222 DISEASES OF CROP-PLANTS 



Diplodia Rot. 



The fungus associated with Diplodia dieback of citrus (which 

 see) is the commonest, in fact the only definitely recognised cause 

 of rotting of citrus fruits in the field in these islands, and reports 

 to the same effect have been made from Cuba, Porto Rico and 

 the Isle of Pines. It occurs to some extent on limes, and pre- 

 sumably other citrus fruits, while they are still on the tree, but 

 is most noticeable in the ripe limes lying on the ground. 



The fruit becomes thoroughly infested and soured by the 

 fungus, the rind turns brown, gum is often exuded, and under 

 moist conditions pycnidia are developed which cover the surface 

 with a sooty deposit of spores ; under circumstances of greater 

 dryness or ventilation the fruit dries up and shrivels. Punctured 

 fruits are especially liable to infection. 



Diplodia rot occurs on lemons in Natal, and on oranges and 

 grapefruit in the Greater Antilles and Florida. It is reported as 

 spreading mostly from the stem end or from an injury. The 

 fungus involved is usually named Diplodia natalensis, but no 

 sufficient reason seems to have been given for distinguishing it 

 from D. TheohromcB. 



Scab. 



Corky irregularities of the rind, not leading to rot or decay 

 or affecting the contents of the fruit. Refer to general descrip- 

 tion of the disease. 



Russeiing. 



Silvery or sometimes brownish scurf general on the rind, 

 giving the fruit a dull appearance. The epidermis is broken up 

 into minute scales. Sometimes very common on oranges and 

 limes for a period of a few weeks in a particular situation, then 

 going off. Here as in Florida both rust-mites and a fungus of 

 the genus Coniothecium are found on such fruits, and it is un- 

 certain how far each is responsible, alternatively or in com- 

 bination, for outbreaks. Dusting with sulphur should be adopted 

 as a remedy where appearance is of value, otherwise there is no 

 harm done. 



Oil Spot. 



One of the commonest of blemishes on local citrus fruits is 

 a small superficial green or brown spot, usually more or less 

 circular, which when closely examined is seen to consist of an 

 area in which the ground tissue is slightly sunken, leaving the 

 oil glands standing out as small green or yellowish papillae. In 

 old examples cork formation has taken place below the spots. 

 The prevalence of this condition caused some alarm on a Dominica 

 orange estate some years ago, as it was thought to facilitate 

 decay. Experiments showed that this belief was unfounded and 



