First report of stemphylium leaf blight of onion caused by Stemphylium vesicarium in Egypt

In 2004, onion plants (Allium cepa cv. ‘Giza 6’) in several commercial fields in upper Egypt (Assiut), exhibited symptoms of blight on the leaves and seed-stalk. Initial symptoms on leaves consisted of tip necrosis followed by small white and/or large purple spots (Fig. 1). A fungus was consistently isolated from diseased tissue and identified as Stemphylium vesicarium (Wall.) Simmons, based on morphological characteristics (Ellis, 1971). Conidiophores were pale to medium brown with dark bands, smooth or minutely verruculose; conidia oblong to ovoid, densely verrucose with 1-5 transverse and several longitudinal septa, 13-21 x 25-40 µm. Ascomata forming in culture contained hyaline, bitunicate, clavate asci with 8 ascospores that were light to medium brown, ellipsoidal, verrucose with 5-7 transverse and several longitudinal septa, usually in incomplete series, 9-17 x 17-46 µm (Fig. 2). Simmons (1969) identified the telemorph of S. vesicarium as Pleospora allii (Rabenh).

To confirm the pathogenicity of 15 isolates, inocula were prepared by growing isolates on PDA medium at 27°C for 15 days. Then ten ml of sterile distilled water was added to each plate and colonies were carefully scraped with a sterile needle. The resulting conidial suspension from each isolate (diluted to 5 x104 CFU per ml) was used to infect 12 onion plants (110-day-old ‘Giza 6’), using an atomizer to spray leaves and seed stalks. After inoculation, plants were covered with polyethylene bags for 48 hours to maintain a high humidity. After this period, the bags were removed and plants were kept under field conditions until symptoms appeared. Symptoms were similar to those observed in onion plants in commercial fields. The fungus was reisolated from lesions of inoculated plants, but not from tissues of any of the control plants. This experiment was repeated with the same results.

The pathogen is widespread in Asia and Europe and has been recorded previously on onion plants in South Africa (Verwoperd & Du Plessis, 1931). It can cause severe damage especially to the onion seed crop and losses of 80-85% on onion by affecting leaves and seed stalk (Tomaz & Lima, 1988). This is the first report for Egypt.