Downy mildew diseases are a major problem in conventional and organic vegetable production. In organic farming systems the use of plant resistance improvers (PRI) is one opportunity to alleviate this problem, but the efficiency of these products is not included in the registration of PRI in Germany. The objective of this study was to screen the activity of preselected PRI against Peronospora destructor, P. p a r a s i t i c a, Bremia lactucae and Pseudoperonospora cubensis under standardised conditions in the greenhouse. The efficiency of these PRI was further characterised in field trials to provide a basis for recommendations in vegetable production. The results of this study demonstrate that plant extracts, salts and microorganisms can reduce disease severity and the incidence of downy mildew in several vegetables under controlled conditions in the greenhouse. Nonetheless, the same products revealed different results in different pathosystems. In the field trials all products failed to control downy mildew epidemics except for one product containing phosphonate. This confirmed the results from the greenhouse test but its classification as a PRI is disputed. For this reason, the application of the tested PRI under field conditions to control downy mildew epidemics is inadvisable.