The short version: Most common vegetable troubles can be read by their cause, and few need a spray. Powdery mildew is a fungal disease that shows as white powdery patches, mostly on cucurbits, and good airflow with watering at the base keeps it in check. Blossom end rot is not a disease but a calcium-uptake problem driven by uneven watering, so steady moisture protects the fruit that sets later.
Many vegetable problems are not a mystery, and few call for the chemicals. The trick is telling apart a fungal disease, a feeding or watering issue, and a plant reacting to heat, because each one asks for a different response. This is a plain guide to the troubles that turn up most often in an Australian backyard patch, and getting the cause right saves a lot of wasted effort.
Powdery mildew: the white film on cucurbit leaves
Powdery mildew is a fungal disease that shows as dusty white or grey patches on leaves, most often caused on cucurbits by fungi in the genus Podosphaera, commonly Podosphaera xanthii. It is familiar on zucchini, pumpkin and cucumber, usually starting on the older lower leaves and spreading until they yellow, dry and die back.
It is favoured by warm days, cooler humid nights and still air, so it tends to show up from late summer into autumn, worst on crowded plants with damp, sheltered air around the leaves. A little mildew late in the season is rarely fatal, and a plant that has already cropped well can be left to finish.
Prevention does most of the work here:
- Space plants well so air moves freely between them.
- Water at the base early in the day rather than over the leaves.
- Pick a sunny, open spot with good airflow.
- Remove the worst-affected leaves, and clear badly infected plants at season's end.
Gentler ways to manage powdery mildew
For an early outbreak, a couple of gentle home sprays can slow powdery mildew, though neither cures it. A milk spray, around 1 part milk to 10 parts water, can suppress mildew when applied early and reapplied after rain, as the Sustainable Gardening Australia guide to powdery mildew explains. Potassium bicarbonate, sold as a registered home-garden fungicide, is another option some gardeners find more reliable. Spray in the cool of the morning so the leaves dry through the day, and treat both as protection on lightly affected plants, not a rescue for one already furred with white.
Choosing resistant varieties is the quieter long-term answer. Many cucurbit seed lines carry some powdery mildew resistance, often noted on the variety description when you browse our vegetable seeds. No spray replaces good spacing and airflow, though, so a plant jammed into a humid corner can keep catching mildew whatever you put on the leaves.
Blossom end rot: a calcium problem, not a disease
Blossom end rot is not a pest or a disease, it is a physiological disorder, and that distinction is the key to fixing it. It shows as a sunken, dark, leathery patch at the base of the fruit, the blossom end, most often on tomatoes, capsicums and zucchini. It can start small and pale, then spread as the fruit swells, a disheartening thing to find on a tomato you have watched ripen.
The cause is calcium failing to reach the developing fruit, usually from uneven watering rather than a true shortage of calcium in the soil. Calcium moves with water through the plant, so when the soil swings between bone dry and soaked, the fruit at the far end of the supply line misses out. This is why adding lime often does little if the watering stays erratic: most garden soils already hold plenty of calcium, and the real problem is getting it to move.
Steady, even moisture usually does more than any spray. Water deeply and regularly so the soil does not dry right out, and mulch well to buffer it. Affected fruit will not recover, so pick them off; the fruit that sets later under calmer watering is usually fine. Containers dry out fast, so they need closer attention than beds through a hot, gusty week.
Yellowing lower leaves: usually feeding or watering
Yellowing lower leaves are most often a nitrogen shortage or a watering problem rather than a disease, especially when the oldest leaves go first. Nitrogen is mobile, so when it runs short the plant shifts it from old leaves to new growth, and the lower leaves pale and yellow.
If the plant is growing slowly and looking hungry, a liquid feed such as a fish and seaweed blend, or fish emulsion, can often green it up over a couple of weeks. Overwatering can cause much the same yellowing, though, because waterlogged soil starves the roots of air. The two look alike, so feel the soil first; if it is constantly wet, ease off and check the drainage before reaching for fertiliser.
Bolting: running to seed in heat or stress
Bolting is when a plant runs to seed early, sending up a flower stalk before you are ready to harvest. It is usually triggered by heat or stress rather than anything you have done wrong, and is common in leafy crops and herbs such as lettuce, rocket, spinach, coriander and many brassicas as the weather warms, when the leaves often turn bitter.
You cannot reverse bolting, but you can delay it. Sow heat-sensitive crops in the cooler parts of the year, keep the water up, pick leaves regularly, and lean on slow-to-bolt varieties. Timing shifts with where you garden: in temperate and cool zones the worst bolting comes with summer heat, in the Mediterranean zone of Perth, southern WA and Adelaide the dry summer is the trigger, and in subtropical and tropical zones the cooler months are the kinder window for leafy greens.
Most of these troubles overlap with keeping pests and diseases in balance, and our guide to organic pest control in Australia covers working with your garden rather than spraying first.
Frequently asked questions
What causes powdery mildew on zucchini and pumpkin?
It is a fungal disease favoured by warm days, cooler humid nights and crowded plants with poor airflow, showing as white powdery patches that start on older lower leaves. Good spacing, base watering and a sunny, open position all reduce the risk.
Does a milk spray actually work on powdery mildew?
A milk spray, around 1 part milk to 10 parts water, can help suppress powdery mildew when used early and reapplied after rain. It works best as protection on lightly affected plants, not a rescue for a badly covered one.
Is blossom end rot a disease, and will adding calcium fix it?
No, it is a physiological disorder, not a disease, so it does not spread. It is caused by calcium not reaching the fruit, usually from uneven watering, and the soil often holds enough already. Steady moisture and good mulch usually help more than lime.
Why are the lower leaves of my tomatoes turning yellow?
Lower leaves yellowing first usually points to a nitrogen shortage or overwatering, not disease. A fish and seaweed blend can help a hungry plant; soggy soil calls for less water and better drainage.
How do I stop my lettuce and coriander from bolting?
You cannot reverse bolting, but you can delay it by sowing in the cooler parts of the year, keeping the water up and picking leaves often. Heat and stress are the main triggers, and slow-to-bolt varieties can buy useful time.
Most vegetable troubles come down to reading the plant and adjusting how you water, feed and space it. Get the cause right and the rest tends to follow.
Related growing guides
- Protect your vegetable garden from possums
- Damping off: what it is and how to prevent it
- A beneficial bug mix for Australian gardens
Ready to plant? Browse our Vegetable Seeds.