496 



FIELD CROPS 



are crop plants. This is true of the root rot of cotton and 

 other plants in the South, which may maintain a foothold 

 in fields by living on V7».ej£ wnen crcp& io does not affect are 

 grown there. Many weeds of the mustard family help to 

 spread clubroot of the cabbage and turnip. When the dis- 

 eases do not actually 

 live on the weeds, the 

 latter may make con- 

 ditions favorable for 

 their development on 

 crop plants. Thus 

 rust and mildew are 

 produced most readily 

 in shady, damp situ- 

 ations, such as are 

 found where the 

 growth of weeds is 

 rank. Weeds may 

 harbor insects by sup- 

 plying them with food 

 when crop plants are 

 not available, or by 

 furnishing them a safe 

 refuge over winter 

 under rubbish alpng 

 fence rows or in fields, thus adding to their own injuriousness. 

 683. Weeds Lower the Value of Crop Products. The 

 presence of weeds or weed seeds in crop products often 

 lowers their value. Buyers of grain quite often make an 

 unjustifiable dockage in weight or price for the presence of 

 any noticeable quantity of weed seeds. If the grower removes 

 the seeds before marketing, they increase the cost of pro- 

 duction by the labor which is required to separate them from 

 the grain. Weeds in hay materially affect the value of that 

 product for the market or for feeding. Weeds in grain crops 



Figure 160. — Canada thistle. 



