Apple scab Venturia inaequalis (Cooke) is one the most serious disease of apples. It occurs in all regions where apples are cultivated. Protection against this disease demands 8 to 12 sprayings. Elaboration of warning bases of scab occurrence by Mills and La Plante was a great achievement in plant pathology development. Criteria of infection periods by V. inaegualis had been and also still are under continuous verification and complementation. The results of studies conducted by Butt and Xu [3], MacHardy and Gadoury [16], Roosje [27, 28], Schwabe et al. [29, 30, 31], Trapman [37] and Triloff [38] concerning new directions in apple scab prediction seem to be of special importance.
Antagonistic activity of 63 bacterial isolates, originating from soil and apple leaves, against Venturia inaequalis was studied in dual culture on PDA medium. Based on measurement of inhibition zones, 16 isolates appeared to be most active. Three of them /57M, 59M (Pseudomonas spp.) and 103M (Collimonas sp.), significantly protected apple trees against apple scab on Gala trees in the net-glasshouse. Of 6 ethanol plant extracts (sage leaf, rhizomes of Potentilla, oak and willow bark and the aboveground parts of St. John's-wort and tetterwort), the highest activity in inhibition of V. inaequalis conidia germination, even after 10- and 20-fold dilution in water, showed extract from rhizomes of Potentilla. Applied at two concentrations (5 and 10%) on Gala apple trees it totally protected leaves against apple scab.