RICE RATOONING - IRRI books - International Rice Research Institute

234 RICE RATOONINGDISEASESRice is prone to several diseases caused by bacteria, fungi, nematodes, viruses, andmycoplasma-like organisms (MLOs). Although many diseases are widespread, anda few can cause heavy damage to the main crop, only some are noticed on the ratooncrop. Damage also is limited by climatic conditions prevailing during the ratooncrop period. However, diseases caused by viruses and MLOs are a severe problem inratoon crop production.Diseases caused by bacteriaBacterial leaf blight (caused by Xanthomonas campestris pv. oryzae ) and bacterialleaf streak (X. campestris pv. oryzicola) are widespread and cause considerabledamage in main crops (13), but are not of much importance on ratoon crops. Theydo not appear in a severe form because of climatic conditions prevailing during theratoon crop season.Diseases caused by fungiAlthough blast (caused by Pyricularia oryzae ) is a widespread and destructivedisease of rice in almost every area, its damage is negligible Dec-May on bothtransplanted main crops and ratoon crops because of the unfavorable climaticconditions during the season. In fact, local experience is that a main crop of S-1092heavily infected with leaf blast can put out regrowth and give a normal yield withoutbeing reinfected at the leaf or neck stages. This means blast may not be a problem onratoon crops in the Karnataka hill valley, but it could be a problem in other areas.Studies for forecasting and for developing preventive measures are needed.Brown leaf spot caused by Cochliobolus miyabeanus (Drechslera oryzae)disease is an important fungal disease on the transplanted main crop and ratooncrop. It causes leaf spot symptoms during the vegetative stage (seedling to flag leaf)and “ear blighting” symptoms similar to neck blast, with brown discoloration of theuppermost internode, which leads to partial or complete chaffiness of the panicle.The fungus also attacks glumes, producing dark-brown or black oval spots. Thesurface of the grain may be blackened and the seeds shrivelled and discolored. InIndia, a crop grown with well-balanced nutrients usually will not be severelyattacked by brown leaf spot (5). Fungi surviving on the stubble and debris left aftercutting are the main source of infection. Secondary spread is through airborneconidia.Spraying the cut stubble (field) with zineb, mancozeb, carbendazim, or anyother suitable fungicide immediately after the main crop harvest may reduce theprimary source of inoculum. Two more sprayings of the ratoon crop, once 20-25 dafter cutting and again at the flag leaf stage, may reduce leaf spot incidence and earblighting and glume discoloration. It is worth experimenting on these lines todevelop a suitable plant protection schedule applicable to local situations.Stem rot ( Sclerotium oryzae ), although not serious on the main crop, maydamage the ratoon crop because of the weakening of the host plant at the time ofcutting. Samson (21) observed that longer stubbles expressed more stem rot