Early symptoms include small, water-soaked, white to light yellow spots on leaves. Typically, these lesions are found in higher numbers on the side of leaves facing the prevailing wind. Over time, these small lesions grow along the leaf blade and coalesce into sunken, oval-shaped or elongated, brown blotches, with tan to brown centers. Concentric zones may also develop in their center. In advanced stages, large necrotic areas form, which may girdle the leaf or seed stem, causing extensive blighting of the tissues.
Stemphylium leaf blight is caused by the fungus Pleospora allii, formerly known as Stemphylium vesicarium, thereby the name of the disease. It survives on infected plant debris and resumes growth during favorable weather conditions in spring. It then produces spores that are spread to nearby plants by the wind. It normally invades dead and dying onion tissue, such as leaf tips, lesions caused by previous diseases, or simply injured tissue (from e.g. insects or hail). Long periods of warm wet conditions encourage disease development. Healthy leaves can thus also be attacked if the weather is warm (18 - 25 °C) and the leaf surface is wet for more than 24 hours. Infection usually remains restricted to leaves and does not affect the bulb. Older leaves are more susceptible than young ones.
Aqueous extracts of Azadirachta indica (Neem) and Datura stramonium ( jimsonweed) can be used for the biocontrol of Stemphylium leaf blight disease with an efficacy close to that of conventional fungicides. Under greenhouse conditions, the preventive or curative application of products based on Trichoderma harzianum and Stachybotrys chartarum results in the reduction of disease incidence and severity (around 70% in both cases).
Always consider an integrated approach of both preventive measures and biological treatments if available. Solution of the active ingredients azoxystrobin + difenoconazole, boscalid + pyraclostrobin, chlorothalonil, iprodione, mancozeb and prochloraz are highly effective at reducing the growth in S. vesicarium. Treatments should be done when conditions are unfavorable for the fungus (cool and dry weather). In general, the effectiveness of the fungicides was enhanced with an alternation of products.