Seedborne Cladosporium variabile and Stemphylium botryosum in Spinach

Pablo Hernandez-Perez; Lindsey J du Toit

doi:10.1094/PD-90-0137

Assays of 77 spinach (Spinacia oleracea) seed lots produced in the United States, Denmark, the Netherlands, or New Zealand in 2000 to 2003 showed that Stemphylium botryosum, causal agent of Stemphylium leaf spot, was present in every lot, at a mean incidence of 29.1% per lot. Either Cladosporium variabile, causal agent of Cladosporium leaf spot, or the morphologically similar species C. macrocarpum, was present in 37 of the 77 lots, at a mean incidence of 1.8% per lot. Some seed isolates of S. botryosum and C. variabile proved pathogenic on spinach. Nonpathogenic isolates resembling C. variabile were identified as C. macrocarpum by the absence of torulose aerial hyphae. Pathogenic isolates of S. botryosum were also detected in each of 12 seed lots stored for up to 11 years at 4.4°C and 60% relative humidity. C. variabile or C. macrocarpum was detected in only 2 of the 11 lots, which had been stored for 3 and 8 years. Component seed assays demonstrated that S. botryosum and C. variabile (or C. macrocarpum) were internal and external in spinach seed. S. botryosum was detected in 5 to 76% of the embryos of five seed lots, but the two Cladosporium species were detected in only 0 to 1% of the embryos of these lots. This suggests greater potential difficulty at eradicating S. botryosum than C. variabile from infected spinach seed using seed treatments. Up to 50% of the spinach seed planted in the United States and up to 20% of the world supply of spinach seed is produced in the Pacific Northwest region of the United States. Washington and Oregon are the primary states for spinach seed production in the United States, where Cladosporium and Stemphylium leaf spots, caused by Cladosporium variabile and Stemphylium botryosum, respectively, can cause losses in seed crops during conducive conditions. In addition, these leaf spot diseases can have economic impact on fresh market and processing spinach crops because of additional expenses incurred for hand-sorting symptomatic leaves from healthy leaves. The possibility of infected seed lots serving as a source of inoculum for Cladosporium and Stemphylium leaf spots was investigated. Assays of 77 spinach seed lots produced in the United States, Denmark, the Netherlands, or New Zealand in 2000 to 2003 showed that S. botryosum was present in every lot at a mean incidence of 29.1% per lot. C. variabile, or the morphologically similar species C. macrocarpum, was present in 37 of the 77 lots at a mean incidence of 1.8% per lot. Some seed isolates of S. botryosum and C. variabile proved pathogenic on spinach. Nonpathogenic isolates resembling C. variabile were identified as C. macrocarpum by the absence of torulose aerial hyphae produced by the latter isolates. Further examination demonstrated that differentiation of C. variabile from C. macrocarpum during freeze-blotter seed assays was only possible ≥5 days after plating the seed, when isolates of C. variabile started forming torulose aerial hyphae from the tips of the conidiophores. Pathogenic isolates of S. botryosum were also detected in each of 12 seed lots stored for up to 11 years at 4.4°C and 60% relative humidity. C. variabile or C. macrocarpum was detected in only 2 of the 11 lots, which had been stored for 3 and 8 years. Component seed assays demonstrated that S. botryosum and C. variabile (or C. macrocarpum) can be internal and external in spinach seed. S. botryosum was detected in 5 to 76% of the embryos of five seed lots, but the two Cladosporium species were only detected in 0 to 1% of the embryos of these lots. This suggests greater potential difficulty at eradicating S. botryosum than C. variabile from infected spinach seed using seed treatments. To our knowledge, this is the first report of S. botryosum on spinach seed produced in Denmark, the Netherlands, and New Zealand. The prevalence of S. botryosum in commercial spinach seed lots might explain the recent first reports of this disease in Arizona, California, Delaware, Maryland, Oregon, and Washington. Development of a polymerase chain reaction molecular seed assay would greatly assist with differentiating S. botryosum and C. variabile from C. macrocarpum and other fungi on or in spinach seed.

Seedborne Cladosporium variabile and Stemphylium botryosum in Spinach

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