CITRUS FRUITS 125 



bark with carbolineum, or by spraying five or six times 

 with Bordeaux mixture. A somewhat similar disease, 

 which may or may not prove to be the same, occurs in 

 California and Arizona. 



Yellows. This disease, which is a yellowing of the 

 foliage, has been reported from Arizona. It is not 

 understood, and is supposed to be due to physiological 

 causes. 



Canker. This is a bark disease which has been 

 reported from Brazil by Noack. It is caused by the 

 fungus Didymella citri, Noack, and appears first as a 

 depression of the bark, followed by a cracking, and 

 finally producing an open wound. The fruits of the 

 fungus are borne in this wound. All diseased parts 

 should be cut and burned as soon as detected. 



Chlorosis. This disease is a yellowing or whitening 

 of the leaves, due to a reduction of the chlorophyll. It 

 may be due to any one of many causes, such as the 

 presence of grubs or fungi on the roots, poor drainage, 

 drought, or a soil which is deficient in the necessary food 

 materials. However, it is frequently difficult to discover 

 any cause whatever, and the presence of parasites which 

 may attack the weakened tree is often confusing to 

 both the grower and the botanist. Of course, the fruit 

 production of the tree is reduced or entirely destroyed, 

 and the death of the plant frequently results. In Malta 

 this disease is said by Borg to be accompanied by an 

 enormous production of bloom, but that these blossoms 

 are usually poorly developed. He also says that the 

 diseased trees sometimes produce an abundance of very 

 small fruit which never matures. As the disease pro- 

 gresses unhealthy, yellowish shoots come from the base 

 of the trunk. Where no specified cause can be dis- 

 covered, good cultivation and fertilization will frequently 

 restore the trees to their normal condition. The fertilizers 

 should usually contain a small amount of sulphate of 

 iron. 



Fleshy Fungi. The large fleshy fungi are no doubt 

 the cause of some troubles on the citrus trees, especially 



