EN
Oak decline phenomenon has been observed in Poland in the 1980s. Especially, Krotoszyn Forest District was affected in the years 1982−1984, when oaks were dying in a mass extend causing economic and ecological problems. Twenty five years later many Phytophthora species were isolated from rhizosphere soil suggesting their important role in fine root damage. This research was focused on application of fertilisers increasing oak resistance to soil−borne pathogens. In total, 60 soil samples were collected c.a. 1 meter from trunks of oaks representing control and treated variant with Actifos® and Kalex® fertilisers. The fine root parameters (smaller than 2 mm) were evaluated with the water scanner and WinRhizo software. The same soil samples were analysed for the presence of macro− (N, P, K, Ca, Mg) and microelements (Al, Cu, Fe, Mn, Na, Zn). The use of ammonium and potassium phosphites in the form of foliar spray had a beneficial effect on the health of oak stands (Quercus robur L.) in the Krotoszyn Forest District. Three parameters: defoliation (ICP Forest), vitality according to Roloff method and synthetic index of damage were applied in order to evaluate the status of oaks health. Two aforementioned crown parameters were obtained from visual assessment from the ground, while the synthetic index of damage was calculated according to the formula developed by Dmyterko and Bruchwald. The parameters of fine roots after treatments, especially their number, length and surface improved. Trees with better quality fine roots had more chance to survive in unfavourable environmental conditions like drought. Phosphite treatments of oak stand positively influenced the chemical properties of soil, decreased acidity, but increased availability of Zn for plants, and probably reduced root damage by toxic aluminium ions. An improvement in the health of fine roots (that were saved by phosphites from damage caused by pathogenic species of Phytophthora genus present in the soil) will have a positive effect on the reconstruction of crown architecture (shoots development from dormant buds).