580 



MAN AS A CONQUEROR 



called "damping off," which attacks seedlings, white "rust," the 

 brown rot of lemons, numerous downy mildews that attack grapes or 

 garden vegetables, and the once dreaded "rot" of potatoes {Phy- 

 tophthora intestans) are among this group. The Ascomycetes, one 

 of the largest classes of fungi, produce spores in a spore case called 



• AOOtdWA' 





MOOaiSTOW^O 



OSHAV 



aafifT 



U. S. Dept. of Agric. 



Map showing spread of Dutch elm disease from July, 1933, to February, 1934. 

 The black circles show centers of infection. 



an ascus. They include the powdery mildews so common on many 

 garden plants, the black knot of plums and cherries, the brown rots 

 of stone fruits, the black rot of tobacco, the wilts of cotton and 

 watermelon, peach leaf curl, apple and pear scab, bitter rot of apples, 

 the blue and green molds, and the yeasts, the latter of which are on 

 the whole useful. 



Shortly before 1910, an importation of Japanese chestnut trees to 

 an estate near New York City introduced a blight which attacked 

 our native chestnuts and spread so rapidly that today, in the eastern 



