SqS microbiai. diseases of plants 



field and garden peas are affected alike, and the symptoms simulate 

 the bacterial stem blight of alfalfa. The stems have a watery, olive- 

 green appearance which soon becomes olive-brown, and in the last 

 stages dark brown. The leaves and stipules appear watery at first, as 

 if bruised, and later turn ocher yellow in color; this is often accompanied 

 by wilting. In young plants, the discoloration of the stems is followed 

 by a shrivelling, and ultima tdy the plants dry up and die; in the older 

 ones, where the infection has taker^ place later, the same condition may 

 result, but on the whole, the disease appears to be less serious, and in 

 some cases the plants seem to outgrow the Wight. Frequently when 

 the first and earKest shoots are destroyed, the plant throws up new 

 shoots from below ground, and a good late crop is obtained, in spite 

 of the trouble. 



Causal Organisms. — Pseudomonas pisi, n. sp., as described by Sackett,* 

 is a short rod with rounded ends, motile by means of a single polar flagellum; 

 neither spores nor capsules observed; filaments formed commonly; stains readily 

 with aqueous stains, and is Gram-negative. 



It produces a flaky surface scum with heavy clouding in broth. On nutrient 

 agar the growth is smooth, glistening, grajdsh white, and the medium is not dis- 

 colored. Gelatin is liquefied rather rapidly. On potato, smooth, glistening, cream 

 to orange-yellow; medium b'ecomes grayish brown. No growth in Cohn's or 

 Uschinsky's solutions. Heavy clouding with white surface pellicle in Fermi's 

 solution; clouding with surface scum in Fraenkel's solution; slight, transient cloud- 

 ing in Naegli's solution. Plain milk is coagulated, and the coagulum is slowly 

 peptonized, the supernatant liquid becoming yellowish green. Litmus milk becomes 

 bluer, and the litmus is reduced, the liquid becoming greenish-gray. Neither 

 indol nor hydrogen sulphide is produced. Ammonia is produced from asparagin 

 and peptone. Nitrates are not reduced. No gas is formed from sugars, but acid is 

 produced from dextrose, saccharose and galactose. Obligative aerobe. Optimum 

 temperature 25° to 28°. Thermal death-point 30°. Habitat, soil. 



Pathogenic for field pea and garden pea (Pisum sativum var. arvense and Fisum 



Method of Intection. — Experimental inoculations indicate that 

 infections take place either through the stomata or through wounds 

 produced by mechanical injuries. 



Control. — There seems to be a close relation between the preva- 

 lence of the disease and a late, cold spring. The low temperatures 

 appear to make the plants more susceptible, and as a result the early 



* Sackett, Walter G., "Stem Blight of Field and Garden Peas — A Bacterial Disease," 

 Bull. 2i8, Colorado Exp. Sta., April. 1916. 



