10 rUXGI AND FUXGICIDES 



them, the fungus is able to multiply with marvelous 

 rapidity during the warm, damp weather most favor- 

 able to its growth. "In an ordinary potato field the 

 abundant foliage, wet with rain and full of juices 

 such as would favor the growth of the mycelium, is 

 swayed by the wind, and leaf flaps upon leaf over the 

 whole area ; quite apart from the wind-blown conidia, 



the active zoospores can 

 soon spread from any one 

 center, and at once infect 

 new leaves. In a few 

 hours fi'esh disease-spots 

 are developed, each put- 

 ting forth new crops of 

 FIG. 7. sECTTox OF TLEAF sHowryG couldla, whlch again ger- 

 MYCELiuM OF FUNGUS, ^lAGxiFiED. niinatc aud send out zoo- 

 spores, and so on." Under these conditions it is easy 

 to see that the attack may be sudden and destructive. 



Besides these conidia, or summer spores, many par- 

 asitic fungi develop in autumn certain spores, by means 

 of which the fungus passes through the winter. The 

 latter are usually better protected than the conidia. 

 They are sometimes called winter-spores. 



Besides the saprophytic and parasitic fungi discussed 

 above, there are some which partake of the nature of 

 both, belonging to one class during a portion of their 

 development, and to the other during the rest. For 

 example, the black rot of grapes is at first a true parasite 

 attacking the healthy parts of the vine, but it completes 

 its growth as a saprophyte, living upon the decaying 

 berries. 



EELATIO:S' OF THE WEATHER TO FUXGOUS DISEASES 



There is a common idea that the weather is respon- 

 sible for a large proportion of plant maladies. In an 

 indirect wav this is often true, but the condition of the 



