DISEASES AND PESTS OF PLANTS. 



37 



Fig. 10. (I.) Tuber affected with common scab. (2.) Tuber showing a mild attack of 

 powdery scab. The pustules are mostly small and isolated. (3.) A more serious form of the 

 disease. The scabs are confluent and* cover a large proportion of the surface. (4.) Shows 

 different appearances presented by the pustules. Those marked + have the covering membrane 

 still intact ; in those marked || the membrane has .lust been 1 broken, while at O the mass of 

 spore-balls is becoming broken up and the spore-balls scattered. (5.) A much-deformed tuber 

 showing a very bad, but fortunately rare, form of the disease. (From original photographs by 

 H. T. Gussoic.) 



