PHYTOPHTHORA 125 



and rotten within a few days, emitting a very disagreeable 

 smell. If a brown patch is examined with a pocket-lens, 

 minute, white, mould-like bodies, the conidiophores of the 

 fungus, will be seen on the under surface of the patch, more 

 especially towards the margin. When the attack is slighter 

 and the foliage is not destroyed at once, the conidia are 

 produced in rapid succession, and are conveyed to neighbour- 

 ing plants by rain, wind, animals, etc., and by such means 

 the disease spreads rapidly. But, as I have proved, the 

 simultaneous outbreak of an epidemic extending over wide 

 areas is not due to infection by conidia, but to the presence 

 of hibernating mycelium of the fungus present in the tuber. 

 It has been stated that the zoospores produced in the conidia 

 are washed down into the soil, and infect the young tubers. 

 This statement, however, has not been proved, and I have not 

 succeeded in infecting young tubers with conidia, even when 

 placed under very favourable conditions for doing so. It has 

 also been stated that the mycelium in an infected stem passes 

 down into the young tubers. I have not succeeded in con- 

 firming this statement. On the other hand, I have proved 

 by repeated experiments that when a diseased tuber is planted, 

 the mycelium from such tuber passes into the young potatoes, 

 which also become diseased, under circumstances where there 

 was no possibility of conidia falling from the foliage on to the 

 soil. 



The brown stains on the surface of a potato infected with 

 Phytophthora are too well known to require description. 

 When the disease is very evident, probably no one would use 

 such tubers for 'seed,' but there are numerous instances 

 where the disease does not show on the surface, or is so 

 slight that it is overlooked, and when such tubers are planted, 

 the young tubers also become diseased, and thus the disease 

 is passed from generation to generation in a vegetative manner, 

 and without the formation of spores on the part of the fungus. 

 The produce of a diseased tuber is always diseased, yet 

 under certain conditions of weather the stem and leaves of 

 the same plant may remain perfectly free from disease. This 

 happens during bright, comparatively dry seasons, the 

 mycelium in the tuber not being able to invade the above- 

 ground portions. On the other hand, every practical potato- 

 grower knows too well that a few cloudy, damp, and sultry 

 days in July will start an epidemic of disease simultaneously 

 over an entire field or over a whole district. The mycelium 



