24 Diseases of Economic Plants 



Root-rot '^' 1^ {Thielavia basicola (B. & Br.) Zopf.). — 

 The roots of the diseased plants, especially the tips of the 

 rootlets, turn black and decay. Sometimes the stem just 

 above the roots is cracked and deformed and the plant is 

 dwarfed. Damping-off may occur in seed beds. This root- 

 rot was first noted in the United States on tobacco in 1904 

 and is now known to be widely distributed. It is found pri- 

 marily on members of the legume, potato, and cucurbit 

 families but has been identified on plants of fifteen other 

 families as well, including the violet, composite, crucifer, and 

 mallow families. In all, something over a hundred species 

 of host plants are known to be susceptible, though to largely 

 differing degrees. In the case of tobacco alone the average 

 annual loss is placed at millions of dollars. There appears 

 to be no biologic specialization and the fungus that can affect 

 one crop can pass readily to the others. For treatment, see 

 under Tobacco, page 253. 

 *'' Root-knot, nematode galls.-^' --■ ^^ — Root galls, varying 

 from a few millimeters to a centimeter or more in size and 

 superficially resembling the root tubercles of the legumes, 

 are common upon a large variety of crops, causing large 

 field loss in all but the most northern states as well as serious 

 injury in greenhouses everywhere. Plants badly affected 

 are dwarfed and are more susceptible to attacks of fungi 

 than are normal healthy plants. Large indirect loss arises 

 from the fact that nematode-infested soil often prohibits the 

 planting of certain profitable crops. These galls are due to a 

 microscopic eel-worm (nematode) which enters the root 

 from the soil and by irritation of the root causes the gall to 

 develop. Following is a list of the more important highly 

 susceptible plants. A complete list would number over five 

 hundred hosts. Field crops: alfalfa, clover, cotton, cowpea 

 (except Iron, Brabham, and hybrids of Whippoorwill crossed 

 on Iron), field pea, flax, pumpkin, soy bean, sugar beet, 

 sugar cane, sweet potato, tobacco, vetch. Ornamental and 

 drug plants: begonia, cineraria, clematis, coleus, dahlia, 

 hollyhock, ginseng, goldenseal, peony, rose, sweet pea, violet. 



FBOfERTY UBRARY 



