First Report of Stemphylium botryosum on Spinach in Texas

Abstract

During the 2009 to 2010 growing season, symptoms of an unknown leaf spot were observed on spinach (Spinacia oleracea L.) in production fields in southwest Texas. Approximately 500 ha were affected, especially cvs. Rakaia and Viceroy. Disease incidence was 30 and 2% for Rakaia and Viceroy, respectively. Diseased plants exhibited small (1 to 3 mm in diameter), tan, necrotic lesions with a circular to oval shape and were void of any signs of a pathogen. Symptomatic leaves were surface sterilized in 1.5% NaOCl for 1 min, rinsed with sterile water, and air dried. Leaf sections (~1 cm2) were cut and placed on acidified potato dextrose agar (APDA), or APDA supplemented with streptomycin (SAPDA). Fungal mycelia growing from the edges of infected leaf sections were transferred to PDA and incubated at 25°C with a 12-h/12-h light/dark cycle. After 14 days of incubation, dark brown mycelia giving rise to unbranched conidiophores bearing brown, deeply septate, ovoid conidia were observed. Conidia measured 16.8 to 27.3 × 13.1 to 19.6 μm. On the basis of these morphological characteristics, the fungus was identified as Stemphylium botryosum (3). Cultures were transferred to clarified V8 juice agar to obtain inoculum for pathogenicity tests. Eight-week-old plants (n = 20) of spinach cvs. Hybrid 310, Wintergreen, Ashley, and Rakaia were sprayed until runoff with a suspension containing 0.001% Tween 80 and 1 × 104 conidia/ml. Noninoculated plants served as a control treatment. Plants were placed in a growth chamber and incubated in the dark at 25°C and 95% relative humidity. Following 36 h of incubation, plants were transferred to a plastic enclosure and maintained at 23 ± 4°C. After 7 to 10 days, tan, oval-shaped lesions were observed on all inoculated spinach plants. All control plants, with the exception of Rakaia, failed to develop symptoms. Isolates of S. botryosum were recovered on SAPDA from symptomatic leaves, confirming Koch's postulates. Previous reports have shown that S. botryosum can be transmitted from infected seed (1), thus, additional plants of each cultivar (n = 36) were grown in the greenhouse to determine the potential for seedborne contamination. After 8 weeks, leaf spot symptoms identical to those observed on the original plants developed on 75% of the Rakaia plants, while symptom development on the other cultivars was negligible. Isolates of S. botryosum were only recovered from symptomatic Rakaia leaves. Similar field observations were made during the 2001 to 2002 growing season; however, attempts to isolate S. botryosum in that season were unsuccessful. Recent outbreaks of Stemphylium leaf spot have been reported in Arizona (4), California (3), Delaware and Maryland (2), and Washington (1). To our knowledge, this is the first report of S. botryosum on spinach in Texas. While the origin of inoculum causing the disease in Texas is unknown, S. botryosum may have been seedborne (2). The implementation within the past few years of very high density plantings of spinach (1.9 to 3.7 million seeds/ha) may lead to an increase in incidence and severity of this disease in Texas. References: (1) L. J. du Toit and M. L. Derie. Plant Dis. 85:920, 2001. (2) K. L. Everts and D. K. Armentrout. Plant Dis. 85:1209, 2001. (3) S. T. Koike et al. Plant Dis. 85:126, 2001. (4) S. T. Koike et al. Plant Dis. 89:1359, 2005.

Multiplex real-time PCR assays for detection of four seedborne spinach pathogens

“…, ; du Toit and Derie , ; Hernandez‐Perez and du Toit ; Reed et al . ). Infested/infected spinach seed could readily be the cause of spread of these two leaf spot pathogens in the USA.…”

“…A relatively high incidence of seed of three seed lots (103SN, 201SA and 201MN) tested in this study was found to be infected or infested with C. variabile and S. botryosum using the real-time PCR assays for these pathogens. Cladosporium and Stemphylium leaf spots of spinach have been reported in numerous states in the USA over the past 10 years, including Arizona, California, Oregon, Maryland, Delaware, Texas, and Washington (Everts and Armentrout 2001;Koike et al 2001Koike et al , 2005Derie 2001, 2012;Hernandez-Perez and du Toit 2006;Reed et al 2010). Infested/infected spinach seed could readily be the cause of spread of these two leaf spot pathogens in the USA.…”

“…, ; du Toit and Derie , ; Reed et al . ). Both of these pathogens are known to be seedborne (Hernandez‐Perez and du Toit ).…”

“…The latter is potentially problematic because some strains of V. dahliae have a broad host range, and spinach is often rotated with high-value vegetables and fruits, including lettuce and strawberry, which can be devastated by V. dahliae (Atallah et al 2010;Iglesias-Garcia et al 2013). Additionally, Cladosporium leaf spot, caused by Cladosporium variabile, and Stemphylium leaf spot, caused by Stemphylium botryosum, have recently emerged as problems for spinach production as well as spinach seed production (Fuentes-Davila and Gabrielson 1996;Everts and Armentrout 2001;Koike et al 2001Koike et al , 2005Derie 2001, 2012;Reed et al 2010). Both of these pathogens are known to be seedborne (Hernandez-Perez and du Toit 2006).…”

Irrigation Scheduling Impacts Vegetative Growth, Seed Yield, and Fungal Diseases of Spinach Seed Crops in a Maritime Mediterranean Climate

“…However, it is also a known disease of spinach seed crops (du Toit and Derie 2001;du Toit et al , 2006. Previous research suggested that Stemphylium botryosum was the primary causal agent of SLS in spinach (du Toit and Derie 2001; Koike et al 2001;Reed et al 2010), but recent analyses by Liu et al (2020) and Spawton et al (2019) found that what was believed to be S. botryosum was more closely related to S. beticola based on recent changes in the taxonomy of the genus (Woudenberg et al 2017).…”