Stemphylium botryosum Pathogenic on Spinach Seed Crops in Washington

Area of Science:

• Plant Pathology
• Mycology
• Agricultural Science

Background:

• Spinach (Spinacea oleracea L.) seed crops in western Washington exhibited leaf spot symptoms in September 2000.
• Initial observations suggested Cladosporium variabile as the causal agent of the observed leaf spots.

Purpose of the Study:

• To identify the fungal pathogens responsible for leaf spot symptoms on spinach seed crops.
• To differentiate the pathogenicity of recovered fungal isolates on spinach cultivars.
• To report the first occurrence of Stemphylium botryosum as a foliar pathogen on spinach seed crops in Washington.

Main Methods:

• Fungal isolates, including Cladosporium variabile and Stemphylium, were recovered from symptomatic spinach leaf tissue.
• Inoculation experiments were conducted on spinach cultivars Winter Bloomsdale and Ozarka II using spore and mycelial suspensions.
• Plants were maintained in a greenhouse, and disease development was monitored for symptom identification and pathogen reisolation.

Main Results:

• Both C. variabile and two isolates of Stemphylium caused typical leaf spot symptoms, characterized by dry, bleached lesions.
• Stemphylium-induced lesions enlarged and coalesced more rapidly than those caused by C. variabile, exhibiting irregular margins.
• Colony morphology and conidia of pathogenic Stemphylium isolates were consistent with Stemphylium botryosum.

Conclusions:

• Stemphylium botryosum is identified as a foliar pathogen of spinach seed crops in Washington, distinct from C. variabile.
• The similarity in symptoms between S. botryosum and C. variabile may have previously hindered diagnosis in Washington.
• This finding contributes to understanding spinach diseases and emphasizes the importance of accurate pathogen identification in seed production.