Family Convolvulaceae 163 



are then trampled upon and crushed and make a good 

 starting point for the spread of black rot. As the 

 seed begins to germinate, the sprouts on or near a 

 mother diseased root will contract the disease. On 

 pulling out this root, nearly every sprout will show 

 the black shank which, upon careful examination 

 with a hand lens, will be found to be strewn with the 

 long-necked pycnidia of the fungus. At the tip of 

 these roots are seen minute waxy droplets which con- 

 sist of the pycnospores. Small mites, white minute 

 spider-like animals, crawl about everywhere, especi- 

 ally on and around the pycnidia. These mites, 

 as well as watering, help to spread the pycnospores 

 in the seed bed and result in the further infection of 

 new sprouts. 



In storage, black rot is first introduced with dis- 

 eased roots and is spread from one to another, fruit- 

 ing best in the presence of moisture. In poorly 

 ventilated houses, it is invariably noticed as soon 

 as the roots begin to sprout. These sprouts turn 

 black and die at the tender tip or throughout. In 

 the first stage, while growing in the interior tissue, 

 the fungus does not produce pycnidia. Therefore, as 

 long as the epidermis on the spot is unbroken and the 

 roots are kept dry, the disease cannot spread. How- 

 ever, if the skin is broken or accidentally bruised, or if 

 the roots are kept in a house which is overheated and 

 damp, the black rot fungus will produce fruit by the 

 formation of pycnidia which appear as minute raised 

 dots in the center of the spot. 



The Organism. The parasite consists of a vegeta- 



