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



from the blighted area and two from the healthy, and determination 

 showed 30 per cent more water in the samples from the healthy area. 

 Altogether, samples taken at 10 different points in diseased stands 

 during three different attacks gave an average moisture content of 3.4 

 per cent for the soil from 2 to 11 inches in depth. The average wilting 

 coefficient of the soil at this nursery, determined from samples taken 

 from the same depths at nine different points located in the same 

 parts of the nursery as the moisture determinations above referred 

 to, was 3.6 per cent, as determined by the indirect method of Briggs 

 and Shantz from moisture-equivalent determinations made by the 

 Laboratory of Biophysical Investigations. While the soil-moisture 

 determinations made were too few to establish the relation of dry 

 soil to the disease, they are to be viewed as contributory evidence. 



(8) High points in beds and the centers of arched beds from which 

 the water runs off or which are missed in flood irrigation are espe- 

 cially liable to damage from the disease. 



(9) Species which normally inhabit moist soil suffer most. Nor- 

 way spruce {Picea excelsa Link) and Douglas fir (Pseudotsuga taxi- 

 folia (Poir.) Britt.) seem to suffer oftener than the pines. Western 

 yellow pine (Pinus ponderosa Lawson) grown from Rocky Mountain 

 seed is more resistant at Halsey than other pines. 



(10) Sufficiently frequent and heavy watering will entirely pre- 

 vent the disease. In three different attacks at Halsey evidence of 

 the preventive effect of watering was obtained. The main practical 

 fact is that for two seasons the nurserymen at Halsey and at Monu- 

 ment, Colo., have practically controlled the disease by increasing the 

 frequency of watering. At these nurseries during the three preced- 

 ing seasons the disease had caused considerable loss. At both these 

 nurseries the disease had been considered parasitic. 



It is not thought that death is usually due to the entire lack of 

 available soil moisture. It is rather probable that it usually occurs 

 when there is still a certain amount of available water left in some 

 part of the soil reached by the root system, but so small in quantity 

 that it can not be taken up by the roots fast enough to supply the 

 demands of the rapid transpiration loss from the needles. This does 

 not necessarily mean slowness of imbibition ; it may simply mean 

 that the capillary water movement from outlying soil particles to the 

 particles adjoining the root surface is too slow when the soil is nearly 

 dry. That this is the case is indicated by the fact that after an at- 

 tack of sun scorch has started as the result of one or two days of 

 rapid transpiration, when the soil is quite dry, the disease may stop 

 spreading on the advent of cooler weather without the addition of 

 any water to the soil from above. 



Some interesting occurrences at Halsey in connection with this 

 trouble, which at first appeared to contradict the relation of the dis- 



