Sweet basil (Ocimum basilicum) is an economically important herb in several Mediterranean countries. Approximately 30 ha are grown annually in France for fresh and processed consumption. During the spring and fall of 2004, a damaging foliar disease was observed in some crops near Saint Tropez in the French Riviera Region. More than 50% of plants were affected in an organically produced field-grown crop at an altitude of 250 m. Leaves of infected plants were initially slightly chlorotic, especially near the central vein. Within 2 to 3 days, a characteristic gray, furry growth was evident on the lower leaf surface and sometimes on the upper leaf surface. The appearance and severity of the disease was affected by overhead sprinkler irrigation. Basal leaves were severely affected. Microscopic observations revealed sporangiophores branching two to seven times. Sporangiophores, with a length of 250 to 500 μm (average 350 μm), ended with sterigmata bearing single sporangia. Sporangia measured 15 to 25 × 20 to 35 μm (average 22 × 28 μm), were elliptical and grayish in mass. The pathogen was identified as Peronospora sp. on the basis of its morphological characteristics (4). Pathogenicity was confirmed by inoculating leaves of 40-day-old healthy plants with a sporangial suspension (1 × 105 conidia/ml). Three containers with 150 plants each of O. basilicum cv Genovese gigante were used as replicates. Noninoculated plants served as controls. Plants were maintained in a growth chamber at 20°C (12 h of light per day) and 90 to 95% relative humidity. The pathogenicity test was carried out twice. After 6 days, typical symptoms of downy mildew developed on the inoculated plants, and Peronospora sp. was observed on the leaves. Noninoculated plants did not show symptoms. To our knowledge, this is the first report of Peronospora sp. on basil in France. Peronospora sp. was previously reported on sweet basil in Italy (1) and P. lamii on sweet basil in Uganda (3). Seed transmission (2) is suspected as the reason for recent outbreaks in Europe.
References: (1) A. Garibaldi et al. Plant Dis. 88:312, 2004. (2) A. Garibaldi et al. Z. Pflanzenkr. Pflanzenschutz 111:465, 2004 (3) C. G. Hansford. Rev. Appl. Mycol. 12:421, 1933. (4) D. M. Spencer. The Downy Mildews. Academic Press, NY, 1981.
