THE MONTHLY BULLETIN. 



255 



the climatic conditions, being almost in direct proportion to the amount 

 of moisture present during the spring and earh' summer months. In 

 extremely dry soil that is none too well adapted to walnut culture, 

 occasionally the twigs are sufificiently blighted back to severely dwarf 

 the tree. Blight must not be confused with an extensive die-back con- 

 dition of the trees which is to be attributed to entirely different con- 

 ditions. 



CLIMATIC CONDITIONS FAVORABLE FOR SPREAD. 



Certain favorable climatic conditions are necessary for the best 

 development of the disease. Heavy fogs — especially foggy nights 

 when the trees are thorouahly saturated — will greatly aid in the dis- 

 tributing of the germs to the healthy nuts. A large amount of cloudy 

 weather during April and ]\Iay is ideal walnut blight weather. In a 

 year having a large amount of sunshine during the spring months 



Fig. 56. — Walnut Blight, PseudoDionas juglandis. on the nuts. 

 (After R. B. Smith, California Agricultural Experiment Station.) 



there will be little of the disease, since the sunlight is a most efficient 

 germicide. Insects are instrumental in spreading the germs, espec- 

 ially certain flies which are often common about the walnut 

 The disease is able to start without any puncture or injury 

 tissue and the flies are simply carriers of the organism. The 

 has been isolated from insects found about the grove. 



grov'es. 

 of the 

 disease 



DISEASE CAUSED BY BACTERIA. 



The disease has been proven to be caused by a definite species of bac- 

 teria, described first by Mr. Newton B. Pierce as Pseudomonas juglandis. 

 The organism is rod-shaped and motile by a single polar flagellum, and 

 when grown on artificial media is of a yellowish color. It secretes 

 enzymes that can destroy the cellulose, starch and other substances of 

 the plant tissue. These enzymes are most active at 65° to 75° F., hence 

 we could conclude that temperatures higher or lower than these would 

 be less favorable for the best development and spread of the disease. 



