Effect of soil moisture and temperature on the survival of phytophthora cinnamomi rands in soil

Soil moisture and temperature affect hyphal lysis and sporangial formation in Phytophthora cinnamomi. Hyphal lysis was most rapid in soils at, or below, field capacity when incubated at 25–27° C. Sporangia were not formed in soils below field capacity and only very slowly in waterlogged soil. Sporangia were not formed at temperatures below 15°C

Effect of soil leachates on the release and motility of zoospores of Phytophthora cinnamomi rands

No abstract availabl

Effect of organic matter on the survival of Phytophthora cinnamomi rands in soil

Soil organic matter collected from beneath an unburnt stand of Eucalyptus marginata was added in increasing amounts to lateritic soil. Phytophthora cinnamomi incubated in soils containing 50% or more organic matter was extensively lysed, and many of the sporangia produced were abortive. With increasing organic matter there is an increase in nutrient concentrations and in the microbial population and it is suggested that these factors are the basis of the antagonism

Phosphorus nutrition of rice in relation to flooding and temporary loss of soil-water saturation in two lowland soils of Cambodia

In the rainfed lowlands, temporary loss of soil-water saturation during crop growth is a common factor limiting rice (Oryza sativa L.) yield but its effects on phosphorus (P) availability are poorly understood. Rice plants were transplanted into pots containing soils that were either continuously flooded, maintained at field capacity or flooded and then dried to field capacity for 3 weeks during the vegetative stage. A black clay soil (Kandic Plinthaquult) and a sandy soil (Plinthustalf) from south-east Cambodia were compared with or without amendments by rice straw and P fertilizer. Under continuously flooded conditions, the growth of rice was vigorous without straw addition and there was a strong response of rice growth to the addition of P fertilizer. The soil underwent reduction, which increased pH from 4.2 to 5.5 or 6.0, in the black clay or sandy soil, respectively. By contrast, a loss of soil-water saturation 3 weeks before panicle initiation (PI) markedly impaired the growth of rice. This was not through any effect of water stress, and the growth reductions were not as strong as with continued loss of soil-water saturation from transplanting to PI. Fluctuations in soil pH and Eh corresponded closely to changes in soil-water regimes. Growth reductions were attributed to reduced shoot P levels resulting from the decline in P availability during the loss of soil-water saturation. The addition of rice straw stimulated soil reduction and lessened changes in soil pH and Eh during the loss of soil-water saturation in both soils. Straw addition enhanced P uptake by the rice plants during loss of soil-water saturation, but its beneficial effects could not be attributed to the direct addition of P, N or K to the soils. Thus the application of rice straw may be effective in lessening the effects of temporary loss of soil-water saturation on rice growth in lowland rice soils by minimising the decline in P availability

The response of upland rice to phosphorus on drained soils subjected to different periods of prior flooding

Low rice yields following intermittent loss of soilwater saturation are believed to involve, on occasions, P deficiency but the possible mechanisms have not been studied in detail. In the present pot experiments, two acid lowland soils from Cambodia (Koktrap -black clay soil and Prateah Lang- sandy soil) were treated with P either before or after flooding to investigate the effect of the timing of P application on its effectiveness for upland rice growth. Phosphate fertiliser (45 mg P/kg soil) was added to both soils before flooding for periods of 0, 1, 2 and 4 weeks, or after drying the flooded soils. After air-drying and crushing, the soils were wet to field capacity and the upland rice grown in them for six weeks. The addition of P 4 weeks before flooding decreased shoot dry matter in the sandy soil and in the black clay soil after only 1 week of flooding. But when P was added after drying the soils, shoot dry matter was not decreased regardless of the period of prior soil flooding. Soil pH increased and redox potential (Eh) decreased during flooding, resulting in an increase in acetate extractable Fe and the phosphate sorption capacity of soils. There was a close relationship between P sorbed and acetate extractable Fe (r2=0.96–0.98). Olsen and Bray-1 extractable P strongly correlated with shoot dry matter and shoot P concentrations indicating that P, the availability of which was controlled by the period of prior flooding, limited the growth of upland rice. It was concluded that phosphate fertiliser added before flooding was relatively ineffective in increasing growth in the upland rice. This was attributed to the increase in occlusion of P within ferric oxyhydroxides formed during subsequent oxidation of the soils. Decrease P availability may also have been associated with a greater phosphate sorption capacity of the soils during flooding and drying of soils. The implications of this for P supply to rice in intermittently flooded lowlands, and for P fertilizer requirements of pre- and post-rice upland crops are discussed

Phosphorus nutrition of rice and temporary loss of soil-water saturation in lowland soils of Cambodia

In the rainfed lowlands, temporary loss of soil-water saturation during crop growth is a common factor limiting rice (Oryza sativa L.) yield but its effects on phosphorus (P) availability are poorly understood. Rice plants were transplanted into pots containing soils that were either continuously flooded, maintained at field capacity or flooded and then dried to field capacity for 3 weeks during the vegetative stage. A black clay soil (Kandic Plinthaquult) from south-east Cambodia were studied with or without amendment by rice straw and P fertilizer. Under continuously flooded conditions, the growth of rice was vigorous without straw addition and there was a strong response of rice growth to the addition of P fertilizer. The soil underwent reduction, which increased pH from 4.2 to 5.5 in the black clay. By contrast, a loss of soil-water saturation 3 weeks before panicle initiation (PI) markedly impaired the growth of rice. This was not through any effect of water stress, and the growth reductions were not as strong as with continued loss of soil-water saturation from transplanting to PI. Fluctuations in soil pH and Eh corresponded closely to changes in soil-water regimes. Growth reductions were attributed to decreased shoot P levels resulting from the decline in P availability during the loss of soil-water saturation. The addition of rice straw stimulated soil reduction and lessened changes in soil pH and Eh during the loss of soil-water saturation in both soils. Straw addition enhanced P uptake by the rice plants during loss of soil-water saturation, but its beneficial effects could not be attributed to the direct addition of P, N or K to the soils. Thus the application of rice straw may be effective in lessening the effects of temporary loss of soil-water saturation on rice growth in lowland rice soils by minimising the decline in P availability