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Garden diseases – Leaf spot

Black spots are a sign of plant fungal infections
Image: matunka

Disfiguring black spots are almost certainly a sign of one of several fungal infections which can damage and kill some plants. If your flowers and plants are developing dark spots on the leaves, here’s what to do about it.

What is leaf spot?

Keep your eye out for leaf spots on your plants
Image: TMsara

An array of fungi can infect ornamental plants, flowers and vegetables causing unsightly spots to develop on the leaves. Some infections, like Peronospora violae, are specific to certain plants like pansies and violas – rotting the crown, killing the plant and spreading the fungus to others nearby. Other infections, like Mycocentrospora acerina, attack the leaves and roots of a wide range of ornamental plants and vegetables.

How to treat leaf spot

Binning infected plants can be one way of stopping the spread of leaf spot
Image: Shutterstock

Though there are several causes of leaf spot, the treatment is the same for all of the infections mentioned above.

Chemical treatment. If you spot a fungal infection, immediately remove the affected leaves and dispose of them with your household waste before treating the remaining plants with a suitable systemic fungicide.

Non-chemical treatments. Treating black spot is simple and easy to do provided you act promptly by taking the following steps:

  • • Remove contaminated material – strip off affected leaves or remove whole plants and dispose of them by burning or putting them out with your household waste. Avoid composting diseased material because the conditions in your compost heap are ideal for the incubation of next year’s fungal infection.
  • • Collect and dispose of any infected leaves littering the ground.
  • • Sterilise any plant containers and tools before using them again.

How to prevent leaf spot

Keep your greenhouse well ventilated to minimise the risk of fungal leaf spot
Image: Linda GeorgeE

Fungal infections which cause leaf spot on pansies and other plants sometimes come from infected wild or cultivated plants growing in the vicinity. The infection spreads either on the wind or through splashes of water infected with spores. If you’ve had a problem with pansy leaf spot, try not to grow pansies in the same place for at least a year because the earth they grew in may well harbour spores.

When growing plants like chilli in the greenhouse or indoors, avoid overwatering. Fungus thrives in moist conditions, especially if there’s inadequate ventilation. Make sure you open your greenhouse door and windows to allow the air to circulate, and always water the roots rather than leaves.

In most cases, you can easily prevent fungal infections by growing your plants in optimum conditions. Strong and healthy plants are less likely to succumb. Camellia leaf spot is often caused by overwatering or nutrient imbalances, but if it’s only affecting a few leaves, simply pick them off and destroy them. Check that your camellias are planted in moist, well-drained, acidic soil, in a sheltered, shady spot.

Leaf spot is a common problem but luckily it’s usually easy to contain if you’re vigilant, act promptly to get rid of diseased material, and avoid replanting the same plants in the same spot.

Black spotted leaves are often a sign of fungal infection, particularly with high humidity. Find out how you can deal with this and take preventative steps right here.

Plants With Spotted Leaves: Fungal Leaf Spot Treatments

From indoor and outdoor gardeners alike, one of the most common gardening questions is, “Why do my plants have spotted and brown leaves?”. And while there are many reasons for plain old brown spots, when those spots look like little brown bull’s-eyes, the answer my friends is fairly simple, organism-wise that is. Those plant leaf spots are caused by one of nature’s most basic organisms: a fungus.

Plants with Spotted Leaves

Fungal leaf spot can be found in your outdoor garden as well as on your houseplant. Spotted leaves occur when fungal spores in the air find a warm, wet, plant surface to cling to. As soon as that microscopic spore gets comfortable in its new home, sporulation (the fungal method of reproduction) occurs and the tiny brown fungal leaf spot begins to grow.

Soon the circle grows large enough to touch another circle and now the fungal leaf spot looks more like a blotch. Eventually the leaf turns brown and falls to the soil where the spores sit and wait for the next available warm, wet, plant surface so the fungal leaf spot process can begin again.

Preventing Plant Leaf Spots

There are a few easy steps you can take to prevent the problem in your garden or on your houseplant. Spotted leaves or the causal fungus need two things to flourish: moisture and poor air circulation.

For your houseplant, spotted leaves can be prevented by watering the soil and not the foliage. Leave enough space between your pots for good air circulation.

In the garden, water in the early morning so the moisture will evaporate from the leaves. Closely packed foliage should be thinned. Always treat pruning and cutting tools with a 1:10 bleach solution after each use. Rake and remove all debris from around your plants before the leaves bud each spring.

How to Treat Leaf Spot Fungus

No matter how diligent you are, the day will come when those tiny brown circles appear on the leaves of your plant so it’s important to know how to treat leaf spot fungus. As soon as you see plant leaf spots, treatment begins.

For houseplants, isolate the pot immediately to prevent the fungus from spreading. Remove any leaf that has been affected. Stop misting.

In the garden, the plant’s leaf spot treatment depends on preference.

For organic treatment, there are several safe and convenient treatments available. Most contain sulfur or copper octanate. Or you can try a more traditional treatment by spraying with a mild solution of bicarbonate of soda (baking soda), using ½ teaspoon per gallon (2.5 mL. per 4 L.) of water.

For those gardeners who have no objection, many all-purpose fungicides are available. Please read the label carefully before applying.

From indoor and outdoor gardeners alike, one of the most common gardening questions is: Why do my plants have spotted and brown leaves? Click here to read what may be causing your plant's leaf spots.