Family Solanaceae 323 



winter unhurt, the rot being checked by the favorable 

 storage conditions. 



The Organism. The mycelium of the fungus is 

 hyaline, non-septate. As shown by Melhus 1 and 

 others, the mycelium may be carried from year to 

 year within the infected tubers. In fact this is but 

 one way late blight is distributed. Through the 

 stomata of the infected leaf emerge the slender coni- 

 diophores (fig. 51 d), bearing the ovoid conidia. Ac- 

 cording to Melhus, a the conidia of Phytophthora infes- 

 tans may germinate either directly by a germ tube 

 or by the production of zoospores (fig. 61 e) as in 

 Pythium. The best germination occurs at the op- 

 timum temperature, which lies between 10 and 13 C. 

 (50-57 P.). The conidia may be killed by exposure 

 for 6 to 24 hours to dry atmospheric conditions such 

 as exist in an ordinary room. Frost which kills the 

 top of the plants will also kill the conidia of Pytho- 

 phthora. Light does not hinder germination and 

 therefore has no inhibiting effect on infection. 

 Phytophthora infestans does not seem to produce sex- 

 ual spores or oospores within the affected tissue of 

 the leaf or tuber. However, Clinton 3 succeeded in 

 developing what appeared to be oospores of the 

 fungus in pure culture on oat agar (fig. 61 f). The 

 oogonia appear as swollen terminal heads, cut off 



1 Melhus, I. E., U. S. Dept. of Agr. Jour. Agr. Research, 5 : 59-65, 



15- 



a Melhus, I. E. f Wisconsin Agr. Expt. Sta. Research Bui. 37 : 1-64, 



3 Clinton, G. P., Connecticut Agr. Expt. Sta. Ann. Rept., 1909- 

 1910:753-774. 



