CHAPTER III 



LEAF SPOTS 



Citrus Canker 

 Pseudomonas citri — Hasse 



The disease was probably intrbduced into the United States on 

 nursery stock from Japan, and since 191 2, has occurred in Florida, 

 ' Alabama, Mississippi, Louisiana, and Texas. 



Symptoms. — According to Stevens,* citrus canker attacks all 

 varieties of citrus trees of any commercial value in Florida, but it is 

 most severe on the grape fruit. Under field conditions a characteristic 

 spotting of the fruit, foliage and twigs is produced which appears as 

 small light-brown spots, 1.5 mm.-6 mm. (J^s to J^ inch) in diameter. 

 These spots may occur singly or several may coalesce to form an irregular 

 area; they are raised above the adjoining tissue and are made up of a 

 spongy mass of dead cells, covered by a thin white or grayish membrane, 

 which ultimately ruptures forming a ragged margin around the spot. 

 The fruit is especially susceptible to the infection, and drops soon after it 

 is attacked. The disease is spread rapidly from one part of the tree to 

 another by insects, rains and heavy dews, so that when once infected, a 

 tree frequently becomes worthless in two or three months. 



Catjsal Organism. — Miss Clara H. Hasse t has described the causal organism, 

 Ps. citri, as a short rod with rounded ends, motile by a single polar flagellum. 



' On nutrient agar, the growth is filiform, shining, dull yellow in color; on potato, 

 bright yellow, shining, viscid. In nutrient broth, a yellow ring is formed at the 

 surface in old cultures. Litmus milk becomes deeper blue, and the casein is pre- 

 cipitated. Gelatin is liquefied. Indol is not produced. No gas is formed from 

 sugars in Dunham's solution. Growth is slight in Uschinsky's solution, and nitrates 

 are not reduced in starch nitrate solution. The organism grows best linder aerobic 

 conditions. 



* Stevens, H. E., "Citrus Canker, I, II, III," Bulls. 122, 124, 128, Pla. Exp. Sta., 1914, 191S. 

 f Hasse, Clara H., "Pseudomonas citri, the Cause of Citrus Canker," Jour. Agr. Res., Vol. 

 IV, No. i, p. 97, 191S. 



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