Early senescence of rice and Drechslera oryzae in the Wageningen Polder, Surinam

Rice is grown in monoculture in the Wageningen Polder, Surinam, South America, which comprises 10 000 ha of heavy clay soil. Fungal diseases caused appreciable losses of the rice crop in several years. Drechslera oryzae van Breda de Haan was found to be the major pathogen of rice. Its most conspicuous symptoms are leaf spots. Infection by D. oryzae was most severe during the reproductive phase of the rice plant. During that phase, the crop also showed a number of other symptoms such as discoloration of leaves, premature ripening with sterility, root decay, and matted roots. These symptoms together are designated as 'early senescence'. Brown leaf spotting caused by D. oryzae is just one of the group of symptoms constituting the syndrome. Severe infection by D. oryzae indicates poor rice-growing conditions.<br/>D. oryzae has the character of a perthophyte. The susceptibility of rice to infection by D. oryzae increases with the age of the plant. The fungus sporulates abundantly only on senescent leaves. Occurrence of epidemics is related to imbalance in development between the vegetative and the generative stages of the rice plant. This imbalance is induced by the incorporation of organic matter in the soil with tillage or the application of large amounts of nitrogen to the crop during the vegetative stage only.<br/>Early senescence of rice in the Wageningen Polder can be prevented by good tillage, balanced application of nitrogen, breeding and selection of cultivars specially adapted to marginal growing conditions.<br/>Control of D. oryzae with fungicides only takes away one of the causes of crop losses. Application of fungicides has not been economic.<p/>