Résumé
Neem is one of the most valuable multi-purpose tree. This tree is very important culturally, medicinally and pesticidally and has gained worldwide recognition for its pharmaceutical and pesticidal properties. Our knowledge about the diseases of Neem is quite insufficient and fragmentary. Only leaf spots caused by Cercospora subsessilis are recorded from Bangladesh. The present work was undertaken to record the occurrence of powdery mildew on Neem leaves that has not been recorded earlier from Bangladesh. The symptoms of the disease were observed thoroughly under field conditions and the infected samples were brought to the laboratory for the detection of the causal organism from the diseased leaves. The causal organism of powdery mildew disease of Neem was observed in the laboratory and was identified as Oidium azadirachtae sp. nov. Initial infection in the leaf surface was found as a tiny powder mass and with the progress of the disease the whole area of the leaf was covered with a powder mass. The infection was also found on the lower surface of the mature leaves. The leaf becomes susceptible to infection when it is half the normal size and remains so till senescence. The disease was more prevalent in mature or old leaves than the young leaves. Because of leaf surface coverage with the powdery mass the photosynthetic activities are reduced and severe fungal attack results in defoliation. The spread of the diseases is favoured by high humidity, low temperature and mist. The specimens with typical powdery mildew symptoms were preserved in the laboratory