Oaks, as the most valuable tree stands of Moldavian forest fund, are in danger, caused by acacia expanding process, presence of oak at the South-Eastern limit of their natural spreading area, climate change, pests and pathogens (insects, fungi), direct action of humans etc. Microsphaera alphitoides is one of the major fungal agent responsible for degradation of oak ecosystems in Moldova. The aim of researches was the assessment of oak powdery mildew (Microsphaera alphitoides) damage in oak stands (Quercus robur, Q. petraea) in correlation to climate parameters and stage in fungus development, with identification of ways to fight against it. Following investigations, in the development of this fungus, two major stages have been identified: 1). the primary infection of the leaves (cleistothecia opening) which was largely determined by spring rains, daily average air temperature of at least 16 °C and a relative air humidity of over 65%, also the presence of the source of the infection (leaves affected from last year, affected young branches etc.); and 2.) the secondary, summer infections of the oak leaves, that were conditioned by the warm, dry and windy weather, that had detached and spread conidia. Thus, the degree of leaves infection by oak powdery mildew, from the investigated experimental areas, has varied throughout the oak vegetation period, with traces of infection at the end of May and early June; and medium strong infection - end of September, early October. Hence, the most effective treatments against this fungus are those applied before it starts sporulation – the I decade of June.