Jan. 21, 1918 Irrigation Experiments on Apple-Spot Diseases iii 



were infested with aphids earlier in the year, and the spots usually 

 appear several weeks before picking time, while bitter-pit is rather 

 evenly distributed over the tree and is found only on mature or nearly 

 mature fruit. Bitter-pit can be distinguished from fruitspot by the 

 fact that with the latter disease there is an almost entire absence of 

 subepidermal browning, and the spots have an irregular outline and a 

 flecked or speckled appearance. 



EXPERIMENTAI, WORK 



The writers were convinced by earlier investigations that bitter-pit 

 was not due to fungi or bacteria. They had frequently seen unsprayed 

 fruit that was seriously affected with the disease, thus making the 

 theory that spray materials were responsible for the trouble seem en- 

 tirely untenable. Drouth had frequently been mentioned as a cause of 

 bitter-pit, and several writers had suggested an excessive or uneven 

 water supply as a possible cause, but little experimental data had ever 

 been furnished in support of any of these theories. The question of the 

 influence of soil-water supply seemed to the writers to be an extremely 

 important one, and a series of experiments were started to determine 

 the effect of irrigation upon the disease. 



The work has been located at Wenatchee, Wash. The climate of the 

 section is arid, but little precipitation occurring from April till October, 

 thus making the trees almost entirely dependent upon irrigation for their 

 soil-water supply during the growing season. Except where otherwise 

 mentioned, the water was applied by the furrow method (PI. 4, A). 

 The contrasts in the amount of water on the different plats were secured 

 by varying the frequency and duration of the irrigations and, in some 

 cases, by differences in the number of furrows supplying water to the 

 row and by variations in the head of water at the flume. 



The amount of water in the soil was determined by means of samples 

 taken with a soil auger. In the beginning of the work samples were 

 taken at depths of 6, 18, 30, 42, and 54 inches — that is, from the middle 

 of each of the first 5 successive feet of soil — but in the final experiments, 

 as reported later, samples were taken only from the one or two depths 

 that seemed most important in determining the condition under which 

 the tree roots were working in the particular orchard. 



Some difficulty was found in securing samples that represented the 

 average moisture conditions of the tree row. The lateral movement of 

 soil water is very slow, resulting in considerable contrast between the 

 amount of moisture beneath the irrigation furrow and a few feet from it, 

 especially in the upper layers of the soil. Samples were usually taken 

 at a distance from the furrow equal to one-fourth the space between 

 the furrows, thus securing soil from a point midway between the wettest 

 and dryest areas. The plan of sampling was always the same for the 

 different plots of a particular orchard. Samples were taken at intervals 

 of 7 to 10 days, and usually just before and i or 2 days after an irriga- 

 tion, thus obtaining a record of the extremes in soil-water conditions. 



