Assessment of the spread of chestnut ink disease using remote sensing and geostatistical methods

Chestnut ink disease, caused by Phytophthora cinnamomi and P. cambivora, is responsible for important economic losses and limits the establishment of new chestnut (Castanea sativa) groves in Portugal. Although the differences in soil properties and in cropping practices affect ink severity, the regional spread of disease is not known. Data for monitoring C. sativa decline were obtained by using field surveys and Small Format Aerial Photography (SFAP), a reliable tool which provides large-scale imagery obtained at low altitude. Visible colour and near-infrared images were obtained with different cameras with an average ground resolution of 22, 14 and 39 cm. The spatial distribution of ink disease in northern Portugal for the years 1995-2004 was estimated through a geostatistical method, and the estimation of precision was determined. From 1995-2002, the chestnut population in the study area increased by 18.5% due to new orchard plantations. After 2002 the population decreased because the new plantations were not sufficient to recover the number of dead chestnut trees, killed mostly by ink disease. The directional semivariograms indicated anisotropy with a greater disease spread in the NE-SW direction. This direction corresponds to site areas at the same altitude, where soil tillage and human mobility are higher.