In 2004, a severe infection by powdery mildew in peach trees occurred in different villages in Al Jabal Al Akhdar. The disease was recorded in previous seasons but in a very low incidence. This study was therefore conducted to investigate progress of the disease with time, distribution in five different villages and causal agents. The results from the first surveys in 2007 showed that first powdery mildew signs were reported in April after one month from leaf bud opening. The disease severity reached its peak in the period between May and June, followed by decline after that until they disappeared after August in all villages. Symptoms of powdery mildew started to appear on peach fruits in April, after fruit setting Throughout the growing season, small white colonies on the fruits enlarged gradually until a large portion of the fruits was covered. Generally, fruits produced during this season have very low marketable value because most are deformed with necrotic spots that lead to surfaces cracking. Low severity of powdery mildew was found in 2008 as compared to 2007. The average twig severity from April to August was between 0.5% and 1% in all villages except in Al-Aien. The fruits produced during the growing season 2008 were in good condition with high marketable value and by the end of June all fruits were harvested. Molecular phylogenetic analysis of the nucleotide sequences of the rDNA ITS region (Internal Transcribed Spacers) of 22 isolates obtained from peach fruits and twigs indicated that all of isolates belong to one species Sphaerotheca pannosa. The isolates of S. pannosa from five different villages in Al Jabal Al Akhdar showed that all isolates have an identical sequence of ITS rDNA. Comparisons of ITS sequences of powdery mildew isolates from Oman with sequences of the ITS region deposited in GenBank (NCBI, National Center for Biotechnology Information ) showed 100% similarity to published sequences of Sphaerotheca pannosa (e.g. AF298543, AB022348, DQ139429) in many places around the world including Australia, Japan and Belgium. A representative ITS rDNA sequence of one S. pannosa isolate from this study was deposited in GenBank (NCBI, National Center for Biotechnology Information) under the accession number HM189673. The pathogenicity of the fungus was proved by using detached leaf assay and representative measurements of conidia confirmed that the sporulating fungus was Sphaerotheca pannosa.