August 1, 1902.] 



SCIENCE. 



193 



PEAB BLIGHT IN CALIFORNIA. 



The discussion of California pear blight 

 given in Mr. Webber's report of the proceed- 

 ings of the Botanical Society of Washington,* 

 induces me to offer some additional notes on 

 this subject. 



Althoiigh supposed cases of pear blight have 

 been reported in California for many years, an 

 examination of the affected orchards has 

 always resulted, until recently, in throwing 

 doubt upon the presence of that disease within 

 the State. In the spring of 1899, however, 

 there occurred a t3Tpical outbreak of blight in 

 southern California, the malady assuming the 

 normal epidemic form of spring development, 

 spreading over several counties in a short 

 time. This was the first case of the kind to 

 come to my knowledge, although the orchards 

 of the State had been under continual ob- 

 servation for upward of ten years previously. 



As early as the spring of 1900 the disease 

 developed seriously near Hanford and at other 

 points in the southern half of the San Joaquin 

 valley, while at present it has spread to a 

 large percentage of the leading pear-growing 

 districts of southern California and of the 

 San Joaquin and Sacramento valleys. There 

 are still special districts within the State 

 which are wholly or nearly free from its rav- 

 ages — as the Santa Clara valley and other 

 coast regions. 



The destructive development of pear blight 

 in the hot interior valleys of California should 

 at once dissipate the somewhat popular theory 

 that bacterial and many other diseases of the 

 East will not thrive under the semi-tropic and 

 more arid conditions of portions of the Pacific 

 coast. The facts are quite to the contrary. 

 In California there is not alone a spring and 

 summer, but likewise a fall and winter epi- 

 demic of pear blight, and the latter form of the 

 disease is by far the more destructive mani- 

 festation of the two. The writer has given the 

 name of 'winter blight' to the fall and winter 

 type of the disease, and by careful inoculation 

 experiments, conducted with pure cultures, has 

 demonstrated the cause of winter blight to be 



* Sci£>-CE, N. S., Vol. XV., pp. 989-991. 



identical with that of spring blight in the 

 East.* 



The leading characters distinguishing winter 

 blight are : Eirst, it rarely if ever attacks a 

 tree at points higher than a man's head, 

 always affecting the trunlv or base of the main 

 limbs, hence the larger and more vital por- 

 tions of the tree; second, the infection takes 

 place about the time the crop is gathered or 

 shortly after; third, it continues in a most 

 active and destructive state during the months 

 of November, -December and January; and, 

 fourth, it may prevail in an orchard showing 

 little or no signs of the spring form of the 

 disease. The experience of growers supported 

 by observation in the orchards shows that 

 winter blight infections usually occur in short 

 spurs developed upon the - base of the main 

 limbs or on the trunk of the trees. In Cali- 

 fornia it is not uncommon for these spurs to 

 develop clusters of flowers in the late fall, 

 when the moisture rises in the soil or after 

 the fall rains begin. The belated flowers rarely 

 occur at points higher than a man's head, and 

 they therefore serve as points of infection 

 for the basal limbs and trunk of the trees. 

 As the spurs are short the time required for 

 the bacillus to pass from the flower to the 

 parenchymatic tissues of the cortex of limb or 

 truniv is brief, and the girdling of the trunk or 

 main limbs is often a matter of a compara- 

 tively brief time. As the winter temperature 

 of many California valleys is sufficiently warm 

 to permit the blight bacillus to grow during 

 November, December and January, and as the 

 organism is so located in cases of winter blight 

 that the affected parts cannot be removed by 

 pruning without removing the more essential 

 portions of the tree, the winter development 

 of this disease has frequently resulted in more 

 serious injury and greater losses of trees than 

 the spring form of the malady in the East. In 

 the latter form of the disease the twigs and 

 smaller limbs are the leading points of infec- 

 tion, and a careful and early removal of the 

 infected parts is commonly accomplished with- 

 out serious injury to the trees. 



In winter blight, as in spring blight, the 



* See California Fruit Grower, May 4, 1901, 

 Vol. XXVI., No. 675, p. 4. 



