First report of Alternaria alternata on Verbesina encelioides in Saudi Arabia.

Abstract

In winter 2009, severe foliar infections were observed on Verbesina encelioides in the greenhouse of the King Saud University at Riyadh (Saudi Arabia). Symptoms appeared first as greyish brown patches around the tips and margins of young leaves then extended towards the midrib, causing curling and shedding of the leaves. Leaf pieces with lesions plated on potato dextrose agar (PDA) yielded fungal colonies with brown septate hyphae bearing straight 1-3 septate conidiophores, 50x3-6 μm in size. Conidia were obclavate to ellipsoidal with a short cylindrical pale-brown to light-brown beak with muriform septation and were usually solitary, occasionally in short chains. On potato carrot agar (PCA), mature conidia were 10-3×512 μm in size, showed 3-7 transverse septa, 1-5 longitudinal septa and were borne in chains with more than 5 elements. Based on these morphological characters, the fungus was identified as Alternaria alternata (Simmons, 1992), confirmed by the Indian Type Collection Centre of the Indian Agricultural Research Institute, New Delhi, and given I.D.No.7912.10. Healthy V. encelioides seedlings obtained from the Botanical Garden of the King Saud University were grown in steam-sterilized soil and used for pathogenicity tests. Detached leaves and whole plants were inoculated with a 5x105 ml-1 aqueous conidial suspension of the fungus (Pryor and Michailides, 2002). Detached leaves were maintained in a chamber at near 100% relative humidity for 3 days and inoculated plants were covered with a polythene bag and incubated at 28±2°C, in the glasshouse. Plants/leaves sprayed with sterile water served as controls. Disease symptoms developed within 3 days on inoculated leaves. Control plants remained unaffected. A. alternata was re-isolated from the lesions, thereby fulfilling Koch’s postulates. To our knowledge this is the first report of a pathogenic A. alternata affecting Verbesina encelioides.