230 THE MONTHLY BULLETIN. 



orchard and. while examining the trees, observed an apparently new and 

 peculiar lesion on the interior sucker growth. At the time this was 

 given little more than a casual examination. Later in the season this 

 peculiar abnormal condition was found in several places on the other 

 side of the Feather River in Thermalito Colony, some eight miles away, 

 and it was realized that here was something new and of possible eco- 

 nomic importance. Accordingly, an attempt to identify the disease was 

 made by the writer, but without success. 



As this disease did not spread to any appreciable extent during the 

 next two seasons, due as we now know to unfavorable weather condi- 

 tions, and the actual damage appeared to be slight, no further attention 

 was given to the matter. It was known to exist, but apparently was 

 of little importance. On several occasions materia] was sent for exam- 

 ination to the Agricultural Experiment Station at Berkeley. 



On returning to Oroville in May, 1915. the writer was immediately 

 struck by the widespread occurrence of a condition ascribed to a late 

 frost by some growers, and characterized by others, where no frost 

 could have occurred, as a •"heavy leaf drop." On examination this 

 condition was found to be due to neither of these causes, but was appai 

 ently the result of the same disease which had been first noticed three 

 years before. The same peculiar lesions were prevalent throughout 

 the mature trees on the year-old growth. During the remainder of the 

 season the disease was studied, notes were taken as to its occurrence and 

 external manifestations, and material was collected for laboratory 

 examination. 



Climatic Factors. 



Normal winter conditions for the citrus districts of northern Cali- 

 fornia are cool weather with intermittent rains, followed by short, dry. 

 clear spells. Under such conditions the disease is not very active. The 

 rains usually cease in February or March and there ensues a hot. dry 

 spring and summer. However, it frequently happens that the winter 

 rains are prolonged and coincide with the early spring growth. Such 

 conditions are ideal for the maximum activity of citrus blast. Leaving 

 its quiescent state, the disease becomes very active and spreads with 

 great rapidity through the orchards, following the direction of tin' pre- 

 vailing winds. Tlie appearance presented is similar to that caused by 

 a rapid blight or heavy frost. 



Disease Characteristics. 



Infection is apparently confined to new spring growth and seems to 

 occur chiefly at the junction of the petiole and blade of the leaf. How- 

 ever, it appears that infection may also occur at the tips of the very 

 young leaves and tender shoots. Once established, the disease progresses 

 rapidly down the shoot toward the older wood. Only the new tender 

 growth is affected, the older limbs apparently possessing greater resist- 

 ance. The young shoots are often killed back to the older wood, includ- 

 ing a portion of the bark about the base of the infected shoot. At these 

 points the characteristic lesions are formed. It is not unusual to find a 

 branch several feet long with every node affected in this manner. Upon 

 infection, the leaf turns pale yellow, then darkens irregularly in spots 

 and wilts in place, where it later shrivels and dries, still hanging to the 



