Soil physical properties and the occurrence of chestnut ink disease : a micromorphological study

Abstract

Ink disease caused by the fungus Phytophthora cinnamomi Rands is a major threat to the sustainability of chestnut ecosystems in northern Portugal. Not only soil factors and host resistance but also management practices can influence the occurrence of this disease. Total soil porosity was measured in chestnut groves affected by the ink disease and in healthy ones, at 2 or 3 depth zones. The orchards were located either in mid-slope position or at the foot of the slope. The purpose of the work was to investigate whether or not the occurrence of ink disease would be related with the presence of a plough pan and with the topographical position of the stand. The areas covered by pores with Ø>3O µm (po30) and by pores with Ø>5OO µm (po500) were directly quantified on soil thin sections using micromorphological and image analysis techniques. This method was chosen because routine methods fail to detect soil porosity differences in the soils with a clear plough pan. The results show that low absolute total porosity does not directly influence the health status of the orchards. However, the presence of a layer with reduced porosity at 20-35 cm clearly affects the occurrence of ink disease. This plough pan strongly hinders root development and soil aeration and thus makes the trees more sensitive to the attack of Phytophthora cinnamomi. The plough pans observed were more pronounced in the foot-slope than in the mid-slope soils. Former studies have shown that lowland groves are also more intensively affected by ink and that there is a close relationship between the number of tillage operations and the incidence of the disease. This study evidences the relationship between the occurrence of Phytophthora root disease and the presence of a plough pan, which in turn depends on the location of the stand on the slope. From a methodological point of view it stresses the importance of micromorphological and image analysis techniques to directly determine total soil porosity and to provide complementary and more accurate data than the indirect methods commonly used for quantifying soil porosity.