Biotic and abiotic causes of yield failure in tropical aerobic rice

Aerobic rice is a new production system for water-short environments. Adapted varieties are usually direct dry seeded and the crop grown under aerobic soil conditions with supplementary irrigation as necessary. Occasionally, yield failures occur which may be related to soil health problems. In the dry season of 2006 and 2007, we conducted a field experiment in the Philippines, to identify the major causes of such yield failure. Four treatments were implemented: (i) Control of direct dry-seeded rice (improved upland variety Apo), (ii) Biocide application, (iii) transplanting into aerobic soil, and (iv) 1 year fallow. Yield in the Biocide treatment was 2 t ha(-1) in both years. In all other treatments, yield was 0-0.3 t ha(-1). Plants grown in the Biocide treatment showed a reduced degree of galling of roots caused by root-knot nematodes (RKN) and better general root health than the Control treatment. Potentially pathogenic fungi were isolated from root samples (Pythium sp.. Fusarium sp., and a Rhizoctonia-like species). Abiotic effects of the Biocide treatment were all increase in KCl-extractable N (initial season 2007) and a decrease in initial soil pH. In the Control treatment, soil pH increased froth 6.5 to 8.0 over the two seasons. In 2007, plant tissue analysis indicated Mn deficiency ill the Control treatment. Plants that were subjected to foliar micronutrient sprays reacted positively to Fe and Mn sprays ill the Control treatment, and Mn spray in the Biocide treatment. We concluded that the Biocide application led to favorable soil conditions by reducing biotic stresses such as RKN and improving nutrient availability. In the other treatments, all interaction of RKN and micronutrient deficiencies with increasing soil pH led to yield failure