Minnesota Plant Diseases. 



27 



cannot usually resist unfavorable conditions ; this is an impor- 

 tant principle commonly made use of in combating fungus 

 diseases. The spraying of fungi is most effective if carried out 

 just after the spores germinate. Under the natural conditions 

 of the sowing of spores, unfavorable dry periods may follow 

 closely on a damp season, in which the spores have just ger- 

 minated, and in this way undoubtedly myriads of spores come 

 to grief. 



Distribution by water. There is a great group of fungi 

 which always live in the water or, if not actually in the water, 

 in very moist conditions; or, as parasites of seed plants, they 

 pass long periods in the resting condition and revive during 

 very moist seasons, as after a rain or heavy dew. Such fungi 

 have spores, with special mechanisms for dissemination through 

 the water. Each little spore is provided with one or two ex- 

 ceedingly delicate whip-like processes which protrude from the 

 end or side of the spore. These lashes whip about and propel 

 the spore with comparatively great speed through the water, 

 until it finally comes to rest and then germinates into a new 

 plant. In this way potato blight is spread and this disease be- 

 comes epidemic only during very wet seasons. In the so- 

 called white rust which so commonly attacks almost, if not 

 all, of the plants of the mustard family, an enormous number 

 of spores is found in white rust-like patches which give the com- 

 mon name to the fungus. These spores are formed in chains 

 and when ripe are blown about by the wind, and are thus borne 

 to the surface of other plants. Here they remain until very 

 moist weather brings about their further development. They 

 then divide up internally into numerous little swimming spores 

 provided, as in the fish and water molds, with propelling lashes, 

 and the chances of infection of the host plant leaves by these 

 swimming spores are thereby many times increased. The white 

 rust therefore uses both the wind and water in the dissemina- 

 tion of its spores. 



Distribution by wind. The great majority of fungi utilize 

 the wind as an agent for carrying spores. The spores of rusts 

 and smuts are shaken out into the wind by the movement of 

 the plants on which they grow. Their position is of advantage 

 just as is the elevated position of the wind-distributed seeds 



