254 DISEASES OF CROP-PLANTS 



The perithecia are borne either on independent cushions or 

 in succession to the conidiophores ; they are yellowish brown, 

 sessile, and crowded closely over the whole surface. The asco- 

 spores are brown, two-celled, fusiform, often slightly constricted 

 at the septum and longitudinally striate, with blunt apices. 



The conidia are bluntly spindle-shaped and four-celled. 

 The two equal-sized median cells are larger, thicker walled and 

 crowded with oil drops of an orange colour. The terminal cells 

 are hyaline of equal size, inflated, and rounded. The long 

 stalks of the conidia are hyaline, septate, and enlarge rapidly 

 towards their attachment to the conidia. Paraphyses are 

 numerous in the heads, exceeding the stalked conidia in length. 



The bark of cacao trees near the base of the stem has been 

 found penetrated by the rhizomorphs of the fungus and covered 

 by its conidial fructifications ; the attack was confined to cacao 

 in mixed cultivations with affected bananas on land subject 

 to periodic flooding. 



Treatment. 



If the diseased plants are present sporadically Ashby recom- 

 mends that the stools be dug out, cut up and charred, or cut up 

 and mixed with freshly slaked lime, and returned to the holes, 

 these being then filled in with soil and left for a few months 

 before replanting. The parasite is not present above the black- 

 ened region If diseased stools are numerous the cheapest 

 method is to cut off the unaffected parts, then chop the stump 

 down to the ground, cover this with freshly slaked lime, earth 

 up and leave for three months. It is desirable in view of possible 

 spread through the soil to isolate the stools with trenches. 



The disease appears to originate most commonly from 

 infested material carried by floods. Thorough cultivation is 

 said to enable the suckers to resist attack. 



Blackhead Disease. 



A disease of the rootstock, which produces brown or dark 

 water-soaked patches either near to the surface or working 

 inwards from it, was found to be due to the invasion of the tissues 

 by the fungus Thielaviopsis paradoxa, the cause of stem diseases 

 of sugar-cane, pineapple and coconut. 



Conidia or bits of mycelium brought into any slight cut or 

 into a slit in the bulb of a young sucker rapidly invade the 

 tissues with the production of rusty discoloration and formation 

 of spore chains in the cells. The tissues though water-soaked 

 remain firm and united. 



As in the case of cane cuttings the disease is most frequently 

 found on planted suckers which are growing with abnormal 

 slowness owing to dry weather, and they may grow away from 

 the attack when rains occur. The cut surfaces of the suckers 



