Vegetable and Field Crops 163 



and dwarfing of the inner leaves. Dense masses of fine root- 

 lets develop below ground, giving rise to the common name. 

 The really distinctive character is the presence of numerous 

 yellowish, beadlike objects, a little over a millimeter long, 

 clinging to the rootlets. This disease has been very destruc- 

 tive in Europe, and, introduced into the United States prior 

 to 1907, is established in several districts in. the West. Fol- 

 lowing is a partial list of plants known to be susceptible to 

 hairy-root. Those marked with a * are highly susceptible. 

 Alfalfa, allseed, barley, bean, dwarf pea, lima bean, garden 

 beets,* sugar beets,* Brussels sprouts,* cabbage,* Chinese 

 cabbage,* cauliflower,* celery, crimson clover, white clover, 

 corn, cress,* dandelion, green foxtail, hemp, hops, kale,* 

 kohl-rabi,* lentils, yellow lupine, annual meadow grass, 

 tall meadow oat-grass, mustard,* oats, parsnip, garden pea, 

 sweet pea, pink, potato, radish,* rape,* rutabaga,* rye, 

 sorghum, spinach, sunflower, timothy, turnip,* vetch, wheat. 



A rotation of four or five years with nonsusceptible crops 

 is the only safe procedure with infected land. Small areas of 

 local infection may be eliminated by a heavy application of 

 unslaked lime well mixed into the soil. 



Black-root-"^ {Rheosporangium aphanidermalus Eds.). — 

 Black-root occurs on both radish and beet, causing damping- 

 off and a browning or blackening and often death of the root. 

 Disinfection of seed is advisable. 



Damping-ofif. — Damping-off of seedlings near the surface 

 of the ground soon after they come up, often before thin- 

 ning, is a frequent cause of loss. The leaves yellow, the 

 roots turn brown, and the plants topple over and die. This 

 is, in some instances, due to the usual causes of damping- 

 off, Corticium, Pythium, Phoma, but sometimes to a heavy 

 crust formed upon the soil, thus preventing the safe emerg- 

 ence of the seedling. 



Minor diseases 



Scab ^^^ {Actinomyces scabies) resembles closely the scab 

 of the Irish potato. It usually covers more completely 



