8 MASS. EXPERIMENT STATION BULLETIN 351 



tion and good growth. This may be especially true with small seeds, usually sown 

 on the soil surface, for they cannot become dry after germination has begun 

 without some likelihood of injury. 



Very small seeds are best watered from below, by the partial immersion of pots 

 in water, for there is then less danger of seeds and seedlings being washed out or 

 buried. Bewley (9) and Gratz concluded that this is also less favorable to damping- 

 off caused by Rhizoctonia than is watering from above. Neither of them asserted 

 that the disease is thus entirely preventable, however; and in the writer's experi- 

 ments with Pythium, there was about as much damping-off of Nicotiana and 

 Canterbury bells when they were watered from below as when they were watered 

 from above through papers. Soil watered from below may receive too much 

 water at one time and, therefore, remain wet too long unless well and promptly 

 drained. This is more likely to happen with soil of high water-holding capacity 

 or one which contains much peat moss, but there is not much risk in the case of 

 a sandy soil. 



TEMPERATURE AND DAMPING-OFF 



Earlier investigators (39, 3, 46, 89, 75) have found that Rhizoctonia may infect 

 seedlings and cause damping-off at temperatures from as low as 45° to 48° F. 

 (7° to 9° C.) to as high as 86° to 95° F. (30° to 35° C), with the optimum tem- 

 perature for infection variouslv identified as between 59° and 86° F. (15° to 

 30° C). 



It has been suggested (54, 9) that temperatures be lowered to help protect seed- 

 lings against Rhizoctonia but, on the basis of the figures presented above, the 

 temperatures might have to be lowered to below 60° F. before damping-off 

 could be expected to be checked, and Bewley (9) indicates that there may be 

 no decrease in the disease causea by this fungus until temperatures have been 

 brought down to below 54° F., which is too low for many species of plants. 



Cardinal temperatures for damping-off caused by Pythium have been reported 

 by other investigators as follows: 



Minimum: Below 59° F. (5), below 55° F. (46), or 46° F. (3) 



Optimum: 86° F. (3), 86° to 68° F. (46), or 75° to 64° F. (5) 



Maximum: 95° F. (3), above 86° F. (80), or 86° F. (46) 



It appears, therefore, that both Pythium and Rhizoctonia may cause damping- 

 off at temperatures from maxima of 95° to 86° F. (35° to 30° C.) down to minima 

 of 48° to 46° F. (9° to 8° C), with the optimum for Pythium between 64° and 86° 

 F. (18° to 30° C.) and the optimum for Rhizoctonia between 59° and 86° F. 

 (15° to 30° C). 



It has been suggested (54) that, in an effort to control Pythium, temperatures 

 be lowered to below 70°F., and the results of others, summarized above, do indicate 

 that less infection by Pythium may be expected at temperatures below about 

 66° F. There may, however, be some infection of seedlings bv Pythium at temper- 

 atures as low as 46° F., and the writer has found no published record of complete 

 control of damping-ofT caused by Pythium by temperature regulation without 

 some adverse effect on germination of seeds or growth of seedlings of some species. 



