Rice diseases - problems and progress

Abstract

Several rice diseases, caused by fungal, bacterial and viral pathogens, as well as nematodes, occur in Sri Lanka. Those induced by fungi include blast (Pyricularia oryzae), sheath blight (Thanatephorus cucumeris), brown spot (Cochliobolus miyabeanus (syns. Drechslera oryz.de, Helminthosporium oryzae)), stem rot (Leptosphaeria salvinii (syn. Helminthosporium sigmoideum)), foot rot or Bakanae disease (Gibberella moniliforme) and sheath rot (Acrocylindrium | oryzae (syn. Sarocladium oryzae)). The most serious disease encountered, blast, caused severe crop losses in the traditional varieties such as Pachchaiperumal 2462/11, formerly widely cultivated, j and overcoming this disease was recognised early as an important research objective.Bacterial leaf ! blight caused by Xanthomonas oryzae (syn. Xanthomonas campestris pv. oryzae) has been the only bacterial disease recorded. Severe epidemics of this disease have occurred from time to time.The presence of some viruses infecting rice crops has been established. They include rice tungro I bacilliform virus (RTBV), rice tungro spherical virus (RTSV), rice grassy stunt virus (RGSV) and ! rice ragged stunt virus (RRSV). Their impacts have yet to be accurately assessed.Two rice I nematodes have been recorded, Aphelenchoides besseyi, which causes the white tip disease, and the j root nematode, Hirschmanniella oryzae (syn. Radopholus oryzae).The principal physiological disease that occurs in this country is bronzing.Yellowing conditions have been observed from time to time. However, the factors causing them have yet to be elucidated.Twenty-seven genera of fungi have been identified as being seed-borne, the most important of which are D. oryzae and Trichoconispadwickii (syn. Alternariapadwickii). P. oryzae has never been detected in a viable form although non-viable spores have been found in a few seed samples. None of the seed-borne fungi cause serious effects in the new improved varieties cultivated when crops are raised from wellfilled mature seed harvested aiid processed under favourable conditions. Weed species able to serve as alternate hosts for some rice pathogens occur in the vicinity of rice fields. They include Panicum repens and Digitaria marginalis for P. oryzae, Cynodon dactylon, Digitaria sanguinalis for C. miyabeanus and Echinochloa colonum for H. sigmoideum and C. dactylon, Cyperus rotundas and E. colonum for RGS V and RTBV. The impacts of diseases on rice crops include reduced yields, poor plant stands, loss of functional leaf tissue and weakened stems which predispose plants to lodging. Strategies to combat rice diseases adopted include the prevention of their occurrence by ensuring that the causal agents are not disseminated, the cultivation of disease resistant varieties, the adoption of cultural practices which minimise disease incidence and the application of agrochemicals. Great emphasis has been placed on the development of disease resistant varieties. Methods have been devised and systems developed for screening against the most important diseases - blast, bacterial leaf blight and sheath blight. Resistant donors have been identified, among them Dissi Hatif, DNJ-129, Ta-poo-cho-z, Engkatek, Remadja, C-46-15, Carreon, Murungakayan 302, Tadukan, Tetep and Tres Marias for blast, BJ1, DZ 192, Malagkit Sungsong, RL Gophar and Zenith for bacterial leaf blight, and Nahn Praya 132, Ta-poo-cho-z, Pankaj, Bahagia and Remadja for sheath blight. Many thousands of hybrids emerging from breeding programmes have been screened. The performance of the most promising selections has been observed in multi-location field trials and those released as improved varieties all have adequate resistance to the major rice diseases under normal cultivation conditions. As a second line of defense against diseases, numerous agro-chemicals have been screened to select suitable fungicides and bactericides to be used if required. Those selected for controlling blast include Kasumin (kasugamycin), Hinosan (edifenphos) and Benlate (benomyl). Highly effective mercurial fungicides identified were never recommended in view of health hazards. The application of chemicals to control other diseases has not resulted in significant yield increases, genet ally. Accordingly, the adoption of chemical control measures has not been recommended except in the case of blast in certain situations. Crop losses attributable to diseases are minimal at the present time. This has been achieved largely by the incorporation of resistance against the important diseases into the improved varieties now widely cultivated. This situation needs to be maintained by continued intensive research and vigilance, especially against the introduction of virulent strains of pathogens into this country from elsewhere by adopting stringent quarantine safeguards. In the recent past, seeds have been imported into the country for crop improvement from the People's Republic of China. The detection of Tilletia barclayana and Ustilaginoidea virens in consignments of hybrid seeds is noteworthy.