DISEASES OF LIME AND OTHER CITRUS TREES 217 



Influence of External Conditions. 



Infection only takes place on young and tender parts of the 

 plant, and is very largely dependent on the prevalence of moist 

 and cloudy weather in periods when new growth is being rapidly 

 developed. Trees in poor condition are but little subject to 

 attack, presumably owing to the scarcity and slow development 

 of new shoots upon them. 



Inherent Resistance. 



According to Stevenson scab was generally prevalent on 

 sour orange and rough lemon stocks in Porto Rico by the year 

 1903, but grapefruit was almost immune until the occurrence 

 of an excessively wet season in 1911-12. Then the grapefruit 

 was attacked suddenly and in most virulent fashion in one 

 section, and since that date the disease has maintained its new 

 character and invaded most of the other citrus districts in turn. 

 This occurrence gives colour to the suggestion that a new and 

 virulent strain has come into existence in the island. 



Prevention. 



Outbreaks of scab are preventable by protective spra3/ing 

 with Bordeaux mixture, but this method has been found to have 

 the serious drawback of also repressing the fungi which keep 

 down scale insects, thus opening the way to serious infestations 

 with the latter. It is found that better general results are 

 obtained from the use of a lime-sulphur spray, which while 

 much less effective against scab is largely free from the objection 

 named. 



Black Melanose. 



The affection known as true melanose in Florida, and ascribed 

 to the fungus Phomopsis Citri does not appear to occur in the 

 West Indies. It is described as a superficial marking of the sur- 

 face of fruits, leaves and stems, consisting of small raised areas 

 with the appearance of wax or of small drops of melted sugar 

 burnt brown. These markings sometimes occur in patches and 

 circles. 



The melanose met with in Cuba, Porto Rico, Jamaica and 

 the Lesser Antilles is a leaf affection distinguished as black 

 melanose or greasy spot. It occurs on various citrus species 

 in these islands, including rough lemon, grapefruit and orange 

 The back of the leaf is covered with slightly raised black and 

 brown markings, somewhat resembling, it has been suggested, 

 spots of dark grease under the epidermis. The affection is very 

 persistent, but does no apparent damage. Its nature is unknown. 



