SCAR 



217 



BLACK KNOT ON PLUM TWIGS. 



worked out. Several rust-proof plants are widely adver- 

 tised, and it is hoped that relief from the pest may finally 

 be found by developing 

 for each crop a rust-proof 

 variety. An excess of 

 manure and nitrogen in 

 the soil, low and poorly 

 drained land, and late 

 planting (so that crops 

 mature late) are condi- 

 tions that favor the 

 ravages of the disease. 

 These conditions the 

 farmer can partly con- 

 trol. 



161. Scab is a fungus 

 known best in the form of "potato scab." The surface 

 of the common potato often shows reddish or brownish 

 spots, early in the season of growth. The growth soon 



develops in a corky crust. 



The only way to control 

 potato scab is to use clean 

 seed and clean soil. Clean 

 seed may be insured by treat- 

 ment with formalin, in a way 

 similar to that described for 

 small-grain seed, but soaked 

 for two hours instead of ten 

 minutes. Clean soil may be 

 secured by crop rotation. 

 Potatoes should not be raised upon a given field oftener 

 than once in three or four years. Scab may also disfigure 

 apples and pears and reduce their market value by causing 

 brownish blotches on the fruit. The most effective remedy 



A WOOD-DESTROYING FUNGUS. 



