MARKET DISEASES OF FRUITS AXD VEGETABLES 25 



The distinguishing characteristics of the disease are gum pockets 

 and bark excrescences on the twigs and smaller branches, stained ter- 

 minal branches, dying back of the branches, and dark-brown to almost 

 black, glossy spots on the rind of the fruit (pi. 15, A) . The latter may 

 be of various sizes and shapes, but usually their first stage on oranges 

 consists of brown gum pockets in the oil glands of the rind. Gum 

 pockets may sometimes be found in the albedo of thick-skinned 

 oranges when no symptoms appear on the surface of the fruit. These 

 are probably an incipient stage of the disease. In grapefruit the gum 

 pockets are found most frequently in the albedo. Gum may also form 

 in the angles of the segments of the fruit along the central axis or core. 



On oranges the affected spots eventually become elevated so that 

 they appear either as pimples or as large irregular scarred areas. The 

 pimples vary from about one thirty-second to one-eighth of an inch 

 in diameter and are sometimes found so close together that they almost 

 touch, over a large part of the surface of the fruit. There are two 

 types of scars, one rough and dark reddish brown like the pimples, 

 and the other fairly smooth and dull grayish brown to almost black. 

 The rough scars are very often deeply cracked, whereas the smoother 

 ones rarely show more than a superficial checking (pi. 3. B). 



Both kinds of scars occur as spots about one-fourth to one-half inch 

 in diameter or as irregular bands or streaks, which may be so large as 

 almost to encircle the fruit. In both of these the rind is rather hard 

 and stiff and as a result often becomes cracked as the orange grows 

 or because of the greater rapidity with which the affected portions 

 dry out during the marketing process. Cracks probably arise also 

 from even ordinarily careful handling of fruit which has this stiff, 

 brittle covering. Oranges affected by the disease do not carry well 

 in transit, the reason probably being that Penicilliurn and occasionally 

 the stem-end rot fungus find easy entrance at the cracks in the 

 affected rind. 



The pimple stage of exanthema on the fruit is sometimes mistaken 

 for melanose. The two can be distinguished by the fact that the 

 pimples of melanose are dark brown surrounded by gray and when 

 examined under a hand lens are seen to be cracked away from the 

 healthy rind around them (pi. 3, A) : those of exanthema are dark 

 reddish brown and, if they are cracked at all, the cracks appear across 

 the top (pi. 3, B). The pimples of exanthema originate in tissues be- 

 neath the epidermis and thus seem to come up through the epidermis. 

 Melanose is on the surface of the rind. 



On grapefruit, exanthema occurs most often as either craterlike 

 or concave depressions about one-sixteenth to one-fourth inch in 

 diameter, or, more rarely, as flat areas half an inch to an inch or more 

 in diameter and very slightly depressed below the surrounding healthy 

 rind. The surface of the spots, large or small, is usually gray to 

 almost white. In the white spongy part of the rind, and easily seen 

 on cutting, are reddish-brown gum pockets having roughly the diam- 

 eter of the gray area on the surface. Similar brown spots can some- 

 times be found in the rind of affected oranges, but they are always 

 near the surface, whereas those in grapefruit may extend clear to 

 the membrane overlying the juice sacs. 



Exanthema may occur anywhere on the surface of grapefruit but is 

 most common at or near the ends. Even in the large flat spots de- 



