310 DISEASES OF FIELD & GARDEN CROPS. [OH. 



Peronospora infestans is in August and September. A 

 potato plant should be selected that has been destroyed 

 or reduced to putridity by the disease. This plant should 

 be taken up with a fork, and the exhausted seed tuber 

 from which the plant has arisen carefully sought out. 

 This seed tuber, or what is left of it, may be frequently 

 found reduced to a sort of transparent jelly, and this 

 jelly-like mass will in many cases be found swarming 

 with the living oogonia and antheridia of the potato fungus. 

 The fungus has attacked the leaves and proceeded down- 

 wards by the stems into the seed tuber from which the 

 plant originally arose, and there, having run its course, it 

 has produced resting-spores for the invasion of the follow- 

 ing year's crop of potatoes. It is much less common to 

 find resting-spores in the hard new tubers even when 

 discoloured by disease ; still it is quite possible to find 

 them even in new potatoes. Eipe resting-spores of the 

 potato fungus may be found with great ease in the spring 

 and early summer, in the fragments of diseased and 

 decayed potatoes picked up in the fields or about manure 

 and refuse heaps by hedge sides. 



A germinating resting-spore may be compared with a 

 germinating seed of dodder. The dodder has enough 

 nourishing material stored up within its outer integument 

 to support an infant dodder plant for a short time. If 

 no suitable host plant is near, the young dodder perishes. 

 The first fruiting branch from a germinating resting- 

 spore of the potato fungus is in an exactly similar con- 

 dition, for, unless the spores or conidia are aided by the 

 wind to reach a potato or some other suitable plant, the 

 first-produced conidia perish at once. The resting-spores 

 of the potato fungus germinate in and upon the ground 

 at the precise time of the year when the potato plant is 

 in the best condition for infection. Habits of this nature 

 are extremely common and well known amongst parasitic 

 fungi. 



We have as far as possible in this work avoided con- 



