DISEASES OF PLANTS. 59 



presence is first iudicated by a sudden wilting of tlie leaves, which 

 continues in bad cases even during wet weather. The whole top may 

 be attacked at once, or only a single branch. The spring following the 

 wilting of the top the branches which have become nearly leafless bear 

 abundant small flowers, which continue to a])pear for 2 or 3 weeks after 

 the normal period for blooming. Little if any fruit is set, and the 

 branches usually die after blooming. The affected trees usually linger 

 for a few j^ears, becoming reduced to stumps by the dying of the 

 branches. The roots for the first year appear normal. 



Die-back, which is estimated to cause at least $100,000 loss annually, 

 receives its name from the behavior of the diseased trees, a few inches 

 of the vigorous growth dying back early in the spring. The disease 

 is apparently caused by malnutrition, improper drainage, improper 

 cultivation, etc., and may be recognized by the very large, dark, pointed 

 leaves and the reddish-brown stains on the new-growth twigs, which 

 finally die back for some distance. Diseased trees bear little fruit, and 

 that formed assumes a pale-green color and rii)ens i^rematurely. The 

 fruits are commonly more or less disfigured by the characteristic 

 reddish-brown stain. It has been found that withholding organic 

 manures, mulching the soil, ceasing cultivation, and, where necessary, 

 thorough drainage, will })revent or greatly reduce attacks of this 

 disease. 



Scab, or verrucosis, a disease which attacks sour oranges and lemons, 

 is due to Cladosporium sp., and its attacks may be prevented on lem- 

 ons by spraying the young fruits 3 to 5 times with ammoniacal copper 

 carbonate solution. The fungus causes small excrescences upon the 

 young leaves and fruits. These excrescences are pale watery green at 

 first, becoming darker with the development of the fungus. When the 

 attack is made upon the fruit while it is quite small the fruits become 

 covered with warts and bumps wliich render them unsalable. The loss 

 occasioned by this disease is estimated at about $r)0,000 annually. 



The sooty mold is due to a black fungus which follows attacks of 

 honeydew secreting insects. A description of this disease was given 

 in Circular 15 of the Division of Vegetable Pathology (E. S. E., 7, p. 

 737), in which resin washes were recommended. Fumigation with 

 hydrocyanic-acid gas is also an effective remedy. 



Foot rot, or mal-di-gomma, is one of the most widespread and destruc- 

 tive diseases of citrus trees. It may be recognized by the exudation of 

 gum from definite patches near the base of the tree. Later the dis- 

 eased bark curls up and falls off. The disease spreads downward to 

 the roots and around tlie tree, finally girdling it and causing the death 

 of the tree. Accompanying sym^itoms are sparse foliage, small yellow 

 leaves, and the dying of small limbs over the tree. While probably 

 C(mtagious, it is thought that improper ai-ration of the roots is condu- 

 cive to the disease. Sweet seedling oranges and lemons are most 

 subject to the disease, while grape fruit is attacked but slightly and 

 4256—^^0. 1 5 



