Strawberry Pests and Diseases: Preventing Gray Mold

What to watch for changes through the year

The cast changes with the season. Autumn brings disease arriving on the plants you buy, winter is quiet, spring brings powdery mildew and aphids, fruiting brings gray mold, and early summer brings spider mites and slugs.

Knowing the order lets you decide what to look at each month rather than looking things up after the damage is done. Gray mold in particular cannot be stopped once it starts spreading, so everything depends on getting the conditions right before flowering.

Check twice a week, and every other day once fruit has set. Look at the undersides of the new leaves and inside the trusses for anything that has changed color, and you will be acting a week earlier.

Gray mold is prevented by conditions, not sprays

Gray mold grows a gray fur on fruit and flowers and moves from berry to berry. It is driven by humidity and by damaged tissue left in place. The spores are everywhere, so the aim is not to eliminate them but to avoid giving them the conditions they need.

1. Keep the fruit off the soil Lay plastic mulch or straw so the berries never touch soil. Soil splash is the direct route of infection, and this alone cuts the problem substantially.
2. Let air through Strip old leaves when the plant gets dense and keep the spacing. Not cramming plants into a container is the same principle.
3. Remove damaged fruit Take moldy berries and spent flowers off as soon as you see them and get them off the plot. Dropped at the base of the plant they simply become the next source.
4. Water at the base Watering over the top wets the flowers and fruit. Water the soil at the base, gently, on a dry morning.
5. Open covers in the daytime A cloche or tunnel raises the humidity inside. Open it on mild days so the plants dry off before evening.

When rain is forecast, picking the colored fruit first is the single most effective thing you can do. One berry left too long is where it starts.

Powdery mildew and aphids in spring

Powdery mildew puts a white bloom on the undersides of leaves and on the fruit, and appears in dry, warm spells. It starts on plants that have had too much nitrogen and have poor air movement, so how much you feed and how you mulch is itself the prevention.

Aphids cluster on new leaves and trusses. They stop growth and they carry viruses. Start checking as the buds move in early spring and remove them by hand or wash them off while the numbers are still small.

Spider mites and slugs in early summer

Spider mites live on the undersides of the leaves and the upper surface takes on a pale, stippled look. They multiply fast in hot, dry weather, so spraying water on the undersides from late spring keeps the numbers down on its own.

Slugs eat the fruit overnight, and the damage they leave is where gray mold gets in. They shelter under pots and under straw mulch, so either go out at dusk or grow the fruit where it cannot reach the ground.

1. Spray the undersides On a dry morning, direct a jet of water at the undersides of the leaves. Mites prefer dry conditions, and this alone changes how fast they build up.
2. Take off stippled leaves Pale, stippled old leaves do almost no photosynthesis. Removing them together cuts the mite population and improves airflow.
3. Go out at dusk Slugs move after dark. Turn over the bottoms of pots, the straw and any bricks and deal with where they are sheltering.
4. Lift the fruit off the ground A hanging basket or a high raised bed keeps fruit away from both slugs and soil splash. Solving it through how you grow lasts longest.

Avoid bringing disease in on new plants

Most of the diseases that kill a whole plant arrive with the plant. Black spots on the leaf stalks, or leaves noticeably smaller on one side of the plant, mean you leave it on the bench however cheap it is.

If you raise your own from runners, taking them from a diseased plant carries the problem into next year. Only use plants that were healthy and cropped well during the season, and never take runners from one that struggled.

1. Black spots on leaf stalks Sunken black spots an eighth to a quarter of an inch across are typical anthracnose. It spreads in heat, humidity and soil splash, so avoid overhead watering while raising young plants.
2. Distorted new leaves New leaves small and twisted, with one side of the plant not developing, suggests a root or vascular disease. It does not recover, so take it out early.
3. Keep pots off the ground Stand young plants on a bench or a board. On the ground, watering and rain splash soil, and with it the spores, onto the leaves.

Growing strawberries in the same ground every year builds up soil-borne disease. Leave two or three years between plantings, and use fresh compost in containers every year.

Dealing with an outbreak

Where gray mold has appeared, take off every affected berry and spent flower, and tidy the surrounding leaves to let air through. Whether it spreads to the rest of the fruit is decided by the humidity from that point. Do not leave what you removed on the plot.

A plant wilting with one side stunted and not recovering has a root or crown disease. Lift and destroy the whole plant rather than letting it spread, and do not reuse that soil.