Downy mildew (<em>Bremia lactucae</em>) in lettuce – the potential of plant strengtheners

Abstract

Downy mildew (Bremia lactucae) regularly causes high reductions or even losses in lettuce (Lactuca sp.) yield in different regions of Germany. Besides denominated races B. lactucae occurs in many unknown, regionally different and often unstable pathotyps. Hence, using resistant cultivars does not always exclude infestation by regional pathotyps. Apart from hygiene measures applications of fungicides are necessary to prevent the development of new virulent races. To reduce application frequency of fungicides or to prevent infestation in organic production plant strengtheners or plant resistance inducers can be used. In the last years various products appeared on the market. However, the efficacy evaluation of these products is not included in the registration procedure in Germany. The objective of this study was to test some preselected products concerning their effect on a reduction or at least a delay of infestation. These findings should serve as a basis for recommendations in vegetable production.

The results of this study demonstrate that some products revealed a clear reduction in disease severity of downy mildew under protected conditions in the greenhouse. Under field conditions the effect was minor or negligible in most cases. Even fungicides failed to control downy mildew sufficiently when the infection had already started. Only plant strengtheners containing phosphonate or foliar fertilizers containing copper oxychloride caused reduced disease severity similar to fungicides when used on a weekly basis. In conclusion, the application of the tested plant strengtheners containing plant extracts, salts or microorganisms under field conditions is not advisable. Finally, the grower himself has to decide if a product is appropriate for practical use depending on the way of cultivation and marketing of the produce.