iio Minnesota Plant Diseases. 



parasites on common garden plants. It is a noteworthy fact, 

 however, that they still require very damp atmospheres in 

 which to develop well and only become epidemic in excessively 

 moist seasons. These fungi have a very well developed system 

 of threads which are much branched and spread through the 

 tissues of the host plant. They are provided with little sucker- 

 threads which penetrate the cells of the host plant and often 

 form here very densely branched systems of threads. These 

 sucker organs steal the nutrient material from the host plant. 

 It is moreover a robber method similar in some respects to that 

 of the damping-off fungus, for the host-plant parts attacked 

 are ruthlessly and almost immediately killed. Such is the com- 

 mon effect of potato blight or downy mildew of vines. On the 

 other hand, these fungi show more power of selection than do 

 the damping-off fungi. Some are capable of attacking differ- 

 ent and even distantly related plants but in general a given 

 fungus of this group is quite constantly found on the same 

 host or on closely related plants, e. g., members of the same 

 genus or family. Wherever the threads of the fungus become 

 abundant, the host plant is killed. The ancestral aquatic habit 

 of these plants is still retained in the method of distribution 

 by spores for these are swimming spores. Hence the fun-, 

 gus can spread rapidly only when there is a great abundance 

 of moisture, as during excessive rains and cloudy, misty days. 

 Under such conditions the swimming spores spread rapidly in 

 the water drops and may be carried in these drops from plant 

 to plant. An epidemic may thus result. On the other hand, 

 these fungi have learned terrestrial methods of spore distribu- 

 tion. We find upon a close examination of the area infected 

 by a potato blight, or false mildew, at first a thin grayish or 

 whitish haze or shimmer spreading over the leaf, and that this 

 fine mold-like growth is caused by an enormous number of 

 usually much branched threads which come out of the air pores 

 of the leaf. They pinch off of each branch end small, round, 

 pear- or lemon-shaped bodies which look very much like spores 

 and are commonly so-called. These bodies are light and easily 

 carried by the wind and thus alight on other plants. In the 

 potato blight and some of its allies this spore-like body does 

 not betray its real nature until the conditions of moisture are 



