Regional and seasonal variation of Fusarium and Oomycetes species associated with apple seedlings decline in Tunisian nurseries

Apple decline, which is responsible for seedlings root and collar rot in the nurseries, is a serious disease that causes reduction in apple plant production. The aim of this study was to evaluate the regional and seasonal distribution of the fungi and Oomycetes, which were associated with the apple decline disease in the Tunisian nurseries. In this study, surveys were conducted from autumn, 2015 to summer, 2016. Apple plants were sampled to detect and quantify the inoculum density of these pathogens. Based on the morphological characteristics; two Fusarium and two Pythiaceae spp. were obtained. The most dominant species were F. oxysporum (33.9 %); Pythium ultimum (33.05 %), F. solani (16.95 %), and Phytopythium mercuriale (16.1 %). Results of the seasonal variation showed that Fusarium spp. and Pythiaceae populations had peaked in June. The populations of F. oxysporum and F. solani were significantly and positively correlated to temperature. In relation to the soil physicochemical characteristics; the Pearson correlation showed that the population of P. ultimum was positively related to the nitrogen (N) content (r = 0.59); sand (r= 0.82), organic matter (r = 0.85), and organic carbon (r = 0.84). However, this species was negatively correlated with the silt content (r = -0.79); clay (r = -0.84), and electrical conductivity (r = - 0.74). The Phytopythium mercuriale population was positively correlated with nitrogen content (r = 0.64), and negatively correlated with soil pH (r = -0.62); clay content (r = -0.47), and silt (r = -0.54).