An integrated disease control strategy for apple scab

The inhibitory effects of natural actives from plants and microorganisms against Venturia inaequalis were investigated. Firstly, the sensitivity of conidia of V. inaequalis was tested against different concentrations of Galenia africana and Tulbaghia violacea extracts.  No conidia of V. inaequalis germinated on water agar plates after a 24 h incubation period, in the presence of purified Galenia africana (0.03, 0.06 and 0.125%) and crude Tulbaghia violacea (5, 10, 15, and 25%) extracts.

Secondly, the protective and curative action of G. africana and T. violacea extracts were investigated under greenhouse conditions. Extracts of G. africana (0.25, 0.5, and 1%) and T. violacea (25 %) showed significant protective action against apple scab. However, the G. africana extract, when applied 24 and 48 h after inoculation, showed a poor curative performance.

Thirdly, the efficacy of microorganisms (Bacillus subtilis and two Trichoderma strains; Eco-T and 77B-T) were evaluated on their ability to prevent the release of ascospores from naturally infected apple scab leaf disks and whole leaves on the orchard floor. Monthly sprays of Trichoderma treatments (Eco-T and 77B-T) reduce the number of ascospores released on leaf disks. In the 2004/2005 season, the two weekly leaf treatments, showed significant higher ascospore counts compare to the once off and monthly treatment. Eco-T (0.1%) was effective in preventing the release of ascopsores in all  spray treatments.

Lastly, orchard trials were conducted for two seasons, 2002/2003 and 2003/2004, at the Bellevue Experimental Farm, Grabouw. The G. africana extract showed good potential for apple scab control on leaves and fruit (at concentrations of 0.5% and 0.25%) compared to Dithane.