332 Diseases of Truck Crops 



In planting infected tubers, the sclerotia germinate 

 and the growing fungus threads attack the young 

 sprouts, causing lesions (fig. 63 b). These may be 

 superficial or so deep as almost to girdle the stems. 

 The lesions are usually numerous? Infected plants 

 attempt to overcome the iU effect of the disease by 

 sending out numerous sprouts above the injured 

 parts, giving the appearance of a rosette. Diseased 

 hills often produce aerial tubers. The disease is 

 spread about by the use of the infected seed tubers. 

 The causative fungus, once introduced into a field, 

 will live in the soil on dead organic matter and attack 

 numerous other crops. Rosette is worse on wet, 

 poorly drained soils, and during seasons of heavy 

 rainfall. Recently Rosenbaum ' has found that there 

 are variations in the strains of Rhizoctonia isolated 

 from diseased potatoes. Some of the strains seem to 

 be more virulent pathologically, and differ morpho- 

 logically from others. For methods of control, see 



P- 336. 

 Southern Blight (fig. 63 d-e), see Pepper p. 305. 



Root Knot 



Caused by Heterodera radicicola (Greef) Muller. 



Root knot on the potato may be easily overlooked. 

 Usually there are no knots on the tubers, and the 

 trouble is merely manifested by minute pimples on 

 the surface of the potato, resembling the pimples 



' Rosenbaiun, J., U. S. Dept. of Agr., Jt?ur. Agr. Research, 9 : 413- 

 419, 1917. 



