Stemphylium is a genus of fungi in the phylum Ascomycota, best known for causing leaf spot diseases on crops like spinach, onion, garlic, and lentils. It also produces airborne spores that can trigger allergic reactions and asthma symptoms in sensitized individuals. With dozens of species spread across every continent, Stemphylium shows up in farm fields, gardens, and occasionally indoor environments.
Basic Biology and Appearance
Stemphylium species are molds that reproduce by releasing microscopic spores called conidia. These spores are oval to oblong in shape, light brown to deep brown in color, and remarkably small: typically 15 to 34 micrometers long and 10 to 19 micrometers wide. Under a microscope, each spore has a distinctive pattern of internal cross-walls running both horizontally and vertically, which helps distinguish Stemphylium from visually similar mold genera like Alternaria and Ulocladium.
The fungus grows on dead or living plant tissue, forming dark, fuzzy colonies. It thrives in warm, humid conditions. The optimal window for spore germination and disease development is an average daytime temperature of 18 to 20°C (roughly 64 to 68°F), morning humidity above 85%, and a thin film of moisture sitting on leaf surfaces for more than 48 hours. Rainy stretches followed by warm days create ideal outbreak conditions.
Crops It Affects
Different Stemphylium species target different hosts, which can make identification tricky. The most economically significant species include S. vesicarium, S. botryosum, and S. beticola. Together, they infect a wide range of vegetable and fruit crops: spinach, onion, garlic, leek, lentils, asparagus, pear, and various herbs.
The disease typically appears as small brown or tan spots on leaves. As the spots expand, they can merge and kill large sections of leaf tissue, reducing the plant’s ability to photosynthesize and ultimately cutting yields. On pear fruit, S. vesicarium causes a condition called brown spot that damages fruit quality. A study on parsley in California found that 10 of 11 tested cultivars were susceptible to S. vesicarium, and the same isolates caused lesions on carrot and celery. Interestingly, those parsley-infecting strains did not cause disease on leek, onion, spinach, or tomato, illustrating how host specificity can vary even within a single species.
For home gardeners, Stemphylium leaf spot is most commonly encountered on tomatoes (where it causes small, dark spots that crack in the center), onions, and spinach. Warm, wet weather in spring and early summer is when outbreaks typically begin.
Fungicide Resistance Is a Growing Problem
Stemphylium has become increasingly difficult to control with conventional fungicides, particularly on spinach. Growers in the southern United States have reported growing trouble managing the disease with a widely used class of fungicides known as strobilurin (FRAC group 11). Research published by the American Phytopathological Society found that resistance to these fungicides is now widespread in S. vesicarium isolates from spinach. All 26 spinach isolates tested in lab and greenhouse assays carried a specific genetic mutation, called G143A, that confers resistance. The same mutation was detected in nearly 83% of isolates from spinach seed lots harvested from 2017 onward in Europe, New Zealand, and the United States.
The resistance appears to be species- and host-specific. Isolates of S. beticola from spinach and S. vesicarium from onion did not carry the mutation, meaning these fungicides still work against those populations. The oldest resistant isolate traced back to a spinach seed lot harvested in the Netherlands in 2003, suggesting the resistance has been circulating for over two decades. For growers, this means rotating fungicide classes and relying on cultural practices like improving air circulation, reducing overhead irrigation, and removing infected plant debris are more important than ever.
Stemphylium and Allergies
Like many outdoor molds, Stemphylium releases spores into the air that can act as allergens. One species in particular, S. herbarum, has been studied for its role in respiratory allergies. In a study of 307 asthmatic patients, 55.8% tested positive for antibodies specific to S. herbarum, and a separate study found Stemphylium among the most common sensitizing agents, with 86.6% of tested individuals showing positive reactions. The fungus has been identified as a sensitizing agent in asthmatic children as well.
That said, Stemphylium allergy is not as well-documented as sensitivity to more common mold allergens like Alternaria or Aspergillus. Its worldwide distribution is established, but specific clinical reports attributing symptoms solely to Stemphylium are limited. For people with mold-related asthma or allergic rhinitis, Stemphylium is one of many potential triggers, especially during warm, humid months when spore counts peak outdoors.
Where It Shows Up Indoors
Stemphylium is primarily an outdoor fungus, colonizing soil, decaying vegetation, and living plants. It can, however, appear indoors. Air sampling studies have detected Stemphylium spores inside buildings, particularly in regions with warm, humid climates. A survey of a research building in Karachi, Pakistan, for instance, identified Stemphylium alongside more common indoor molds like Cladosporium, Alternaria, Aspergillus, and Penicillium.
Indoor Stemphylium growth is far less common than these other genera. When it does appear, it typically enters through open windows, ventilation systems, or on contaminated plant material. Standard mold prevention strategies, keeping indoor humidity below 60%, fixing water leaks promptly, and ensuring good ventilation, reduce the chance of any mold establishing indoors, Stemphylium included.