6o4 MICROBIAL DISEASES OF PLANTS 



the early stage, the lesions appear as small,, circular, raised, discolored, 

 water-soaked areas; later, these spots increase in size and turn black. 

 Under favorable conditions, the' disease may extend through the hull 

 and shell-forming tissues into the kernel which at length becomes 

 blackened and finally destroyed. 



Causal Organism. — According to Smith, C. O., Pseudomonas jttglandis, Pierce, 

 is a rod with rounded ends; single or in pairs, rarely in chains; measures i.S/t to s-oiju X 

 0.3/j to o.Si/tj stains readily with the ordinary aniline dyes; Gram-positive; spores and 

 capsules not observed;, motile by means of a single polar flagellum; agar colonies 

 nucleated, circular, moist, shining, pale yellow with regular margins; startiform 

 liquefaction in gelatin; potato, abundant, moist, shining, slimy, raised, white chang- 

 ing to yellow; uniform turbidity and ring in bouiUon, slight flocculent precipitate; 

 indol produced; nitrates not reduced; enzymes, diastasic, cytohydrolytic, reimet, 

 proteolytic; milV coagulated, curd digested; litmus milk wine colored; viability, 

 nine and one-half months on potato; methylene-blue milk reduced. 



Method of Intection.— It has been shown that the causal 

 organisms live over winter in the old lesions of the wood and bark and 

 that in the spring they exude to the surface and are carried to the new 

 growth, to which they gain entrance through the stomata. The disease 

 is most severe during seasons when the fogs and rainfall are heaviest, 

 and in those localities where rain and fogs are abundant. "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." Distribution by this means is thought 

 to be one of the most important, if not the most important, methods of 

 spreading the trouble. Insects probably play some part in the dis- 

 semination of bUght. 



Pathogenesis. — Pathogenic for Juglans regia. (English) under 

 natural conditions; pure culture inoculations give positive lesions on 

 Juglans nigra (eastern black), Juglans hindsii (nothern Cal. black), 

 Juglans cahfornica (southern Cal. black), Juglans cinerea (butternut). 



Control. — Systematic spraying experiments with Bordeaux mix- 

 ture, lime-sulphur, and a sulphur spray have demonstrated that spray- 

 ing is impracticable and has little value as a means of control. Applica- 

 tions of lime to the soil have resulted in no benefit. It has been 

 observed that individual trees exhibit great differences in their natural 

 resistance to the blight, and at the present time the selection and 

 propagation of varieties which are more or less immune promises 

 the most practical solution to the problem. 



