Yield of irrigated rice affected by asymptomatic disease in a long-term intensive monocropping experiment

Sustained high yields of rice (Oryza sativa L.) are required in intensively cropped, irrigated systems to ensure food security and avoid expansion of cultivation into natural ecosystems. We examined whether good management practices and adapted cultivars could maintain high yields during 15 years (2001-2015) of continuous rice cultivation with three crops per year in the dry season (DS), early wet season (EWS), and late wet season (LWS). Results are for the highest-yielding fertilizer N rate of 195 kg N ha(-1) in DS and 90 kg N ha(-1) in EWS and LWS at the long-term continuous cropping experiment (LTCCE) in the Philippines. Our study addressed two critical questions remaining after previously reported research in the LTCCE: Can DS yield be sustained at >80 % of the climate- and genotype-specific yield potential, and can yield be increased in wet seasons? Management in DS, when cropping was synchronized with surrounding fields, controlled diseases and maintained yield at >80 % of simulated yield potential during 15 years. In EWS and LWS, when cropping was not synchronized with surrounding fields, tungro viruses were transmitted by insects from surrounding fields into the experiment. Management and adapted cultivars within the experiment did not prevent yield loss associated with reduced spikelet filling, which was attributed largely to tungro disease. In EWS, when planting was earlier than surrounding fields, yields were about 60-80 % of yield potential. In LWS, when planting was later than surrounding fields, yields declined to <65 % of yield potential. Enzyme-linked immunosorbent assay confirmed high infection in EWS and LWS with rice tungro spherical virus, which was not accompanied by symptoms of plant yellowing and stunted growth. In DS, when tungro was negligible, good management with adapted cultivars consistently minimized yield loss from nutrient deficiencies, soil problems, pathogens, and pests. However, good management with adapted cultivars could not maintain a small yield gap when tungro viruses were transmitted from infected surrounding fields into the experiment. In such case, closing the yield gap requires management interventions to reduce the pathogen and vector in surrounding fields. Negligible visible symptoms on most tungroinfected plants highlight the ease with which a biotic stress can be overlooked in experiments and growers' fields.