338 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



1907 and 1908. In 1907 there was an abundant amount of bacterial 

 infection and quite a severe loss of nuts. There was at the same time a 

 large crop of nuts. In 1908 there was almost an absence of blight and 

 a very much lighter crop of nuts. It is rather difficult to always fully 

 account for these variations because of the great number of conditions 

 that must be considered. 



Fogs and Moisture. The amount of moisture present in the air has 

 without question considerable influence on the quantity of disease that 

 may develop, as humid conditions are especially favorable for its spread. 

 It has been proven again and again by our experiments that infection is 

 produced when water containing the disease germs is sprayed on the 

 surface of the young nuts, while the untreated ones are free from the 

 disease. Then, for infection to take place under natural conditions, it 

 only becomes necessary for virulent germs to come in contact with 

 immature nuts, and water is apparently the principal agent in convey- 

 ing the germs from the diseased lesions to the young growth or small 

 nuts below. Rains very rarely occur in the spring after the nuts appear, 

 but foggy weather is often more or less frequent. What are called high 

 fogs, a cloudy condition of the sky, but with little moisture, which in 

 some sections occur during the spring months, do not cause infection, 

 although such cloudy conditions are favorable for bacteriosis to develop 

 where infection has already taken place. Low fogs and especially foggy 

 nights are very favorable for the dissemination and new infection of 

 the small nuts. During one of these fogs the trees become saturated, 

 water dripping from one portion of the tree to another which could 

 easily carry the disease organisms to healthy tissue. Observations go to 

 show that the secondary infection, in which large numbers of the small 

 nuts become diseased, is very likely to follow one of these foggy periods. 



Insects. It is more difficult to say just what part insects play in the 

 spread of this disease, as it is not necessary for the nuts to be bitten in 

 order to cause infection. Insects are present in quite large numbers 

 about the tree. Several species of flies and beetles are most numerous. 

 Flies have been observed on diseased nuts, probably attracted by the 

 exuding of organic matter mixed with walnut blight germs that is some- 

 times present, and these flies have been collected and culture media 

 inoculated by placing them in it, with the result that the characteristic 

 organism has developed in the cultures. This shows that the living 

 germs of walnut blight were on their bodies and only needed to be 

 placed in contact with the nut to cause infection. A species of aphid 

 is often abundant on the leaves, rarely on the nuts and branches. This 

 insect, however, appears some time after the nuts, and probably does 

 not cause much infection of any part of the tree except possibly the 

 leaves. The honeydew is secreted and deposited on leaves and nuts, 

 and in this a sooty mold grows. 



