There is a particular disappointment reserved for basil growers. The plants look glossy and upright for weeks, and then the leaves nearest the compost start to fade, as though something has drained the colour out of them. Turn one over and there is a faint grey dust on the underside. That dust is basil downy mildew, and once a plant is badly infected it will not recover. The good news is that it is far easier to prevent than to cure, and much of the work comes down to two things: where you grow your basil, and which variety you sow.

What downy mildew looks like

Downy mildew on basil is caused by an organism called Peronospora belbahrii, which behaves more like a mould than a fungus. It rarely announces itself. The first clue is usually a change in colour on the older, lower leaves, followed a day or two later by a soft grey or purplish-grey fuzz on the underside of the same leaves. The fuzz is thin and easy to miss unless you actively look for it, and it is most visible early in the morning while the air is still damp.

As it progresses, the yellowing spreads upwards through the plant, leaves begin to drop, and growth slows to nothing. In a bad case, stems darken and the whole plant collapses within a week or so. What makes it distinctive is that the yellowing is often patchy and hemmed in by the veins rather than spread evenly across the leaf, and the undersides look dusty rather than shiny.

Why it spreads so fast in the UK

Downy mildew needs two things: high humidity and wet leaves for several hours at a time. British summers and British greenhouses supply both. A polytunnel with the door shut, a crowded windowsill above a kitchen sink, a row of pots watered overhead in the evening — these are ideal conditions. Spores are light and travel on air currents, so an infection can arrive from a neighbouring plot or from a plant you carried home from the shop. It can also come in on the seed itself, which is why saved seed from an affected plant is never worth the risk.

Once one plant is sporulating, anything that brushes past it picks up spores and moves them along. That is why a single yellowing plant in a crowded trough tends to become a trough full of yellowing plants.

Is it definitely downy mildew?

Yellow leaves have plenty of other causes, and reaching for the bin when a plant is merely hungry wastes good basil. A few lookalikes worth ruling out:

• Nitrogen deficiency: older leaves pale evenly across the whole leaf, growth is slow, and the plant picks up within a week of feeding.
• Sun or glass scorch: bleached, papery patches on leaves facing the window, usually after a hot, bright day.
• Grey mould (Botrytis): a thicker, fluffier grey mould, but normally on damaged tissue, stems and flowers, in cool damp conditions rather than on healthy leaf undersides.
• Fusarium wilt: one side of the plant wilts first, and cutting the stem reveals brown streaking inside.
• Spider mite: fine webbing and pale speckling, with leaves that feel dry and stiff.

The giveaway for downy mildew is the combination: yellowing on the upper surface, grey fuzz underneath, and the oldest leaves affected first.

Prevention, in order of importance

Variety choice does most of the heavy lifting. After that, it is about keeping foliage dry and air moving.

1. Grow a resistant variety. This single decision removes most of the risk.
2. Space plants out. A little airflow through the canopy dries leaves far faster than you would expect.
3. Water the compost, not the leaves. Morning is better than evening, because any splash has time to dry before night.
4. Ventilate. Open greenhouse vents and doors on mild days, and keep kitchen pots away from steamy corners.
5. Feed lightly. Soft, overfed growth is tender and infection-prone; a weak liquid feed every fortnight is plenty.
6. Isolate new arrivals. Split and pot up supermarket basil, then keep it apart from your own plants for a fortnight before mixing them.
7. Buy fresh seed. Don't save seed from a plant that showed symptoms.
8. Tidy up properly. Bin infected material in general waste rather than composting it, and clean pots and trays at the end of the season.

What to do when you spot it

Act before you touch anything else. Put a plastic bag over the affected plant, tie it closed, then cut the stem at compost level and put the whole lot in the bin. If a large potted plant has only one or two suspect leaves, you can strip those leaves carefully, bag them, and move the plant somewhere drier and airier — then check the undersides every couple of days for a fortnight. If symptoms return, bin the plant.

Afterwards, clear fallen leaves from the compost surface and the greenhouse floor, wash your hands, and clean tools and surfaces with hot soapy water. Don't replant basil in that pot straight away; empty it, wash the pot, and start again with fresh compost.

Resistant varieties worth growing

Seed packets often carry the letters DMR, for downy mildew resistance. Look for:

• Everleaf series — 'Everleaf Genovese', 'Everleaf Emerald Towers' and 'Everleaf Thai Towers'. Bred for resistance and slow to bolt, which makes them well suited to pots.
• 'Prospera' — compact with large leaves, good for containers.
• 'Eleonora' — a classic Genovese-style leaf with resistance built in.
• 'Devotion' — slow bolting and reliable in a damp season.
• Rutgers DMR lines — sold under names such as 'Obsession', 'Passion' and 'Thunderstruck'.
• 'Marian' and 'Pesto Party' — frequently listed as resistant and worth trying.

Resistance is not immunity. Downy mildew exists in different strains, so a variety that shrugs it off in one garden may still catch it in another. If your resistant plant does show symptoms, treat it exactly as described above. Thai, lemon and holy basils vary in how badly they suffer, but they taste quite different, so if you want true sweet basil flavour, 'Everleaf Genovese' or 'Eleonora' is the closest match.

A simple plan for the season

Sow a resistant variety in late spring, grow it in a pot you can move, and keep it in the sunniest, breeziest spot you have. Water at the base in the morning, feed lightly, and pick from the top regularly so air keeps moving through the plant. Keep an eye on the undersides of the lower leaves once a week — a two-minute check is all it takes. At the end of the season, bin the plants rather than composting them and wash the pots before storing. Do that, and you give yourself a genuinely good chance of a summer of clean, fragrant leaves.

Photo: monicore / Pexels