🐛✗ Tested and false“Onion downy mildew and onion white rot are basically the same disease under different names.”
These are two separate diseases caused by two unrelated fungi. Onion downy mildew is caused by Peronospora destructor, spreads through summer on rain splash and wind, and forces a roughly five-year gap before replanting alliums on the same ground. Onion white rot is caused by a different fungus, Stromatinia cepivora, attacks bulbs and roots from the base rather than starting on leaves, and its resting structures can persist in soil for at least fifteen years, far longer than downy mildew's soil legacy.
Our read The confusion is understandable since both are soil-persistent allium diseases with no chemical cure for gardeners, but treating them as the same problem risks under-resting ground after a white rot outbreak or over-resting it after downy mildew, when the two call for different rotation lengths.
What to do: Identify which disease you actually have before setting a rotation plan: downy mildew shows first as yellowing leaves with grey-purple fungal fuzz, while white rot shows as wilting foliage with white fungal growth and black specks at the bulb base.
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- RHS, Onion downy mildewOnion downy mildew is caused by the fungus Peronospora destructor and spreads via rain-splashed and wind-blown spores through summer.
- RHS, Onion white rotOnion white rot is caused by the fungus Stromatinia cepivora and its sclerotia can remain in the soil for at least fifteen years.
“If onion sets look firm and healthy at planting time, they're free of downy mildew and safe to plant.”
The fungus can survive as dormant infection carried inside a bulb with no outward sign, only becoming visible once the set sprouts and produces fresh spores in spring. A set that looks sound in the bag can already be carrying next season's outbreak. University extension guidance backs this up: infected bulbs and sets are listed as a primary source of seasonal reinfection alongside soil-borne resting spores and infected debris.
What to do: Buy sets from a reputable supplier, and once planted, watch for any that sprout unusually early or weakly and pull those rather than assuming firmness at purchase was proof enough.
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- RHS, Onion downy mildewThe fungus can remain dormant within affected bulbs, producing spores the following spring once conditions turn favourable, and recommends discarding any sets that sprout prematurely.
- UC IPM, Downy mildew (onion and garlic)Infected bulbs, sets, seeds, and plant debris are listed among the sources of inoculum that carry the pathogen between seasons.
“Scotland's cool, overcast summers make onion downy mildew less of a threat here than in warmer parts of the UK.”
Cool and wet is precisely the combination this fungus needs, not a condition that suppresses it. Spore production requires extended leaf wetness and high humidity, and severe outbreaks are specifically linked to wet summers; the disease is actually slowed down by heat, with spore production dropping above roughly 24C. A cooler, damper Scottish summer removes the one factor, sustained warmth, that would otherwise hold the fungus back.
Our read Assuming Scotland's climate offers natural protection is the opposite of what these sources describe: it's closer to a best-case scenario for the fungus than a worst case for it.
What to do: Treat airflow, spacing and site drainage as non-negotiable in a Scottish garden rather than optional extras, since the local climate is not doing any of that protective work on its own.
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- RHS, Onion downy mildewExtended periods of leaf wetness are required for spore production and infection, so severe outbreaks are only likely to occur during wet summers.
- UC IPM, Downy mildew (onion and garlic)Spore production needs high humidity and cool temperatures, declining above 75F and becoming suppressed above 82F for extended periods.
“Growing a resistant onion variety like Hylander or Santero means downy mildew stops being a concern.”
The resistance described for these two cultivars is partial, not immunity. They are reported to show good protection against the disease, which is a real and useful advantage over an ordinary variety, but that is a different claim from being unaffected by it, and neither is described as immune.
Our read Treating a resistant variety as a substitute for the cultural measures, rather than an addition to them, is where this claim goes wrong.
What to do: Choose a resistant variety as one layer of protection, but keep spacing, airflow, sanitation and healthy sets in place regardless of which variety is in the ground.
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- RHS, Onion downy mildewThe cultivars Hylander and Santero appear to offer good protection against onion downy mildew.
“If an onion crop showed no visible downy mildew symptoms before harvest, the bulbs are safe to store long-term.”
The fungus can infect a bulb systemically without producing the obvious leaf symptoms gardeners watch for, and that hidden infection tends to surface only in store, where affected bulbs sprout prematurely, shrivel, or turn soft and watery. A clean-looking crop at harvest is not the same test as a crop that is actually free of the pathogen.
What to do: Cure and store onions as usual, but check stored bulbs regularly through winter and remove any that soften, weep, or sprout early, rather than treating a symptom-free harvest as a guarantee.
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