1 1 2 Minnesota Plant Diseases. 



chiefly in the leaves where, in the roomy air-spaces, which are 

 charged with moisture, they realize most nearly their prefer- 

 ence for moist conditions. Perhaps the most famous fungus 

 in this group is the potato blight, which causes rot of the plant 

 in the field and a dry brown rot of the tubers in the cellar. 

 Epidemics of this disease follow excessively moist and warm 

 seasons and have been known to cause great damage. Toma- 

 toes and other plants of the potato family also suffer attack 

 from this fungus. Another very famous fungus of this group 

 is the downy mildew of vines, which attacks the vine fohage 

 and fruit, both of the old and new world, and causes great dam- 

 age. Others exhibit habits similar to damping-off and attack 

 seedlings. Some are found on bean plants, on grasses, and a 

 very conspicuous one inhabits members of the carrot family. 

 They are also found on sun-flowers and again a very impor- 

 tant one is known on melons and cucumbers and their allies. 

 Lettuce, beets, clovers, onions and tobacco plants, besides a 

 large number of wild plants, as violets and anemones, are 

 known as hosts to these parasites — in fact, almost every family 

 of plants has its downy mildews. (Figs. 2, 39, 40, 44, 45, 166 

 to 171, 196 to 198.) 



White rusts (Peronosporinecs in part). Very closely allied to 

 the downy mildews are the white rusts. In their egg-spores 

 and general habits they are quite similar. Just as in the potato 

 blight, spore-like bodies are produced which later show their 

 spore-case nature, but these spore-cases are not cut off singly 

 from the ends of threads. They are formed in chains and these 

 chains are arranged together in such dense clusters that they 

 form a white rust-like mass when the host-plant skin has been 

 broken and thrown back. 



The most abundant of the white rusts is one common on 

 the weed plant known as shepherd's purse, and found also on 

 other mustards. Another is found on pig weed and on portu- 

 laccas, others on morning glories and on a great many other 

 plants. As in the downy mildew the egg-spores are often found 

 in swollen portions of the host where the fungus has excited 

 the host plant to unusual effort. The advantage to the para- 

 site is evident, since it is in the egg-case-producing part of the 

 plant that much nutrition is needed. (Fig. 45.) 



