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First Record of Verticillium dahliae on Potato in Malta

Potato is an important and highly valued crop throughout the Maltese Archipelago. Much of the production is exported to Holland. In January 2005, minor wilts and chlorosis of potato plants were observed in a field at Hal-Farrug, Luqa (Malta). Verticillium dahliae Kleb (1) was isolated on potato dextrose agar (PDA) from vascular tissue excised from the base of the plants. Three different isolates were obtained, all of which were typically distinguished by verticillately shaped conidiophores and the abundant production of microsclerotia on PDA. In May 2005, colonies of these three isolates were cultured in potato dextrose broth (PDB), from which conidial suspensions of each isolate were prepared with sterile distilled water to a concentration of 10⁷ ml^-1. For each isolate, 10 7-day-old potato seedlings were inoculated via root immersion in the inoculum suspension and transplanted to 20-cm diameter plastic pots containing a soil/peat mixture (1:1 [v/v]). Seedlings treated in the same way with sterile distilled water were used as a control. All plants were kept under controlled glasshouse conditions (20 ± 3°C) and watered to field capacity as required. Minor chlorosis and wilt of the pair of lower-most leaves was noted 7 days after inoculation. During subsequent weeks, wilt began to appear in the typical half-leaf form, while chlorosis was noted on all organs of the plants, including the principal stem (3). Symptoms were absent on the control plants. Measuring the weight of the new tubers produced by each plant revealed no apparent difference between inoculated and healthy plants; nevertheless, inoculated plants resulted in more tubers with a smaller diameter in respect to those of the uninoculated plants. V. dahliae was never isolated from tubers. Little to no variation in symptom severity was noted among plants inoculated with the three individual isolates. At the end of June, V. dahliae was reisolated on PDA from all inoculated plants, in particular, from vascular tissues originating from principal and lateral stems, crowns, and roots. All attempts to isolate the pathogen from control plants were unsuccessful. Molecular detection of the pathogen by using species-specific primers and real-time Scorpion PCR (2) confirmed the results obtained by the classical isolation method. The low symptom severity observed by the growers in the field, usually mistaken for normal dieback of aged plants, might explain why V. dahliae was never reported before on potatoes in the Maltese Archipelago.

References: (1) D. L. Hawksworth and P. W. Talboys. No. 256. Descriptions of Pathogenic Fungi and Bacteria. Commonwealth Mycological Institute (CMI), Kew, Surrey, UK, 1970. (2) F. Nigro et al. Pages 454--461 in: Proc. Convegno Internazionale di Olivicoltura. VI Giornate Scientifiche SOI, Spoleto, 2002. (3) W. R. Stevenson et al., eds. Compendium of Potato Diseases. 2nd ed. The American Phytopathological Society, St Paul, MN, 2001.