324 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



any other one factor. In the year 1903, one of the first bad blight years, 

 the total crop of the State fell off to only a little more than one half 

 of that of the previous year. It is probably conservative to state that 

 in the seedling groves of southern California an average loss of at least 

 50 per cent of the crop which would otherwise have been harvested has 

 been caused by the blight during the past ten years. Many individual 

 trees may be seen some years which show a loss of practically all the 

 crop from blight. This disease affects the quantity and, to some extent, 

 the quality of the crop much more than it does the growth of the tree, 

 being of less importance in that respect than pear blight, for instance, 

 and many other plant diseases. While the leaves and young shoots 

 become affected to some extent, as we shall describe later, this injury 

 in otherwise thrifty trees is soon overcome during seasons when the dis- 

 ease is less prevalent, and is not usually a serious matter. In occasional 

 instances, especially on light, dry soils where the trees are at best none 

 too thrifty, sufficient twig infection sometimes occurs to actually stunt 

 and dwarf the trees to a considerable extent by a continual killing back 

 of the young twigs. Damage is also done in such cases by the loss of 

 the fruit spurs which should produce the catkins and nut-bearing shoots 

 the following year. 



It should be clearly understood that walnut blight is not conspicu- 

 ously a die-back disease of the tree, but is more especially a source of 

 loss in the amount of the crop, varying widely in prevalence from year 

 to year, and not, in the majority of instances, seriously affecting the life 

 or growth of the tree. 



Earlier Studies. Reference has already been made to the work done 

 by Pierce. He first isolated the organism and printed quite a full and 

 accurate, technical description 1 of the disease and its physiological and 

 pathological characteristics. He also published several more popular 

 accounts of the trouble. 2 Various experiments in spraying were made 

 by him in trying to control the trouble by such means, the results of 

 which will be more fully discussed under another heading. He also 

 observed that certain walnut trees seem to possess a certain immunity 

 to the disease, but did not indicate that the time of blooming had any- 

 thing to do with the amount of blight that developed. 



In 1905 the California legislature made a special appropriation to 

 the Experiment Station for carrying on an investigation of this disease, 

 and Mr. A. M. West was appointed to work on the problem. He isolated 

 and studied the organism and confirmed in most respects the work 

 previously done by Pierce. 



^Botanical Gazette, 1901, 31, p. 273. 



^Pacific Rural Press, 1896, 57, No. 25, p. 387. 



California Fruit Grower, 1896, 19, Nos. 13, p. 243; 16, p. 316. 



