124 DISEASES OF CULTIVATED PLANTS 



consequent exposure to tropical conditions for so long a 

 period, destroyed the hibernating mycelium; whereas when 

 a quicker means of transit, due to the replacing of sails by 



FlG. 27. Phytophthora infestans. i, a cluster of conidiophores 

 with conidia, emerging through a stonia of the leaf of a potato 

 plant ; 2. a free conidium, the contents of which are breaking up 

 into zoospores ; 3. a conidium liberating zoospores ; 4. a zoospore 

 that has come to rest and is germinating ; 5. a conidium germinat- 

 ing by the protrusion of a germ tube. All highly mag. 



steam, became general, the hibernating mycelium present in 

 the tuber survived the voyage. 



The first indication of the disease is the appearance of 

 small brownish blotches on the leaves ; these quickly increase 

 in size, and the leaves curl, and under favourable weather 

 conditions for the fungus both stem and leaves become black 



