The white, powdery coating that shows up on pumpkin, courgette, cucumber and other cucurbit leaves late in the growing season is powdery mildew, and it's less about a specific mistake than about a season doing exactly what seasons do: cooler nights, heavier dew and thicker, more crowded foliage in late summer and autumn all favour it at once.
Unlike most fungal problems, powdery mildew doesn't need standing water on a leaf to spread, in fact it often thrives when leaves are dry but the surrounding air is humid, which is exactly the combination a lot of New Zealand gardens get as summer turns to autumn. That's part of why it tends to appear seemingly overnight on a plant that looked fine a few days earlier: the conditions for it were building for a while before the first visible patches showed up.
It starts as small white spots, usually on older leaves first, spreading into a fuller dusty coating that can eventually cover most of the leaf surface. Badly affected leaves photosynthesise less efficiently and can yellow and die off early, which shortens the harvest window on a plant that was otherwise still producing.
It starts as small white spots, usually on older leaves first, spreading into a fuller dusty coating that can eventually cover most of the leaf surface.
Airflow is the most effective prevention available at planting time: spacing cucurbits further apart than feels necessary, and pruning back excess leaf growth on established plants, both reduce the humid, still microclimate the fungus needs to establish. Watering at the base rather than overhead, and earlier in the day rather than evening, helps for the same reason.
Once it's showing, a diluted milk spray, roughly one part milk to nine parts water, applied to affected leaves every week or so, is a genuinely research-supported treatment rather than a folk remedy: several studies have found it measurably slows the fungus, thought to be down to naturally occurring compounds in milk that interfere with spore germination when exposed to sunlight. Potassium bicarbonate spray is the other well-established option where milk isn't practical. Removing badly affected leaves as you spot them, rather than leaving them on the plant, also slows how fast it spreads to healthy growth nearby.
Powdery mildew is one of those problems that's cheap to catch early and considerably more damaging once it's covered most of a plant, which is exactly the gap a premium check-in is built to close: a nudge to look at a specific plant before those first small spots have had a chance to spread.