First Report of Black Leg of Hydroponic Basil in the United States Caused by Plectosporium tabacinum.

During August 2007 and again in January 2008, compact sweet basil (Ocimum basilicum 'Genovese') plants grown hydroponically in Indiana displayed dark, irregular, stem lesions extending 2 to 3 cm above the interface of the nutrient solution. These necrotic stem lesions (black leg), observed on 20 to 30% of the basil plants caused very weak, brittle stems so that they could not be marketed fresh. Although no wilting was noted, reduced plant height was observed. Similar symptoms of blackleg and poor growth have been reported from Italy on greenhouse-grown basil infected with Microdochium tabacinum (1,2). Diseased plant samples were sent to diagnostic clinics at Purdue University and the University of Massachusetts. Stem samples were surface sterilized and plated on potato dextrose agar (PDA) acidified with 1 ml of 85% lactic acid per liter as well as onto one-quarter-strength PDA. A fungus morphologically consistent with Plectosporium tabacinum (van Beyma) M.E. Palm, W. Gams, & H.I. Nirenberg (synonyms M. tabacinum (von Arx, 1984) and Fusarium tabacinum (Gams & Gerlagh, 1968) (3) was cultured from the basil stems and identified as P. tabacinum by R. Wick. Cultures sent to J. McKemy and J. Bischof (USDA/APHIS/PPQ) and W. Elmer (Connecticut Agricultural Experiment Station) also were identified as P. tabacinum. Amplification of the 323-bp internal transcribed spacer (ITS) region (ITS1, 5.8S rRNA gene, ITS2) and subsequent BLAST alignments of the resulting sequence indicated a 98% match for Plectosphaerella cucumerina (anamorph P. tabacinum) (GenBank Accession No. U17399; MIDI Inc., Newark, DE). Inoculations were performed on basil plants grown in peat-based soilless medium in a greenhouse for 6 weeks. Immediately before inoculation, the roots were washed with tap water to remove the peat-based medium. A single basil plant was placed in each of eight, 125-ml Erlenmeyer flasks. Four flasks were filled with 100 ml of deionized water as negative controls and four were filled with a 1 × 10 CFU/ml water suspension of P. tabacinum so that the liquid reached the crown of the basil plant. Basil plants in the Erlenmeyer flasks were incubated on a laboratory bench at 23°C. After 24 h, the solutions in all flasks were discarded and each flask and root system was rinsed three times with deionized water. The plants were then incubated in deionized water on the laboratory bench for four to five additional days. Within 4 days, dark brown-to-black stem lesions similar to those observed originally on basil plants in the hydroponic production greenhouse developed on the plants at the water interface and extended up the stem. Lesions extended a mean of 22 mm above the water level on inoculated plants. Control plants remained symptomless. P. tabacinum was recovered from symptomatic tissue of inoculated plants to complete Koch's postulates. The experiment was repeated several times with similar results. Further evidence of pathogenicity was obtained by stem inoculation of basil plants growing in a soilless medium. These data indicate that P. tabacinum was the causal agent of the symptoms observed on the hydroponic basil. To our knowledge, this is the first report of P. tabacinum causing 'black leg' and reduced growth on basil in the United States and the first report in the world of P. tabacinum on hydroponic basil. References: (1) A. Garibaldi et al. Plant Dis. 81:124.1997. (2) A. Matta. Riv. Patol. Veg. Ser. IV 14:119, 1978. (3) M. Palm et al. Mycologia. 87:397.1995.