Sweet basil (Ocimum basilicum L., Fam. Lamiaceae) is an important annual culinary herb crop used for both fresh and dry
consumption and as a source of essential oil and oleoresin for manufacturing perfumes, food flavors and aromatherapy products and as ornamental and cut- flower plant. Symptoms of downy mildew (Figs. 1A & 1B and 2C) were recorded firstly during July, 2015 on basil plants (Ocimum basilicum L., Balady or local and Grand vertcvs) growing in some fields at
Naser (Bani Adi), Beba (Safet-Rashin) and Somusta (Bedahel) counties, Beni- Suief Governorate, as yellowing of basil leaves of more than 60% of the growing plants. Small to large chlorotic lesions were found on more than 40% of plant leaves with necrotization from the middle, variable in diameter and the necrotic irregular spots are limited by the main veins. A grayish white furry growth was observed on the abaxial leaf surface. The fungus, however, was identified as Peronospora belbahrii Thines on the basis of morphological and microscopical characteristics. Sporangiophores (Fig. 2D) were hayline with a long of 396 to 806μm (averaged 583μm), each had a straight trunk and monopodially branched 4 to 9 times (averaged 5.6). Sporangiophore ended with two curved acute branchlets. Olive to brown, rounded or ovoid sporangia were found to measure, on the average, 34.6 X 27.0μm. According to the available literatures, this is the first time to report this fungus and its disease in Egypt.