8 BULLETIN 44, U. S. DEPARTMENT OF AGRICULTURE. 



son to think that the high temperature of July 27 injured the trees 

 directly. While 102° F. is unusually high, the temperature has gone 

 to 107° at this nursery without injury to the pines. 



The death of the roots, especially characteristic of sun scorch in 

 transplants, has led to the belief in some quarters that root parasites 

 were the immediate cause of the disease. The evidence obtained con- 

 tradicts this view. The most that can be said is that in many places 

 some of the common soil fungi may act as facultative parasites, kill- 

 ing weakened portions of root systems and so making the plants some- 

 what less able to withstand drought injury. Even this has not been 

 demonstrated, and from the nature of the case it is practically im- 

 possible to demonstrate it. An attempt was made to secure evidence 

 on this point by carefully washing the roots of healthy 1-year-old 

 jack-pine seedlings and planting part in autoclaved soil and part in 

 untreated soil. All the soil used came from part of a bed at the 

 Halsey nursery which had been affected with sun scorch. After the 

 plants had had a few weeks to become established all the pots were 

 allowed to dry out. Death occurred in a manner fairly character- 

 istic of sun scorch, coming at practically the same time in both steri- 

 lized and unsterilized pots. Microscopic examination indicated about 

 the same fungi in the dead roots in both sterilized and unsterilized 

 soil. The results were entirely negative. Because of the impossi- 

 bility of securing a growth of stock large enough to exhibit typical 

 blight symptoms and of keeping any of it free enough from common 

 soil organisms to use as controls for inoculation tests, there is little 

 chance of learning anything definite as to what part fungi may play 

 in causing sun scorch. 



Taken as a whole, the evidence is believed sufficient to establish a 

 lack of balance between water absorption and water loss as the chief, 

 if not the only, cause of the disease. While there have at times been 

 occurrences rather difficult to explain, 13 different attacks of the 

 disease have been seen at Halsey, and in all of them evidence has 

 been obtained of the relation of the trouble to drying weather, shade, 

 crowding, or soil moisture, and in most cases to two or more of these 

 factors. The Halsey nursery has been under fairly continuous ob- 

 servation by the writer during the summers of 1909, 1910, 1911, and 

 1912, and it can be said quite positively that all the serious losses 

 which occurred in established stock over 1 year old during the 

 growing seasons in this period could have been averted by maintain- 

 ing sufficient soil moisture and mostly averted by shading. While 

 the conditions are in many ways different from those at other nurser- 

 ies, the evidence here obtained has been checked up by observations 

 made at many other nurseries from the Eocky Mountains to the 

 Atlantic coast. While some of the other diseases listed in this paper 

 undoubtedly are concerned in the spring and summer losses at many 



