DAMPING-OFF IN FOREST NURSERIES. 39 



In both artificial cultures and in the tissues of coniferous and 

 dicotyledonous hosts the numerous strains observed showed no con- 

 spicuous differences in the size or other characters of the spores pro- 

 duced, though noticeable and constant abnormality was found in one 

 strain in the readiness with which spores were produced and in two 

 strains. in the ratio between chlamydospores and oospores in agar 

 cultures. In the first-mentioned strain, obtained from pine in 

 Kansas, and in cultures reisolated from seedlings inoculated with it, 

 both chlamydospores and oospores are produced tardily and so 

 scantily that it is often difficult to find them. In most strains, on 

 the other hand, almost the entire contents of the mycelium are 

 promptly emptied into the spores as soon as the limits of rapid vege- 

 tative growth are reached. In another abnormal strain from pine 

 from the same locality, and in still another furnished by Hawkins 

 from a California potato, chlamydospores are produced in large 

 numbers, but oospores are few. In many other strains, including 

 several from California, the opposite condition obtains, oospores in 

 plate cultures being decidedly more numerous than chlamydospores. 

 These peculiarities of particular strains seem to be fairly constant 

 characters, the first abnormal strain mentioned having been under 

 observation for more than three years without any change in its tend- 

 ency to scanty fruiting, and the low ratio of oospores to chlamydo- 

 spores having been constant during the shorter periods over which 

 the observation of the other strains extended. In view of the small 

 variation between different strains in the matter of speed of growth, 

 a purely vegetative character, this variation in reproductive habit 

 is somewhat surprising. The strain which produced spores infre- 

 quently was unquestionably parasitic, though it killed fewer seed- 

 lings than the average Pythium debaryanum strains. The strains 

 with the high ratio of chlamydospores were both of at least average 

 virulence on pine. 



Oospores in the strains the writer has had in culture, whether ex- 

 amined in agar, in water cultures, or in root tissues, have ordinarily 

 been somewhat larger than the diameter of 14 or 15 to 18 \x given in 

 a number of the descriptions. The maximum range has been 12.8 

 to 20.6 fx, the same strain' sometimes being well down within the usual 

 size range and sometimes ranging from 17 to 20 \x. The largest 

 oogones observed were 26 \l in diameter. Various stages of fertili- 

 zation are shown in Plate I, figures 2 to 4. Chlamydospores attain 

 a maximum diameter in the case of the limoniform intercalary forms 

 of 32 [/,, and spherical chlamydospores sometimes reach a diameter 

 of 28 [jl. There is no lower limit for these bodies, as under unfavor- 

 able conditions — e. g., in sour-prune agar — they are sometimes all less 

 than 15 ^ in diameter, and the smaller ones are little larger than the 



