BLIGHTS. 501 



WALNUT BLIGHT. 

 Pseudomonas juglandis Pierce. 



DISTRIBUTION AND SYMPTOMS. The English walnut of California* 

 is subject to a blight which is so serious in the southern part of the state, 

 that one-half of the crop is frequently lost. Pierce has shown the cause 

 to be a microorganism, Ps. juglandis, which produces black, cankered 

 spots on the young nuts, either causing them to fall prematurely, or spoiling 

 the kernel. Similar discolorations also occur on the young, tender growth 

 and kill back the terminal branches, which should have the crop the next 

 year. As the wood hardens, the disease is checked, and the vitality of 

 the tree is not affected to any extent, the crop suffering rather than the 

 tree. Little is known of the method of infection and spread of the dis- 

 ease. 



CAUSAL ORGANISM. According to Pierce, Ps. juglandis is a short rod with rounded 

 ends, i.ofi to 2.o// by 0.5/4; actively motile by a single polar flagellum. It produces a 

 bright chrome yellow growth on the ordinary culture media. Marked diastatic action 

 on potato, distinguishing it from Ps. stewarti, Ps. hyacinthi, and Ps. campestris of feeble 

 diastatic action. Liquefies gelatin. Does not produce gas in various sugar solutions. 

 Aerobic. Distinguished from Ps. campestris by abundant bright yellow pigment on 

 surface of extracts of leaves of Juglans regia, Magnolia macrophylla, Ricinus communis 

 and Ficus carcia. 



Pathogenic to nuts, leaves and tender branches of Juglans regia. 



CONTROL. Spraying twice with Bordeaux mixture, after first re- 

 moving affected twigs, has reduced the disease 50 per cent. Trees 

 differ greatly in their susceptibility, so that an immune variety may be 

 found which will solve the difficulty. 



* Pierce, N. B. Bot. Gaz., 31: 272-273, 1901. 



