Sweet basil (Ocimum basilicum L.) is the most important annual culinary herb in the United States. It is grown commercially for the fresh market as a food flavor and as a source of essential oils and oleoresin for manufacturing perfumes and aromatherapy products. In November 2010, grayish to dark, necrotic lesions were observed on the stem of sweet basil plants of the cv. Genovese grown in 10-cm diameter pots containing “Fafard” soilless medium in a greenhouse in Homestead, FL. Symptoms started 3 weeks after planting at the soil line and extended 2 to 5 cm up the stem. Nearly 99% of the plants were symptomatic after another 1 to 2 weeks. The lesions coalesced and reached as high as 12 to 15 cm up the stem. Severely affected plants were stunted (as short as 10 cm compared to 25 cm for non-infected plants), became weak, and lodged as infection progressed. Stem sections from five diseased plants were surface sterilized with 0.6% sodium hypochlorite for 15 min, plated on potato dextrose agar (PDA), and incubated in the dark at 25°C. Pale to pink, slimy, appressed colonies with whitish, sparse aerial mycelia grew on PDA. Hyaline, smooth, ellipsoidal conidia, each tapering gradually to a broadly rounded apex and base, with one to two guttulation droplets, were observed microscopically. Conidial dimensions averaged 9.4 (4.2 to 13.2) × 2.5 (2.2 to 3.2) μm. The isolated fungus was morphologically similar to Plectosporium tabacinum (van Beyma) M.E. Palm, W. Gams et Nirenberg (synonyms M. tabacinum (van Beyma) von Arx, 1984 and F. tabacinum (Gams & Gerlagh, 1968)) (3). Fungal DNA was extracted and an 834-bp fragment of partial 28S rDNA was amplified with primers LROR and LR5 (4), and the amplified fragment was sequenced (GenBank Accession No. JQ999955). Sequence comparison using BLAST revealed 99% identity with that of Plectosphaerella cucumerina (GenBank Accession No. U17399). Pathogenicity tests were carried out three times by inoculating three 2-week-old, healthy basil seedlings in each test with a conidial suspension of the isolate sprayed onto the plants at 1 × 106 conidia/ml. Non-treated control plants were sprayed similarly with deionized water. Lesions identical to the above descriptions appeared on stems of all inoculated plants 2 to 4 days after inoculation (plants were held at 25°C), and the same pathogen was reisolated on PDA. Additionally, leaves of inoculated plants showed extensive chlorosis and necrosis. All noninoculated plants remained symptomless. Black leg, caused by P. tabacinum, was reported on hydroponically grown basil in Indiana (1). It was also reported to cause necrosis on the stem of basil plants in Italy (2). In Homestead, FL, seven cultivars (Cinnamon, Genovese, Broad leaf Italian, Red Rubin, Lemon, Thai, and Common basil) of basil grown in plastic pots have shown black leg symptoms in the greenhouse. To our knowledge, this is the first report of P. cucumerina (anamorph P. tabacinum) causing black leg on greenhouse grown basil in Florida. Approximately 50% of the 12-week-old plants in a field trial at Homestead, FL, showed identical symptoms in spring 2012. The high incidence of this disease in Homestead and the potentially aggressive nature of the pathogen on foliage suggest a need for development of effective management strategies.
References: (1) D. Egel et al. Plant Dis. 94:484. 2010. (2) A. Garibaldi et al. Plant Dis. 81:124. 1997. (3) M. Palm et al. Mycologia. 87:397. 1995. (4) Rehner and Samuels. Can. J. Bot. 73:816. 1995.