26 DISEASES OF CROP-PLANTS 



(d) Rots of' non-woody vegetative parts, as injured or 

 weakened stems of sugar-cane and the basal part of failing 

 coconut leaves. 



{e) Rots of stored vegetables, such as sweet potato and 

 dasheen. 



The mycelium of the common species of Diplodia is freely 

 septate, light to dark brown when mature, and imparts a slaty 

 or smoky colour to infested wood. The spores (pycnospores) 

 develop in small flask-shaped bodies (pycnidia) formed singly or 

 in groups, usually thickly set, below the surface of bark or rind 

 and later breaking through. The spores are oval, hyaline and 

 non-septate when young, dark and one-septate when mature. 

 They may be extruded and germinate in either condition, and 

 mistakes in identification are liable to arise from this habit. 



The best form of control of the dieback diseases is cultural, 

 i.e., the maintenance of the trees in high vigour, and, with cacao 

 and limes, avoidance of undue exposure. Careful pruning of 

 failing branches, and prompt treatment of wounds have great 

 value in preserving oldish trees. In relation to fruit diseases the 

 fact mentioned above, that these fungi are only common on 

 dying or recently dead material, gives some force to the usual 

 recommendation regarding the destruction of diseased wood and 

 of such intestable material as cacao pods. 



The Rhizoctonias. 



The Rhizoctonias are soil-inhabiting fungi, distinguished by 

 the possession of mycelium and sclerotia of characteristic tj^pes. 

 One or more species are very abundant in temperate countries, 

 and several are reported to be prevalent in India. In the tropics 

 they have been very little noticed, but from occasional observa- 

 tions made it seems certain that at least one species is widely 

 distributed in the West Indies. 



Rhizoctonia Crocorum (Pers.) DC. (violacea Tul.) is a fungus 

 occurring on numerous hosts but particularly on alfalfa. The 

 disease occurs in the fields in patches, in which the plants appear 

 yellow or chlorotic and eventually wilt rather suddenly. The 

 mycelium completely invests the root system with a weft of 

 hyphse which at first is pale buff to violaceous, later red-violet 

 and finally violet-brown. The sclerotia are flattened or rounded 

 bodies varying in diameter from a few millimetres to several 

 centimetres, deep violet-brown when mature and thickly clothed 

 with a persistent velvety felt. 



No spore-producing stage of this fungus is known. It has 

 rarely been grown apart from the host and appears to be strictly 

 parasitic. It attacks developed plants and is not associated with 

 damping-off. It has not been recorded from the West Indies. 



Rhizoctonia Solani Kuhn, is now known to be the mycelial 

 stage of Corticium vagum, B. et C. {Hypochmis Solani P. et D.). 

 It is a fungus of very general distribution in arable soils and 



