The castor- oil plant (Ricinus communis) is an annual herb or spreading shrub to about 6 m tall that belongs to the Euphorbiaceae. It is perennial in moist areas and annual in seasonally frosty or dry areas. It has been cultivated in Asia and Africa from ancient times. This plant is now widely naturalized in moist tropical and subtropical zones of the world (Francis 2004).

During a survey of plant pathogens conducted on R. communis in August 2010, typical symptoms of a powdery mildew disease were observed on nearly 10% of the plants in the surveyed area in the Mahammadiah region (32° 52′ N/59° 13′ E), south Khorasan province, eastern Iran. Symptoms included extensive abaxial cover of older leaves by white patches of external superficial mycelium and conidiophores, along with chlorotic and necrotic regions on the upper leaf surface. Colonies were also present on the upper leaf surface (Fig. 1). These symptoms were mainly observed on castor- oil plants subjected to water deficiency stress, 2 weeks prior to harvest. However such symptoms were not observed in an ornamental castor- oil variety, planted close to the diseased plants. A sample was collected, dried and deposited in the herbarium at the Plant Pests and Diseases Research Institute (IRIPP) with voucher number IRAN 15030 F.

Fig. 1
figure 1

Powdery mildew signs on upper leaf surface of Ricinus communis infected with Leveillula taurica

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Fungal structures were detached from the diseased leaves of with a razor blade, mounted in water or lactic acid and examined with a light microscope.

Conidiophores emerged through leaf stomata, singly or branched, usually in groups of two to three, and formed dimorphic conidia (Fig. 2). Examination of 50 of each type of conidia yielded the following description of their morphology: primary conidia lanceolate with distinct apical points, 12.5–19 × 37.5–70 μm; secondary conidia cylindrical, 12.5–20 × 37.5–77.5 μm; both conidia hyaline with angular/reticulated wrinkling of the outer walls (Figs. 3 and 4). These morphological features are typical of the anamorphic stage description of Leveillula taurica (Lév.) Arnaud (Braun 1987).

Fig. 2
figure 2

Conidiophore (C) of Leveillula taurica emerging through a stoma (S). Bar = 50 μm

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Fig. 3
figure 3

Leveillula taurica on Ricinus communis. Primary (top) and secondary (bottom) conidia, Bar = 20 μm

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Fig. 4
figure 4

Leveillula taurica conidiophores: a) with a primary conidium and b) with a secondary and cylindrical conidium. Bar = 20 μm

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Leveillula taurica has been reported on R. communis worldwide [Ramakrishnan and Narasimhalu (1941), Chiddarwar (1954), Amano (Hirata) (1986), Farr and Rossman (2010)] but this is the first report of this powdery mildew species on Ricinus communis in Iran (Ershad 2009; Khodaparast and Abbasi 2009). Other recent records of L. taurica from Iran were published by Mirzaee et al. (2007) and Mirzaee and Mohammadi (2010).