Stemphylium vesicarium (Wallr.) E.G. Simmons · formerly or also: Pleospora allii (sexual stage), Helminthosporium vesicarium
Stemphylium leaf blight causes tan to brown lesions, often at the tips of onion and garlic leaves, that turn dark olive to black as the fungus sporulates. Severe attacks kill large parts of the foliage, giving smaller bulbs and weaker storage. It has re-emerged as a leading onion leaf disease in several regions, partly because populations have become resistant to common fungicide groups. The same fungus causes brown spot of pear and purple spot of asparagus.
Stemphylium leaf blight of onion and garlic is caused by Stemphylium vesicarium (Wallr.) E.G. Simmons, a fungus (leaf-spotting mould) (order Pleosporales, family Pleosporaceae).
Older or alternative scientific names: Pleospora allii (sexual stage), Helminthosporium vesicarium.
Worldwide in temperate and subtropical regions. It has become much more important on onion since the 2010s in north-eastern North America and is a long-standing problem of onion seed crops in South Asia and of pear orchards in southern Europe.
| Region | Status |
|---|---|
| North America | Re-emerged as the dominant onion leaf disease in the north-east, linked to fungicide resistance |
| South Asia | Important on onion bulb and seed crops |
| Europe and the Mediterranean | Brown spot of pear in southern Europe; leaf blight on onion and asparagus purple spot |
| Oceania and South America | Present on onion, garlic and asparagus |
A fungus that produces olive-brown, oblong spores with cross and lengthwise walls, borne singly on short, swollen-tipped stalks; masses of spores make old lesions look sooty olive-brown to black. On overwintered debris it forms small, dark fruiting bodies (pseudothecia) of the sexual stage, Pleospora, which release airborne ascospores in spring.
| Crop stage | What you see |
|---|---|
| Early infection | Small, yellowish to pale water-soaked spots, often on the side of leaves facing the prevailing wind or on leaf tips. |
| Developing lesions | Spots enlarge into tan to brown, oval to elongated lesions; the centres turn dark olive-brown to black with spores. Lesions often start on leaves already damaged by thrips, hail, sand or other diseases. |
| Severe infection | Lesions merge into dieback of whole leaf tips and leaves; fields look prematurely brown. On seed crops, flower stalks are girdled and fall over. |
The fungus survives on infested onion and asparagus debris, on weeds and other hosts, and probably on seed, volunteers and transplants. In spring, pseudothecia on debris release ascospores, and conidia from debris and weeds start infections in the crop. Conidia produced on lesions are spread within the field by wind, rain splash and possibly thrips. Infection needs long periods of leaf wetness in warm weather. The fungus readily attacks leaf tissue that is ageing or damaged.
| Stage | Duration | Notes |
|---|---|---|
| Survival on debris, weeds and other hosts | Between seasons | Pseudothecia mature on debris over winter. |
| Primary infection by ascospores and conidia | Spring to early summer | Needs long wet periods. |
| Lesion development and sporulation | About one week or more | Wind, rain splash and thrips spread conidia. |
| Repeated cycles | Through the season | Epidemics peak in warm, wet weather as leaves age. |
Infection is favoured by temperatures of about 15-27 °C with extended leaf wetness; longer wet periods are needed at the cooler end of this range. Forecasting systems based on hours of leaf wetness and temperature have been developed for asparagus and pear and adapted for onion.
Several to many conidial cycles in one season; one ascospore cycle in spring.
Long dew periods, frequent rain or overhead irrigation, warm temperatures, ageing foliage late in the season, injury from thrips, hail, wind-blown sand or herbicides, nearby onion debris, and fungicide programmes that rely on groups to which local populations are resistant.
No numeric economic threshold has been published for onion. Spray decisions follow scouting for first lesions and weather-based disease forecasts; local programmes are revised each year according to fungicide-resistance testing.
Thresholds are local. They depend on crop price, control cost, variety and natural-enemy activity, so use the figure your local extension service publishes where one exists.
Loss of green leaf area reduces bulb size and quality. Healthy leaves are also needed late in the season for bulbs to mature properly and store well, so early defoliation can shorten storage life. In onion seed crops, infection of flower stalks reduces seed yield. On pear the same fungus spots and rots fruit.
Control works best as a sequence: prevent, monitor, then act only when a threshold or a risk forecast says so, using the least disruptive option that works.
Foliar fungicides slow the epidemic but resistance is widespread; programmes must be chosen from local resistance data. What is registered differs by country and changes over time; follow the label and your local plant-protection authority.
| Scheme | Group | Mode of action | Notes |
|---|---|---|---|
| FRAC | 3 | Demethylation inhibitors (DMI) | Reduced sensitivity reported in some populations. |
| FRAC | 7 | Succinate dehydrogenase inhibitors (SDHI) | Resistance to some members found in onion populations. |
| FRAC | 11 | Quinone outside inhibitors (QoI) | Widespread resistance in some onion regions. |
| FRAC | 9 | Anilinopyrimidines (AP) | Resistance found in some onion populations. |
| FRAC | 2 | Dicarboximides | Used in pear and onion; resistance risk. |
Onion, garlic, leek and shallot are the main field hosts. The fungus also causes brown spot on European pear and purple spot on asparagus, and has been recorded on sunflower, parsley, radish, tomato, soybean and mango. Weeds such as nutsedge, pigweed and purslane can carry it near fields.
How to grow the crop well is the first line of defence. Laisha's free grow guides cover each crop from sowing to harvest:
Written by Laisha from the sources above, in its own words. This page is general reference, not a prescription: confirm identification locally and follow the label and your plant-protection authority.
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