330 Minnesota Plant Diseases. 



this fungus can be shown to be truly parasitic but in others is 

 doubtfully so. It is usually capable of unlimited growth in a 

 saprophytic manner. 



The elimination of unfavorable conditions of temperature 

 and moisture are recommended. Freshly decaying vegetable 

 matter should be removed. The soil may be limed as an aid 

 but is not an absolute preventive. A complete sterilization of 

 the soil should entirely prevent the disease. 



Grey mold of lettuce. See Diseases of Greenhouse and Or- 

 namental Plants. 



White rust of mustards, cabbage, etc. [Albugo candidus (P.) 

 Ktzc.]. This is one of the most widely distributed fungi known. 

 It occurs on all kinds of plants belonging to the mustard family 

 on both wild and cultivated forms. It is most commonly found 

 on the wayside weed known as shepherd's purse, but is also 

 found on many other wild mustards. Amongst cultivated 

 plants the following are frequently infected : radishes, horse- 

 radish, cress, cabbage, turnip, water-cress and wall flower. 

 Plants closely related to the mustard, e. g., caper plants, are 

 also known to become infected. 



The fungus attacks the plants in the seedling stage through 

 the seed leaves. The mycelium very frequently causes abnormal 

 and distorted growths in the host plant and in these regions the 

 summer spores are formed. These are produced in extensive 

 patches which at first have a porcelain-like appearance. Later, 

 by the bursting of the superficial tissue of the host plant, the 

 spores are set free as a white powder, hence the name white rust. 

 The spores are formed in chains and the spore powder is blown 

 by the wind to other plants. The spores require moist con- 

 ditions for germination, and under these conditions break up 

 into tiny swimming spores which scatter the infection by mov- 

 ing about in the drops of water on the leaf. When they come 

 to rest they germinate into a tube which infects the leaf. The 

 winter spores are formed as a result of a breeding act between 

 two swollen organs formed on the fungus threads and are pro- 

 vided with a thick coat. They are produced within, the tissues 

 of the host and are set free in the following spring by the de- 

 cay of the tissues. These winter spores produce swimming 

 spores in a similar manner to the summer spores. 



