POTATO DISEASES 697 



(i) Cat off a short piece of the stem including the shrivelled portion and 

 mount in water on a glass slide. Examine with a low power, and look for 

 the transparent hyphjE oi Pythium on the surface of the stem. 



(c) Place a similar piece of stem to that used in {b) in water on a glass slide : 

 split it longitudinally with needles and tease out the parts. Cover with a 

 cover-glass and press the latter firmly down on the slide, then examine with 

 a high power. Observe the branched hyphse, occasionally septate, among the 

 cells of the stem issues. Are they inter- or intracellular hyphse ? 



(d) In some cases, round conidia, sporangia, and oogonia will be 

 noticeable. 



(e) To obtain conidia, sporangia, and oogonia for examination place a 

 diseased seedling in water in a watch-glass, and leave the whole under a bell- 

 jar for a few days. 



Oospores may also be found among the withered tissues of seedlings which 

 have 'damped-off' and been allowed to remain until the whole plant has 

 turned brown. 



3. The genus Phytophthora of the Oomycetes is very closely allied 

 to Pythium. The inter- and intra-cellular mycelia of the various 

 species vegetate within, and speedily kill the tissues of their 

 hosts, but the conidiophores and sporangiophores are exposed to 

 the open air through the stomata of the infected plants. (Fig. 

 234). Conidia and zoospores are produced, the latter possessing 

 two cilia each. 



In one species, namely Phytophthora omnivora de Bary, 

 spherical fertilised oospores occur, while in P. infestans 

 oospores are said by some authorities to be absent. 



P. omnivora is very destructive to seedling conifers', beech, 

 and other trees, and must be combated by methods similar to 

 those employed for Pythium de Baryaniim. 



P. infestans causes very extensive disease to the potato 

 crop : its life-history, and the character of its mycelium and 

 asexual reproductive organs are described below. 



4. Potato Diseases. — It is important to point out that the 

 potato plant is subject to several distinct ailments, one of which 

 is still unfortunately styled ' the potato disease ' as if there were 

 no other. Although some of the fungi destructive to the potato 



