No. 4.] INSECTICIDES AND FUNGICIDES. 195 



less familiar to all as an illustration of a parasitic fungus, 

 let us consider its life history. This fungus, like all others, 

 starts from a seed-like body called a spore so minute as to 

 be invisible to the naked eye except when seen in masses, 

 and perhaps not more than one one-thousandth of an inch in 

 diameter. One of these spores falls on the leaf, and under 

 conditions of actual moisture, like a rain drop or heavy dew, 

 in hot, close weather germinates, sending its roots, i. e., the 

 cotton-like threads, into the leaf, running through it and 

 taking the nourishment, which the potato plant has elaborated 

 for its growth and building it up into its own structure. 

 When the fungus has matured its gro^vth sufficiently, 

 branches are thrown up through the other breathing pores, 

 upon which are produced new spores in such numbers often 

 as to give the surface of the leaf a rusty or powdery appear- 

 ance. From the one spore mentioned may in a few days, if 

 the weather is close and hot, be developed hundreds, even 

 thousands, of new spores, which in turn, coming in contact 

 with other leaf surfaces, grow and produce their increasing 

 numbers, until in a very few days our potato vines have 

 blighted, our grapes have been ruined by mildew, or our 

 pear trees denuded of their foliage by the leaf blight, etc. 



Most fiingi produce two kinds of spores, one that we will 

 call the rust or mildew stage, and the other the resting or 

 winter spores. The first are of short duration, lasting only 

 during the warm weather of summer ; while the latter remain 

 persistent through the winter, ready to germinate the next 

 season, should conditions be favorable to their development. 



The spores from which blights, rusts or mildews come may 

 have been produced upon our own grounds, may have come 

 from land miles away, or may have been developed upon 

 some wild plant in the fields or woods ; and the immense 

 number of spores produced as illustrated by the dust from a 

 dry puff l)all, makes it certain that there are spores enough at 

 any time for the production of serious injury, if the condi- 

 tions of weather are favorable. 



The life history of this fungus illustrates, with more or 

 less variation in structure and modification by the action of 

 the host plant or conditions of weather, that of most of the 

 common diseases of our farm and garden crops. While our 



