When Not to Spray: Most Plant Problems Fix Themselves

The instinct when a plant looks unwell is to spray something on it. It is usually the wrong instinct, and occasionally an expensive one.

Here is a rough guide to when treatment earns its place.

Rule one: a fungicide only works on fungus

This sounds obvious and is the single most common mistake in home gardening. A fungicide will do precisely nothing about:

• aphids, mites, caterpillars or beetles
• nitrogen or iron shortage
• overwatering or drought
• herbicide drift
• a virus
• sunscald

Given that watering faults and nutrient shortages between them account for a very large share of unhappy plants, spraying without a diagnosis is more likely to be useless than useful. Work out the cause first.

Rule two: most fungicides are protectants, not cures

Copper, chlorothalonil, sulphur and the rest work by coating healthy tissue so spores landing on it cannot establish. They do not heal a lesion that already exists.

The practical consequence: applying a fungicide to a badly diseased plant in August mostly wastes the product. The spotted leaves stay spotted. The protection only applies to leaves that have not yet been infected, and by late season there are not many of those left.

If you are going to use one, it has to start early, before or just as symptoms appear.

Rule three: cosmetic damage is not a problem

A great deal of what looks alarming costs the plant nothing meaningful:

• Leaf spots on a mature shade tree. A maple can lose a large share of its leaves to fungal spotting and leaf out normally the following spring. Spraying a full-sized tree is impractical, expensive and unnecessary.
• Powdery mildew on squash in September. The plant is at the end of its life anyway. You are treating a plant that is about to be pulled.
• Flea beetle shot-holes on an established plant. Ugly, irrelevant. On seedlings it matters; on a three-foot tomato it does not.
• Japanese beetle damage. Six weeks of skeletonised leaves on an established shrub or tree does no lasting harm.
• Sooty mould. It is not even infecting the plant — it is growing on insect honeydew. Deal with the insects and it stops.

The honest question is not "is something wrong?" but "will this reduce the harvest or kill the plant?" Often the answer is no.

Rule four: some things cannot be treated at all

Spending money here is purely wasted:

• Viruses. Mosaic, spotted wilt, rose rosette. There is no treatment. The plant is a reservoir for infecting others, and removal is the only useful action.
• Fusarium and verticillium wilt. Once inside the plant, nothing reaches it.
• Bacterial wilt in cucurbits. Same.
• Squash vine borer, once inside the stem. No spray penetrates. Surgery or nothing.

Recognising these early saves both money and the several weeks of hoping that usually precede pulling the plant anyway.

Rule five: sprays have costs beyond the price

• Broad-spectrum insecticides kill predators too. Ladybirds, lacewings, hoverfly larvae and parasitic wasps are doing free pest control, and they reproduce more slowly than aphids do. Wiping them out reliably produces a worse pest problem three weeks later.
• Anything applied to open flowers risks bees. If a plant is flowering, spray at dusk when bees are not foraging, or do not spray.
• Pre-harvest intervals are on the label for a reason. On edible crops this is not optional.
• Oil and sulphur do not mix, and neither should be applied in high heat. Both can scorch foliage badly.

When treatment is genuinely worth it

• Late blight on tomatoes or potatoes — though the action is removal, not spraying.
• A serious pest infestation on a young plant that cannot outgrow the damage.
• Spider mites on a houseplant, which will kill it and spread to the rest of your collection.
• Scale or mealybugs indoors, where there are no natural predators to help.
• A protectant programme started early on a plant that gets the same disease every year in a wet climate.

The most effective interventions are not sprays

Ranked roughly by how much difference they make:

1. Water at the base, in the morning. Most fungal and bacterial diseases need wet leaves for hours to infect.
2. Mulch. Stops soil-borne spores splashing onto lower leaves.
3. Space plants properly. Airflow dries foliage and is the reason crowded beds get mildew.
4. Rotate crops. Most soil-borne disease is defeated by not planting the same family in the same soil.
5. Buy resistant varieties. For cucumber and squash powdery mildew, and for tomato wilts, this makes more difference than any product you can spray.
6. Clear debris in autumn. Removes the overwintering stage.
7. Hand-pick large pests. Ten minutes on a tomato plant beats any spray for hornworms.

None of these cost much, and all of them work on next year as well as this one.