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First Report of a Canker Disease of Walnut Caused by Botryodiplodia theobromae in Egypt

Botryodiplodia spp. are known to produce cankers and dieback of several woody hosts. Botryodiplodia diseases were observed in 7-year-old orchards in Rhafah, north of Sinai, Egypt, in July 2005 and 2006. Symptoms appeared as dieback and cankers with dead leaves that were covered mostly with grayish white fungal growth; black pycnidia appeared on the surface of the infected branches. Plant foliage was discolored and partially or completely dry. When the outer bark was removed, the affected tissue appeared dark brown, in contrast to the yellowish green of healthy inner bark. On the basis of morphological characteristics (3), these fungi were identified as Botryodiplodia theobromae Pat. by the Plant Pathology Department, National Research Centre. Sporulating lesions were black and had a rough surface caused by the erumpent, confluent arrangement of pycnidia formed in infected tissue. The pycnidia were smallest in naturally infected twigs in nutritionally rich medium such as oatmeal agar (190 to 887 × 155 to 705 μm). Conidia were initially hyaline and unicellular, subovoid to ellipsoidal with a granular content. Mature conidia were two-celled, cinnamon to light brown, and often with longitudinal striations. Conidia measured 20 to 30 × 12 to 15 μm. Pathogenicity of isolates from symptomatic branches was determined by branch inoculations on rooted cuttings made from 7-year-old walnut trees growing in plastic pots. One isolate was inoculated on wounded and unwounded twigs using 100 μl of a suspension of 5 × 10⁵ conidia per ml. Control branches were sprayed with water. All inoculated and control plants were kept in a greenhouse and watered as needed. There were three replicate plants for each isolate and inoculation technique that was used. After 3 weeks, cankers and grayish necrotic lesions developed on all inoculated plants. Samples of tissue from 10 infected walnut branches were plated on water agar. B. theobromae was recovered from all sampled plants. Control plants did not display any symptoms. B. theobromae has been reported on species of walnut from the Hermosillo Coast of Mexico (1) and India (2). To our knowledge, this is the first report of walnut dieback and canker caused by this pathogen in Egypt.

References: (1) J. A. Arredondo. Rev. Mex. Fitopatol. 12:138, 1994. (2) B. Kusum. Indian J. Mycol. Plant Pathol. 21:295, 1991. (3). S. Masilamani and J. Muthumary. Mycol. Res. 100:1383, 1996.