i9o6.] The Spread of Fungus Diseases. 



259 



Examples. — " Potato blight " caused by Phytophthora infestans, 

 De Bary, and potato " leaf-curl " caused by Macrosporium 

 solani, Cooke. 



3. Hybernating mycelium located in the seed is alone present, 

 the production of spores being completely arrested. 



This phase illustrates the perfection of the method of infec- 

 tion and distribution of a fungus by means of hybernating 

 mycelium ; the older method by means of spores being com- 

 pletely obliterated. 



Exaviples. — Darnel, rye-grass, Italian rye-grass. Owing to the 

 absence of fruit, the affinities of the fungus cannot be determined. 



It has generally been assumed that the infection of cereals by 

 "smut" {Usttlago) spores, could only be effected during the 

 youngest seedling stage of the plant. Such infection was 

 considered to be due to spores present in the soil, or more 

 generally by spores adhering to the "seed" and consequently 

 sown along with it. As a preventive against such infection it is 

 the common practice to treat " seed " before sowing with a 

 solution of sulphate of copper, formalin, or some other 

 fungicide. Such treatment sometimes proves beneficial, some- 

 times not. 



Brefeld, a German mycologist, whose researches on the 

 " smuts " {Ustilago), " bunts" ( Tilietia), and other groups of fungi 

 are well known, has recently published the results of his 

 investigations on the infection of cereals and other plants by 

 " smut " spores. Without entering into minute details it may be 

 stated that Brefeld has proved, as the result of an elaborate series 

 of experiments, that in the case of wheat, barley, and some other 

 plants of no economic importance, infection takes place through 

 the flower, and not during the seedling stage in the ground. 

 " Smut" spores carried by wind, insects, or in the case of aquatic 

 plants by water, are deposited on the stigma of the flower, where 

 they germinate and pass down into the ovary or young seed. 



There an amount of hybernating mycelium is formed which 

 remains in a resting condition until the seed is sown, when the 

 fungus grows along with the plant, and finally produces the well- 

 known sooty masses of spores in the seed. 



Other plants, including oats, are only infected during the 

 seedling stage in the ground ; others again are infected by both 



Y 2 



