130 SOILS AND PLANT LIFE 



how much would the owner have made by treating his 

 seed? 



The scab of potatoes may be controlled in a similar 

 way by allowing the seed to remain for two hours in a 

 solution of one pint of formalin in thirty gallons of water. 



(2) Spores in the Soil. — Soil, which has become in- 

 fested with the spores of such diseases as the potato scab 

 or onion smut, should be planted to some other crop for 

 several years until the ground becomes free from the spores 

 of the disease. Crop rotation, then, becomes an important 

 step in disease control as are also thorough drainage and 

 deep cultivation. 



(3) Spores on Dead Plants. — Most disease-producing 

 plants have two kinds of spores : those which are able to 

 withstand the severe cold of winter, called winter spores; 

 and others which are produced during the warmer months 

 and are called summer spores. These are formed much 

 more rapidly than the winter spores but are more easily 

 destroyed. Owing to the presence of winter spores, it is 

 exceedingly important that all of the preceding year's 

 plants in infested fields be destroyed before seeding time 

 arrives. The dreaded black rust of wheat has been shown 

 to live over the winter on old grass and weeds. Field 

 sanitation, then, is an important matter. 



(4) Spores on Diseased and Fallen Fruit. — The winter 

 spores are not usually directly responsible for the rapid 

 spread of disease on fruit. When a winter spore germi- 

 nates, a tiny plant develops. This little plant in turn 

 sends up a mass of summer spores, and from these summer 

 spores come the plants which really produce disease. 



There are certain chemical mixtures which will destroy 

 both winter spores and summer spores when they begin 

 to germinate without injury to the plants upon which 



