20 BULLETIN ii, U. S. DEPARTMENT OF AGRICULTURE. 



that imported stock will bring in serious tree diseases make it espe- 

 cially important that methods of controlling these blights be found, 

 in order to encourage the growing of planting stock in this country. 

 The writer has not only experimentally determined the cause and 

 distinguishing features, but also the control methods for sun scorch, 

 the most serious of these blights, and by following his recommenda- 

 tions the disease has been controlled in nurseries where it had 

 before done serious damage. Distinguishing characters and pre- 

 ventive measures for the two other commonest blight types, winter- 

 killing and mulch injury, have also been determined. An account 

 of what the writer has been able to learn from his own work and the 

 experience of others with the other types of blight is also given. 

 Distinguishing characters and control methods have not been found 

 for all of the types. 



It is seldom that a case of nursery blight can be diagnosed merely 

 by examining a few specimens of diseased trees. It is hoped to carry 

 this investigation so far that each nurseryman will be able to identify 

 for himself, by general observations, all of the types of blight which 

 attack his nursery beds and to take the necessary steps for preventing 

 further losses. 



The writer wishes to acknowledge his indebtedness to Mr. W. H. 

 Mast and Mr. R. G. Pierce for their cooperation in the work at the 

 Halsey nursery, to Dr. H. L. Shantz for helpful advice, and to Dr. 

 Perley Spaulding, under whose general direction the work was con- 

 ducted, for advice and the use of his specimens and unpublished 

 data. 



SUMMARY. 



The following are the types of blight most likely to cause losses 

 of coniferous nursery stock in the United States : 



(1) Sun scorch. — This is the commonest summer trouble. The 

 roots die before or at the same time as the tops. Death is caused 

 by excessive water loss. It usually occurs when the air is hot and 

 dry and the soil around the roots is dry. The disease is worst on 

 sandy soils, in crowded beds, and on raised parts of beds. On 

 sandy soils it may kill suddenly and in definite patches. Successful 

 preventive measures tested by the writer are watering, shading, and 

 avoidance of crowding. In nurseries located on mineral soils the 

 humus content should be increased. 



(2) Winterkilling. — The tops of the plants dry out when the soil 

 is frozen so that the plants can not take up water. The preventive 

 measures most used consist of a light straw mulch on the beds and 

 windbreaks. 



(?>) Mulch injury. — The tops die in winter as a result of being 

 mulched. This happens while the mulch is still on, or occasionally 



