44 Pflanzenkrankheiten. 



by earlier writers. A temperature of 23° to 27° C. and a well- 

 watered soll were found to be the most favorable for the mycelium 

 to spread in the tuber and grow out into the sprouts, both when 

 partially and when whoUy covered with soil. Under these conditions 

 the sprouts may become infected from 4 to 20 days after planting, 

 regardless of their size and age. The time required is doubtless 

 influenced by the proximity of the mycelium to the buds and the 

 external conditions. 



The mycelium of P. infestans may remain alive in seed tubers 

 planted in the soil for a least 45 days, and it is very possible that 

 under conditions less favorable for the soft rots which follow 

 P. infestans in the tuber the fungus may live much longer. None 

 of the author's results or observations tend to show that the potato 

 fungus is latent in the stems and leaves of plants growing from 

 diseased tubers, as stated by Massee. 



Laboratory tests showed that the fungus infects not only the 

 sprouts but also the shoots that break through the soil. The mycelium 

 grows from the tuber into the slem, where it travels up to the 

 surface of the soil and sporulates, as held by De Bary and Jensen. 

 This usually takes place in the small dwarfed shoots in a hill. 



Potato tubers infected with P. infestans used for seed purposes 

 and planted under field conditions may cause the development of 

 an epidemic. The mycelium grows from the parent tuber up into 

 the stem exactly as shown in the laboratory experiments. It later 

 sporulates and foliage infection results. Ten such cases were found 

 and followed in northern Maine during the growing seasons of 

 1913 an 1914. All except two of these became centers for foliage 

 infection, and severe epidemics of P. infestans followed. 



Conidia of P. injestans may be borne on the cut surfaces and 

 sprouts of tubers when planted under field conditions. As the cut 

 surface corks over or the tuber decays, the fructification of the 

 fungus decreases. Spores taken from tubers two or three weeks 

 after they were first planted showed only limited germinating 

 capacity. No evidence was obtained tending to show that the conidia 

 borne in the soil are instrumental in starting foliage infection. 



The mycelium of P. infestans spreads most rapidly in the 

 cortical tissues of the stem, where it travels up more rapidly 

 than down. 



Epidemics of late-blight may Start from a Single shoot or hill 

 naturally or artificially infected with P. infestans. The infection 

 spreads radially from the initial point of infection during the early 

 stages of the development of an epidemic. These spots of infection 

 in the fields probably come into existence through the planting of 

 seed potatoes infected with P. infestans. Jongmans. 



Valleau, W. D„ Varietal resistance of plums to brown-rot. 

 (Journ. Agric. Research. V. p. 365—396. PI. 37-39. 1915.) 



The brown-rot fungus in Minnesota seems to be identical 

 with that found in other parts of the Un ited States and with 

 Sclerotinia cinerea of Europe. Chlamydospore tufts vary in color 

 from gray to bright ocher. For the production of the ascus stage 

 the Sclerotium apparently must be buried in the ground for two 

 winters. Mummies which have hung on the trees for one year are 

 still capable of producing apothecia. 



Infection may take place through the uninjured skin at any 



