T92 DISEASES OF CROP PLANTS 



the bud. It is caused by the attacks of the " pineapple " fungus 

 familiar on sugar-cane, Thielaviopsis paradoxa, v. Hohn. The 

 disease has not so far been recognised in the Lesser Antilles. 



Ashby's account of the symptoms is as follows : — 



" The younger green unfolded and unfolding leaves appear 

 bitten off at the ends or show successive leaflets lower on the stalks 

 broken or reduced to stumps. Those nearest the heart show 

 spots on successive green leaflets which bend over or break and 

 hang down at these points. 



" Leaves just pushing up can be detected with black tips ; if 

 the end of the heart leaf is cut out with the white leaflets over- 

 lapping and pressed on the stalk, characteristic discoloured 

 patches and spots will be seen. Infection usually starts as a 

 yellow discoloration with a brown edge along a narrow line beside 

 the ribs of the leaflets ; the infection runs across all the leaf- 

 lets overlapping at that point. These long, narrow spots widen 

 to wedge-shaped patches with dark brown broad edges and a pale 

 brown centre, with the skin raised into a blister so that it can 

 be torn off to expose the fibres, with the tissue between them 

 more or less decayed away. 



' ' If infection is high up only the tips of the leaflets become 

 black. A heart leaf may show one or more wedge-like patches of 

 disease. Spots are also present on the leaflets, at first minute, 

 round or oval, yellowish with depressed centre, later with pale 

 yellow centre and brown edge ; by union these become patches ; 

 in bad cases the leaf stalk (rachis) beneath the diseased leaflets 

 is attacked by a dark brown rot so that when pushed out the end 

 of the limb may break off. Spots and patches show black specks 

 consisting of spore masses of the fungus. The frequency of affec- 

 tion beside the ribs of the leaflets is due to a thin line of cells there 

 which regulate the opening and closing of the mature leaflets 

 in dry and moist air, so as to control evaporation of water from 

 the leaf pores which are confined to the under sides of the leaflets. 



" The disease, as already pointed out, starts on the heart leaves 

 while still white and tender before they push into the light. 

 Just what conditions favour this infection cannot be stated yet. 

 Check to growth during a period of drought may have started it 

 at some estates while the last two unusually wet years have made 

 it epidemic. 



" The diseased patches attract the beetle Metamasius sericeus 

 (weevil borer of sugar-cane) which increases the extent of the 

 damage. 



" Cutting out the visibly diseased parts will sometimes remove 

 the infestation, but to be certain of preventing its recurrence 

 it is found necessary to chop back the bud until no more dis- 

 coloured leaflets are met with. About a pint of a powdered 

 mixture of one part each of salt, lime and copper sulphate is 

 applied to the cut ends. In most cases the trees recover rapidly 

 from the treatment." 



