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  • Writer's pictureDash Lawn Care

Outdoor Plant Diseases to Look Out For

Plant diseases can come during any season, but most of all, they thrive in the heat and humidity of summer. Diseases can destroy entire plants, gardens, and even trees. They spread rapidly, but if you know the signs to look out for a lot of them can be contained or prevented.

Here are some of the most common outdoor plant diseases to look out for and signs that they’re spreading.

Downy Mildew

Downy mildew appears as yellow and white patches on the upper side of older leaves and white and grayish fuzzy fungi on their backs. It’s most noticeable after rain or other wet weather while the patches disappear in sunny weather (but not the disease). Eventually, as the disease takes over, the leaves will grow brown and dry and fall from the plant no matter how much water it has.

“Downy mildew occurs in cool, moist weather usually in early spring or late fall. Spore production is favoured by temperatures cooler than 65˚F. and by relative humidities approaching 100%. This disease overwinters on plant debris and in the soil. Fungal spores can be carried by insects, wind, rain or garden tools.” - Planet Natural

The best way to treat downy mildew is to avoid the conditions that help it develop in the first place. Prune your plants, water them early in the morning so they have time to dry during the day, and remove and destroy plants with any sign of the disease so it doesn’t spread.

Grey Mould

Grey mould is a very common plant disease and, just like downy mildew, it thrives during excessive rain or high humidity. Moisture helps many diseases spread in plants. The spores that cause grey mould are often already on plants when you buy them, but they remain dormant until conditions are just right for them to blossom.

“Flowers that often get botrytis [grey mould] include zinnia, peony, geranium — plants with blossoms that are full and packed with petals.” - HGTV

The best way to prevent this disease’s spread (and often many others) is to remove dying and dead leaves, flowers and fruit. Mulching your soil is also a good idea as it helps keep rain from splashing spores from the soil onto your plants.


This disease often affects vegetable gardens and, more notably, tomatoes and potatoes. During the early stages, you’ll commonly see it as brown patches on the leaves of these plants. Then, these patches will spread down the stems and, finally, to the tomatoes and potatoes themselves.

“Assuming you’re growing organically there’s not much you can do about it once you’ve got it, so we recommend cutting off infected foliage as you see it – this will slow down the spread of blight and hopefully you’ll manage to get a crop.” - Rocket Gardens

You want to do the most you can to completely prevent this. This includes leaving enough space between plants so that the air circulation is good as well as, for tomatoes, cutting off the leaves near the ground so that they don’t get wet when you water it.

Pests and Disease

Often, preventing garden pests will help prevent diseases. Lots of common pests, like aphids, carry plant diseases. An infestation along with spreading disease are the last things you want in your garden.

“Also, the dew they [aphids] excrete onto plants encourages the growth of mould which supports the spread of disease.” - Dash Lawn Care

With the right care, you can keep your garden disease-free. On the occasion that one does appear in your garden, there are often ways to treat it and, if there aren't, there are ways to keep it from taking over.

Do you need help with your landscaping? Contact us today to get a FREE quote!

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