SOIL TREATMENTS AND SEED GERMINATION 7 



Going to the other extreme, high pH values, it should be noted that lime as 

 applied to soil by Johnson (54) did not protect seedlings against infection by 

 Pythium. Similar results were obtained here with seedlings of Delphinium and 

 several species of Dianthus when hydrated lime, up to 2 tons per acre, was ap- 

 plied to soil (inoculated with Pythium) which had an initial pH value of 5.0. 



Buchholtz (15) was able to reduce the severity of damping-off by a heavy 

 application of lime, it is true, but his soil had an initial pH value of 6.2, and it is 

 reasonable to assume that this may have modified the effect of liming. In a soil 

 with an initial pH of 4.5, an application of lime which changes the pH value to 

 6.0 might be expected to make damping-off caused by Pythium not less but more 

 severe. 



Turning now to Rhizoctonia, it has been established by several investigators 

 (93, 39, 52, 63) that this fungus can grow, at least in culture in artificial media, 

 at pH values ranging from a minimum of pH 2.0 to 3.0 to a maximum of pH 9.0 

 to 10.0, with the optimum at about pH 6.0 or 7.0. This is a wider range and a 

 higher optimum than that reported by others for Pythium. It does not appear 

 that damping-off caused by Rhizoctonia would be at all readily or practically 

 controllable by the adjustment of soil reaction. Peltier (73) observed that it 

 grew equally well in acid and alkaline soils; Gloyer and Glasgow (38) found it 

 growing well in soil with a pH value of about 7.4; and in experiments of Jackson 

 (52), there was some damping-off until pH values were lowered to 2.5, although 

 the disease was more severe at or near neutrality. 



These relations are further mentioned in connection with the use of certain 

 chemicals, especially sulfur, aluminum sulfate, and calcium salts. 



SOIL MOISTURE AND DAMPING-OFF 



Soil used in seedage is often watered unnecessarily heavily, as a result of which, 

 especially if the soil is inclined to heaviness, it so shrinks, bakes, and cakes as to 

 interfere with germination about as much as fungi do. 



Chemical treatments mentioned below were applied to soils which were mod- 

 erately dry. Soils were watered immediately after seeding. Water was usually 

 applied through papers laid on the soil, so that seeds would be neither washed 

 out nor buried. These papers were left on until seedlings began to emerge. This 

 helps to prevent a too prompt recontamination of the soil, as by dirt blowing 

 upon it, but is unnecessary as far as holding fungicides in the soil is concerned. 



Damping-off fungi are sometimes present in water, in which case they may 

 be destroyed by boiling, and some investigators (71, 5) have done that before 

 watering sterilized soil. This was unnecessary with the water used in these ex- 

 periments, however; for, in sterilized soil, there was as little damping-off of seed- 

 lings watered with tap water as of those to which only boiled water was applied. 



It has been suggested that damping-off may be at least partly controlled by 

 less heavy or less frequent waterings and by more ventilation, but the indica- 

 tions are that this would be neither easy nor safe. Abdel-Salam (3) concluded 

 from his experiments that a high degree of dryness of the air does not stop the 

 spread of the disease once it has appeared. Alexander and his associates (5) came 

 to the similar conclusion that damping-off caused by Pythium and Rhizoctonia 

 cannot really be controlled in practice by regulating soil moisture; and Gratz 

 (39) found that Rhizoctonia infected plants at any soil-moisture content favorable 

 to their growth. Abdel-Salam had similar results, a strain of Rhizoctonia causing 

 as severe damping-off in relatively dry as in much moister soils. It would appear, 

 therefore, that a soil too dry for damping-off is probably too dry also for germina- 



