Shiso (Perilla frutescens) is a leafy herb in the Lamiaceae family and is widely used in Japanese and other Asian cuisine for cooking, pickling, oil (from the seeds), and garnish. A number of shiso types are used, though the most common are green shiso (ao-shiso) and red shiso (aka-shiso). In the winter months of 2010 and early spring 2011, a foliar blight disease developed on greenhouse-grown green shiso produced in Ventura County, CA. Initial symptoms were angular, dull green leaf lesions on older foliage. Such lesions often were initiated along leaf edges. As the disease progressed, lesions turned gray green, expanded, and could affect most of the leaf surface. Lesion tissue became dry and papery in texture; signs of a pathogen were not present. Tests for bacterial agents were negative. However, a fungus was consistently isolated from symptomatic leaves. Isolates of this fungus were grown on potato dextrose agar (PDA) in petri plates incubated under fluorescent lights and were identified as Botrytis cinerea (1). On PDA, mycelial growth was gray brown and conidiophores measured 2 mm or longer and were branched at the terminals. Conidia were aseptate, hyaline, ellipsoidal, and measured (6.5–) 8.4 to 9.2 (–12.0) × (6.1–) 6.8 to 8.0 (–9.5) μm. Sclerotia were not present. Pathogenicity of four isolates was tested by spraying conidial suspensions (1 × 105 conidia/ml) until runoff onto sets of potted green and red shiso plants. Each set consisted of six wounded (leaf tips cut) and six unwounded plants. Plants were enclosed in plastic bags for 48 h and then maintained at 22 to 24°C in a greenhouse. After 4 days, leaf lesions developed on both wounded and unwounded leaves of green and red shiso. The resulting symptoms were similar to those observed in commercial production and B. cinerea was recovered from symptomatic tissue. Non-inoculated, wounded, and unwounded red and green shiso plants were sprayed with distilled water and did not develop symptoms. This experiment was conducted two times and results were the same. To our knowledge, this is the first report of gray mold of shiso in the United States caused by B. cinerea. The disease caused significant damage to the shiso crop because symptomatic leaves are unacceptable for market. In 2010, the greenhouse facility that contained the diseased shiso had numerous leaks in the roof; winter rains that occurred during this time therefore resulted in higher free moisture and humidity in the growing area, which likely provided optimum environmental conditions for the pathogen to infect and cause disease on shiso.
Reference: (1) M. B. Ellis and J. M. Waller. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 431, 1974.
