278 



PLANT DISEASES 



leaves do not produce a thread immediately, but break 

 up into other little spores which swim about in the water. 



These little swimming spores 

 swim around in the drops of 

 rain or mist for a time, but 

 finally come to rest. Then 

 they send out little threads 

 which penetrate the leaves 

 and a new- infection is 

 started. This scattering of 

 spores from leaf to leaf and 

 the growth of a new my- 

 celium in each leaf may be 

 very rapid and, as mentioned 

 above, may destroy a whole 

 field in a very short time. 

 Some measures for combat- 

 ing this disease are already 

 apparent. Diseased tubers 

 for planting should always be discarded. The most 

 effective method, however, for fighting this disease is by 

 spraying on the plant a solution which is particularly 

 poisonous to the fungus. This solution is known as 

 Bordeaux mixture and is prepared by mixing a solution 

 of blue vitriol with limewater. (For detailed directions 

 see Experiment Station bulletins.) This Bordeaux mix- 

 ture is sprayed on in as fine a spray as possible, the for- 

 mation of large drops being avoided. The spraying is 

 done at regular intervals, commencing early in July be- 

 fore any disease appears on the potatoes. After the 

 plants are thoroughly sprayed with the mixture, an ex- 

 amination of the leaves shows very fine spots of the Bor- 

 deaux mixture all over the leaf. If now some spores of 



FIG. 143. Potato Blight (Phy- 

 lophthora infestans, one of the 

 downy mildews), thread with 

 spore-like swimming-spore cases. 

 (After DeBary.) 



