Insects, diseases trouble gardens in late summer

  • Mike Groll—AP

For the Monitor
Saturday, August 26, 2017

Overall it has been a great garden season. Nighttime rains followed by bright sunny days provide ideal conditions for flowers, fruits and vegetables. We’ve enjoyed bumper crops of berries and the tomatoes are finally ripening.

Every garden is different though, and there has been trouble in paradise. Late summer is the time when diseases and insects become most prevalent.

Brown tomatoes

If you have been losing bottom leaves on your tomato plants, chances are fungal diseases like early blight and septoria are at work. True to its name, early blight appeared early in the season in our garden affecting not only the lower leaves but the plant stems and a few of the ripening fruits.

Septoria comes a little later with its scatter shot of brown spots and eventual withering of the bottom leaves. Neither is usually fatal but they may reduce the amount of fruit the plants produce.

The general advice for preventing these fungal diseases is to clean up plant debris and dispose of it in the trash rather than composting it. We religiously adhere to that advice yet we still get these diseases each year.

Like many fungi, the dry spores are light and can travel on the wind. No sign of the dreaded late blight yet. That fungal disease is usually fatal, with plants turning to a disgusting mush practically overnight. It slowly creeps up from the south each year. If you want to check on its progress across the country go to the website usablight.org.

Squash attacker

Squash vine borer is a problem in our garden every summer, and I have resorted to some extreme measures to try and thwart this pest. Panty hose wrapped around the stems, foil covering the soil at the base of the plant, rhubarb leaves as a repelling mulch – with mixed success.

This year, we planted the cole crops in the bed that had summer squash last year. Since we cover the cole crops with a shade cloth tunnel to exclude the cabbage moth, when the vine borer moth, whose larvae overwinter in the soil, emerged in June, they were trapped under the cloth and could not get out. Success! Now I will be following summer squash with cole crops in our rotation plan every year.


Insects are the largest class of living things on Earth with close to a million species. In New Hampshire, we only have 10,000 to 20,000 species of insects to deal with and most of them are beetles. Some years, I feel that all of them live in my garden.

If you weren’t diligent about crushing the brick-colored egg masses on your squash leaves, you are probably seeing a new generation of squash bugs on your plants. The newly hatched ones move a little more slowly than the adults, gather together in large groups, and are easy to catch and squish.

Japanese beetles are easy to find. They usually hang out in the roses, raspberries or beans at my house. I keep a small bucket of soapy water at the ready and knock the sleepy ones to their sudsy death during my morning rounds. We have been cautioned against buying one of those commercial beetle traps that uses pheromones to lure the beetles. It will not only attract the ones in your yard, but draws more of them from far and wide, making matters even worse.

If your Asiatic and Oriental lilies have been looking chewed, check for the bright red lily leaf beetle. They are active from early spring onward with both the larvae and the beetles feeding on your precious plants. It is only attracted to true lilies and will not affect your daylilies.

Skeletonized leaves on your viburnums, especially the native ones, mean the viburnum leaf beetle is at work. The adults are active right now and will be laying egg masses soon. The best way around this pest is to plant varieties they don’t like such as doublefile and Korean spice.

There is a new beetle showing up in our garden – the wee harlequin bug. Also called the twice-stabbed or two-spotted stink bug, it is black with two red spots. Right now they are staging a love-fest in the snapdragons but don’t seem interested in anything else. They supposedly like verbascums, grapes, columbine and raspberries, too.

Ash borer

Another new bug infiltrating the state, that thankfully I have not seen yet, is the emerald ash borer. This destructive pest has destroyed millions of ash trees in the U.S. and Canada.

The larvae tunnel under the bark and the adult beetles feed on the leaves. Eventually the foliage wilts, branches die, and the canopy thins. Trees can die in three to four years if heavily infested.

The emerald ash borer has shown up in more than 20 towns in four New Hampshire counties. A shiny metallic green, it should not be confused with the much larger green tiger beetle which is a predator of small insects. Consult the website emeraldashborer.info for advice on identification and prevention.

There are still a lot of glorious gardening days left. Try to make the most of them despite what may be bugging you!