Defense against summer diseases and pests


The long, sultry days of summer roll on, bringing the heat, humidity, and occasional storm.

The vegetable garden is churning along, relishing the heat and pumping out a bounty of flavors and colors. The cucumbers produce a profuse number of fruits from their trellised vines, needing picked before they get too mature and yellow.

Green tomatoes turn whitish, then pink, then red. First a few, then more. The kale hums along, growing abundant, crinkly leaves for salads, sautees and to be cooked with bacon.

And the peppers hang or stand upright, slowly making their way to red or orange or yellow ripeness.

Of course, that isn’t all that’s going on in the garden. On closer inspection, cabbage worms are dining on a fresh leaf of kale, Septoria leaf spot is making its way from leaf to leaf up the tomato plant, flea beetles are sampling from a variety of plants and powdery mildew is starting to whiten, then kill the cucumber leaves.

The heat of summer, it seems, is not only the time that gardens produce the most, it is also the time when the things that want to eat or destroy your gardens are at their busiest.

So while the heat and humidity may have you wanting to be a couch potato rather than tending your potatoes, you should be out checking on your plants, scouting for diseases and insects, and treating them appropriately.

Of course, the disease and insect load will be much less if you take some preventative measures to help reduce the pest population.


Prevention is especially key for plant diseases because it is almost impossible to eliminate them once a plant has them. So here are some common diseases and pests infiltrating gardens right now and tips on how to prevent them and treat them.

It never fails that this is one of the most common questions I get each summer. The gardener automatically assumes that they have blight and calls every leaf disease they find on their tomatoes blight.

In fact, there are three distinct diseases that are found on tomatoes. The most common is called Septoria leaf spot. It appears as small brown spots, usually on the bottom leaves first.

The leaves then turn yellow, then brown and fall off. Early blight may appear, and starts as brown splotches ringed with yellow on the leaves.

Late blight, the most rare but the most devastating, appears as black splotches ringed in purple. All of these diseases will make their way up the plant, eventually killing the whole plant and, in the case of late blight, spoiling the fruit.

Another fungal issue is downy mildew on cucumbers and squash. This appears as white patches on leaves, which ends up turning leaves yellow, then kills them. The disease can affect the whole plant, eventually killing it.

Using mulch to reduce disease spread is the first line of defense. I use both straw and shredded newspaper. Some gardeners may opt for a preventative spray to reduce the likelihood of disease, a plant prophylactic, if you will.

For a preventative spray, most people turn to an organic or less toxic option.

A new treatment, called Serenade is an interesting option. It is a bio-fungicide, consisting of a bacteria called Bacillus subtilus, which acts as an antagonist for disease organisms.  Basically, it out-competes the bad organisms.  Think of it like yogurt – it has good bacteria that helps you reduce the chances of getting sick.

Another organic option is copper sulfate, which shouldn’t be overused because of health concerns of it never breaking down suppressing good organisms in the soil.

Stopping the spread

Once you get these diseases, the job switches to slowing their spread since you can’t actually eliminate them.

First things first, remove any affected leaves or plant parts. This can help reduce the number of diseased spores that spread through the plant. Once you get the leaves off, get them away from the garden — throw them away, bury them, burn them.

The next step is getting something on the plant to prevent the spread of the disease. The organic options will work, but some gardeners opt for something stronger. One thing that I often recommend is chlorothalonil, an effective broad-spectrum fungicide.

Bugs, worms and crawlers

Another issue altogether are the six- (or more) legged pests that find their way to the garden to ruin perfectly good plants. Just as with disease, an ounce of prevention is worth a pound of cure.

When making a decision to control an insect pest, checking to see how many you have in the garden is key. There’s no need to go for the nuclear option if you only have one or two offending insects.

For pests like cabbage worms on kale, using a row cover or mesh cover over the plants can keep insects at bay. You can use covers on other plants as well, just remember to remove them when the plant is in bloom so pollinators can get to the blooms.

There are several ways to prevent or control insects in the garden. One of the most common controls for insects is pyrethrum, an extract of chrysanthemums.

For those caterpillar/worm pests like the cabbage worms, a product containing Bacillus thuringiensis, or Bt, can offer effective control. Several sprays and dusts contain this control option.

For beetle pests, like bean beetles, cucumber beetles and potato bugs, spinosad, a bacterial extract, is a great option for control of these pests.

So, while the garden may be too hot to handle, it is time to be out looking for pests and diseases. Prevent where you can, and keep things under control. You’ll want to keep your garden as healthy as possible through the rest of the season, and that starts now.

This post was originally published in the Charleston Gazette-Mail on on July 24, 2016.

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