spore sac or perithecial stage of the 

 fungus is developed. Upon the coming of warm showery weather 

 about early June, new spores are produced from either mummies or 

 cankers and new infection may occur upon the new fruits. The 

 problem of the control of this disease, therefore involves a knowledge 

 of its manner of survival. 



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 Gibberella 

 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 in- 

 stances there seems no real need for the advent of another host plant. 

 In other groups of fungi, notably among the Uredinese or rusts, we 

 discover in certain species, that survival is accompanied by a neces- 

 sary change of host plant. The apple rust is known in summer to 

 attack the leaves and fruit of apple, thorn apple (Crataegus) , jiine- 

 berry and mountain ash. This is the aecidial or cluster-cup stage 

 of the apple rust and has its counterpart in the aecidiospores or clus- 

 ter-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 trees as branch enlargements called cedar 

 apples (Gymnosporangium) . The dry looking apples upon the 

 cedar trees take on a new form during spring showers when they be- 

 come 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 is a 

 practical matter for each orchardist. It may be better to make fire- 

 wood 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 



