2i8 DISEASES OF CROP-PLANTS 



Citrus Canker. 



History and Distribution. 



The existence of the serious infectious disease now known as 

 citrus canker became known through its introduction into 

 Florida, though it had been present in some of the Western 

 States of the Union, notably Texas, for some years previously. 

 It appears to have come in from Japan on Citrus trifoliata stocks, 

 upon which the disease is not prominent. The date of first 

 introduction into Florida is authoritatively stated to be Novem- 

 ber, 1912, which is a matter of importance to those who have 

 received nursery stock or bud-wood from that State. In the 

 following year the disease began to cause alarm in the grapefruit 

 industry and a special organisation was created to deal with it. 

 The very drastic efforts which have since been made to entirely 

 eradicate the infection have had the effect of thoroughly sup- 

 pressing the disease, and their ultimate success remains to be 

 tested by time. 



The distribution of citrus canker, as known at present, is as 

 follows : — United States (Texas, Louisiana, Missisippi, Alabama, 

 Florida), Japan, China, the Philippines, Java, Singapore, South 

 Africa and Australia. 



Symptoms of the Disease. 



The following description of the appearance of citrus canker 

 on leaves, twigs and fruit is quoted from H. S. Stevens : — 



" The distinguishing feature of citrus canker as observed 

 in the field is the characteristic spotting produced on the 

 fruit and foliage. As usually seen the infection appears as 

 small light brown spots, from less than one-sixteenth to 

 one-quarter of an inch in diameter. The spots are usually 

 round, and may occur singly, or several may run together, 

 forming an irregular area. This last usually occurs on fruits. 

 The spots project above the surrounding healthy tissue, and 

 are composed of a spongy mass of dead cells, covered by a 

 thin white or greyish membrane. The membrane finally 

 ruptures and turns outward, forming a lacerated or ragged 

 margin around the spot. 



" On the leaves, infections first appear as small, watery 

 dots, with raised convex surfaces. These dots are usually 

 of a darker green than the surrounding tissue. Sometimes, 

 however, the surfaces of the spots are broken as soon as they 

 appear. Spots may appear on either surface of the leaf, but 

 they do not at first penetrate through the leaf tissue. They 

 gradually increase in size, change to a light brown in colour, 

 and become visible on both sides of the leaf. In the older 

 spots one or both surfaces may be bulged or raised, and such 



