OBSTRUCTIONS TO DEVELOPMENT 115 



F. Above 78 F. and below 50 F. there is pradlically 

 no germination of the spores. 



Prevention. i. Spraying with copper compounds 

 as, Bordeaux mixture, copper sulphate and soda mix- 



FIG. 35 THE MATURATION OK A SPORE SAC (CONIDIA) AND 

 GERMINATION OF A SPORE (ZOOSPORE) OF ROT 



(Phytophthora infestans} 



(After Marshall Ward) 



a Ripe spore sac in water, b The protoplasmic contents break up into 

 blocks and escape as kidney-shaped zoospores (c and rf). e Each have 

 two thread-like arms, called cilia, which are lost as the spore comes to rest 

 (/and g) ; k, i,j\ and k show stages of germination. Moist, warm, or still, 

 muggy days are best for the growth and development of these spores. On 

 such occasions the disease spreads rapidly. 



ture. If the surfaces of the leaves and stems be covered 

 with a thin film of some copper compound, we either 

 prevent the entrance or injure the vitality of the ger- 

 minating spore tubes, so that the potatoes enjoy a 

 certain degree of immunity from the di.sease. 



This is the philosophy of the use of Bordeaux mix- 



