Apple scab caused by the fungus Venturia inaequalis is economically the most destructive disease in apple production worldwide. Apple scab control requires multiple applications of fungicides during spring and summer. A potential biological control agent – H39, based on a particular strain of the fungus Cladosporium – has been tested in several European field trials during the last two years. H39 appeared to very successfully reduce apple scab in leaves and fruits. The same control levels could be reached as with common fungicide spray application schedules. A biological control company is currently evaluating the possible commercial use of the H39 antagonist. An important step which might lead to reduction of the dependency on chemical fungicides in the near future.
Apple scab can cause – if not controlled – extensive yield losses (70 percent or higher) in regions with humid, cool weather during spring and summer months. Losses are the direct result of a high number of infected and damaged fruits or indirectly from repeated defoliation which can reduce tree growth and yield.
Control of apple scab in commercial orchards currently depends on multiple applications of fungicides: preventive and curative. Resistance of the Venturia fungus to an increasing number of chemical fungicides in major apple growing areas has arisen over recent years. Furthermore, government regulations, e.g. in the European Union, restrict more and more the use of fungicides. The increasing demand for fruit without or with low pesticide residues is another reason why alternatives for chemical fungicides are needed.
The antagonist Cladosporium cladosporioides H39 was found to significantly reduce sporulation of Venturia inaequalis, the fungus causing apple scab. The potential of this antagonistic isolate has during two years been tested by several institutes in orchards with different apple cultivars in Hungary, Poland, Germany and the Netherlands. Treatments with H39 were conducted as regular calendar sprays or after infection periods. In the Netherlands (Randwijk) the trials focused on the effect of timing of application of the antagonist H39 before or after infection periods.
The overall results of the field trials consistently showed – for the first time – that stand-alone applications of H39 reduce apple scab in leaves and fruits. This has been demonstrated in an organic growing system as well as in conventional orchards by spray schedules applied during spring or summer. In both systems the same control levels could be reached as with common fungicide schedules. Efficacies of control reached 42 to 98% on leaf scab incidence and 41 to 94% on fruit scab.
The next step is to develop H39 into a commercial biological control product. A company is currently evaluating the possible commercial use of the H39 antagonist.