ORANGE DISEASES 



1489 



usually lies underground, and most often 

 denotes unfavorable soil conditions or 

 lack of water. 



Exanthema, Florida Die Back 



The branches die back from the ends 

 and numerous axillary buds develop, 

 forming bushy tufts of small twigs all 

 through the top of the tree. Corky out- 

 growths develop on the bark of the twigs 

 and gum exudes from these places. In 

 the first stages of the disease there de- 

 velop near the center of the tree abnorm- 

 ally large, dark-green leaves, giving 

 the tree a false appearance of unusual 

 thrift. The fruit takes on a pale-yellow 

 color while still small and immature and 

 has an insipid sweetness with no develop- 

 ment of acid quality. Dark-brown spots 

 or patches appear on the rind and from 

 these as centers the oranges crack and 

 split. 



This disease occurs in California mostly 

 upon coarse or gravelly soils or subsoils, 

 following the application of stable ma- 

 nure or other nitrogenous material in an 

 organic form. It is most apt to occur 

 when such fertilization is practiced on 

 trees which have not previously been re- 

 ceiving it, especially if they have been 

 suffering somewhat from lack of plant 

 food and water. The disease may be lik- 

 ened to a form of indigestion. 



On these soils citrus trees should re- 

 ceive very careful irrigation to the end 

 that the ground may be kept continuously 

 moist and not be allowed to become al- 

 ternately diT and wet, as is the tendency 

 on such porous soils. Fertilization should 

 be uniform without the sudden applica- 

 tion of large amounts of manure or or- 

 ganic fertilizers. 



Florida Die Back. See Exanthema, this 

 section. 



Gxim-Spot Leaf Spot 



Dai-k colored, slightly raised spots or 

 areas appear on the back side of the leaf 

 in places where it is turned up and ex- 

 posed to the sun. These spots are form- 

 ed by the deposition of a gummy sub- 

 stance in the tissue. The trouble ap- 

 pears to be in a form of gumming result- 

 ing from sunburn on the under side of 

 the leaf. Not serious. 



Mai di Gomma 



A virulent decay of the bark of the 

 loots from the surface of the ground 

 downward. This is occasionally found on 

 trees in extremely heavy wet soil or 

 where too much water is used close about 

 the trunk of the tree. 



Affected trees can rarely be saved but 

 may be replaced successfully if soil con- 

 ditions can be improved. 



Mottled Leaf 



What has been said in the last case 

 applies to this also. Affected trees show 

 a yellowing of the leaves between the 

 veins, with the green color only along 

 the mid-rib and the lateral veins, giving 

 the leaf a mottled appearance. Examina- 

 tion shows that leaves once green never 

 become typically mottled thereafter, al- 

 though they may become lighter in color 

 or even bright yellow. Typical mottled 

 leaves are found only toward the ends 

 of the shoots and represent leaves in 

 which the green color has never com- 

 pletely developed, rather than those in 

 which the chlorophyll has once existed 

 but then disappeared. Mottled leaf is a 

 case of non-development or slow develop- 

 ment of chlorophyll. Along with the 

 mottling of the leaves there also occurs, 

 if the trees are badly affected, a decided 

 shortage in the amount of fruit, and in 

 typically bad cases the fruit present is 

 of very small size, becoming fully colored 

 when only an inch or two in diameter. 

 Considerable die back also occurs and the 

 foliage is thin and weak, giving the tree 

 a brushy look, with many small, dead 

 twigs at the extremities. 



All observations point to one con- 

 clusion, namely, that the most prevalent 

 and typical form of mottled leaf is due 

 to an irregular supply of moisture and 

 plant food. 



Nailhead Rust- See Bcaly Bark, this 

 section. 



IVavel Rot — ^Black Rot 



AUernaria citri 

 Affected oranges color prematurely in 

 the fall and are affected with a dry, black 

 rot in the tissue below the navel end. 

 This rot is not very virulent and often 

 remains confined to one section of the 



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