348 



DISEASES OF CULTIVATED PLANTS 



Onion smut ( Urocystis cepulae, Frost) is only known to 

 attack cultivated onions, and develops during the seedling 

 stage. As a rule the first leaf shows the disease before the 

 second leaf appears : the disease attacks the leaves in the 

 order of their appearance. The outer coat of the bulb is 



FlG. 104. Urocystis occulta. 1, upper part ol rye plant 

 diseased ; 2, spores, one of which has germinated and produced 

 a cluster of secondary S| \ of the germ-tube ; 



3, a spore that has germinated and produced secondary spon 

 two of which have germinated. Figs 2 and 3 highly mag. 



also attacked. The fungus hursts through the epidermis as 

 large, black, powdery streaks. 



Spore-clusters 18-25 I 1 diam., usually only one, brown, 

 central, fertile cell, surrounded by many pale, sterile cells. 



White varieties are most susceptible to the disease. Infec- 

 tion occurs during seedling stage by spores in the soil. It 



