In December 2011, symptoms typical of Cladosporium leaf spot caused by Cladosporium variabile (4) were observed in organic “baby leaf” spinach (Spinacia oleracea) crops of the cultivars Amazon, Missouri, Tasman, and Tonga in the Imperial Valley (Imperial County, CA and Yuma County, AZ). Leaves had small, circular lesions (1 to 3 mm in diameter), some of which had progressed to necrotic, bleached lesions surrounded by a thin dark margin. The incidence of symptoms in affected crops was ≤20%. Fungal isolates resembling C. variabile were recovered by surface-sterilizing sections (5 mm2) of symptomatic leaf tissue in 0.6% NaOCl, triple-rinsing the sections in sterile water, and plating the sections onto water agar and potato dextrose agar amended with 100 ppm chloramphenicol (cPDA). Single-spore transfers made onto cPDA were maintained at 24 ± 2°C with a natural day/night cycle. Each isolate produced slow growing cultures consisting of dense masses of dark conidiophores (≤350 μm long) with chains of up to three dematiaceous (olive) conidia, and almost no mycelium. Torulose (coiled) aerial hyphae developed from the apices of conidiophores after 5 to 7 days, and distinguished the isolates as C. variabile, not C. macrocarpum (2,4). Pathogenicity was tested for each of six single-spore isolates using 36-day-old plants of the spinach cultivar Carmel. The plants were enclosed in clear plastic bags overnight and inoculated the next day with the isolates of C. variabile by atomizing approximately 30 ml of a spore suspension (1.0 × 106 conidia/ml in sterile water amended with 0.01% Tween 20) of the appropriate isolate onto the upper and lower leaf surfaces of each of five plants/isolate. Five control plants were inoculated similarly with sterile water + 0.01% Tween 20. The plants were resealed in plastic bags for 72 h and then placed on a greenhouse bench. Pinpoint, sunken lesions developed within 4 to 7 days on the leaves of plants inoculated with each of the six test isolates. Lesions developed into dry, circular spots typical of Cladosporium leaf spot. Symptoms were not observed on control plants. After 20 days, C. variabile was reisolated from lesions caused by all six isolates, but not from control plants. Although Cladosporium leaf spot has been reported in the Salinas Valley of California (4), to our knowledge, this is the first report of the disease on spinach crops in the Imperial Valley of California and Arizona, the primary winter, fresh market spinach production region of the United States. Inoculum of C. variabile may have been introduced to this region on spinach seed lots (3), because even seed infestation levels <0.1% could lead to seed transmission (1) under the dense planting populations (≤9 million seeds/ha) and overhead irrigation typical of “baby leaf” spinach crops in this region. Fungicides can be used to manage Cladosporium leaf spot in conventional spinach crops (1), but management in certified organic crops may be more challenging.
References: (1) L. J. du Toit et al. Fung. Nemat. Tests 59:V115, 2004. (2) M. B. Ellis. Page 315 in: Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Surrey, England, 1971. (3) P. Hernandez-Perez. Page 79 in: Management of Seedborne Stemphylium botryosum and Cladosporium variabile Causing Leaf Spot of Spinach Seed Crops in Western Washington, MS thesis, Pullman, WA, 2005. (4) P. Hernandez-Perez and L. J. du Toit. Plant Dis. 90:137, 2006.