Stemphylium vesicarium

Stemphylium vesicarium is most reliably identified through its microscopic spores rather than any visible fruiting body typical of larger fungi.

The organism produces dark, muriform spores—divided internally by both horizontal and vertical septa—that give them a distinctive checkered appearance under magnification.

Observation data for Stemphylium vesicarium shows records occurring across every month of the year, with counts rising from a low in January toward a pronounced peak in March and staying relatively high through the middle months of the year before tapering off toward December. This pattern suggests the fungus is detected more often in spring and summer, though these figures reflect presence observations rather than abundance or a complete picture of its range.

As a pathogen of onion, garlic, asparagus, and pear, its occurrence is tied to the cultivation of these crops, appearing wherever these host plants are grown.

1. Stemphylium vesicarium is not limited to one type of crop: it infects bulb vegetables such as onion and garlic, a stalk vegetable in asparagus, and a tree fruit in pear.
2. Its authority citation, (Wallroth) E.G. Simmons, reflects a taxonomic history in which the species was reclassified after its original description.