Hornworm

Tomato hornworm with frass on leaf.

By Jim Leser

Growing tomatoes seems like a straightforward process: buy transplants, pluck them in the ground, water and fertilize when needed and almost like magic, a bountiful harvest of juicy, red tomatoes is at hand. If only it was always that simple. Unfortunately, the garden gremlins often seem bent on preventing your success. I’ve already covered much of what is involved in producing a bountiful crop of homegrown tomatoes last week in the Home and Garden Supplement and my “What’s bugging You” column. But what are the potential stumbling blocks to denying you your harvest?

There are three groupings of problems that might circumvent your success. These are physiological or environmental, diseases and pests, notably insects.

First there are the physiological problems such as poor pollination and resultant few fruit set due to low nighttime temperatures and even high temperatures during the summer. Believe it or not you can help pollination by lightly shaking plants as you walk by your plants. This will help pollen shed even when natural pollinators are scare.

Another problem includes cat facing, misshaped fruit with scars and holes on the blossom end. This can be caused by cold weather, too much nitrogen and especially if you plant too early. Blossom end rot appears as a water-soaked lesion on the blossom end of the tomato that turns black and leathery. This can be caused by cold temperatures and fluctuations in soil moisture.

Sometimes you will find the top leaves of some of your tomato plants curling or twisting, often with some purplish color. This can be due to 2, 4-D hormone herbicide drift from you or one of your neighbors. There is nothing you can do. But hopefully these damaged plants will grow out of this and resume normal growth.

Diseases are many but often can be avoided or minimized by planting resistant varieties, rotating planting sites from year to year, increasing air circulation, minimizing wet leaves, and keeping your site clean and weed free. Early blight is one disease that occurs during hotter weather months. Black-brown target-like spots appear on older leaves. You can minimize this malady through crop rotations, wider plant spacing and anything that increases air flow.

Septoria leaf spot is a fungus that appears as white or gray spots surrounded by a black or brown margin. Again, sanitation is key as well as removing the older bottom shaded leaves.

Both fusarium and verticillium wilt are both soil-borne diseases affecting the vascular transport system. There are varieties for verticillium wilt resistance but crop rotation out of the infected soil site may also be warranted.

Some diseases are vectored by insects. Keeping your weed population down will be somewhat helpful but many vector insects are fliers and can move long distances into your garden.

Curly top virus is transmitted by the beet leafhopper. Leaves turn yellow, stop expanding and upper leaves roll and turn purplish, especially along veins. Plants stop growing. Net row covers can help shield plants from the early season beet leafhopper infestations. Removing infected plants will help prevent spread to adjacent plants. There is no chemical control or prevention for viruses and spraying for the beet leafhopper is generally a futile practice.

Cucumber mosaic virus can look the same as some herbicide injury symptoms. Plants yellow, become bushy and stunted. Again, dispose of infested plants. Tomato spotted wilt virus causes brown or purple spots on leaves, spreading to stems. Wilting, yellow rings or spots on fruit are other symptoms. This virus is vectored by western flower thrips.

Insects are generally not a direct problem to growing tomatoes. Sucking insects such as aphids and whiteflies can appear but rarely reach levels justifying control. An insecticidal soap application or even a forceful stream of water can take care of them. One thing you might not know is that a light colored straw mulch can reduce the incidence of some insects such as aphids and whiteflies. Light reflecting from the straw acts as a repellent and also reduces their reproduction and hence rate of increase. Don’t mulch too early or you stand a chance of keeping the soil too cold.

Now tomato hornworms can get your attention because of their large size (up to 3 or so inches) and their voracious appetites. And they are not satisfied with eating just leaves. They can move to chewing large holes in your prized tomato fruit. I once had a neighbor whose dog could locate these beasts and quickly devour them. If you don’t have such a dog and don’t want to spend your time picking them off yourself, there are Bacillus thuringiensis (BT) products that can be sprayed that are extremely safe to us and kill only caterpillars, nothing else.

This concludes the tips I have for successfully growing tomatoes in your western Colorado home gardens. The rest is up to you. Experience is often the best teacher. Do your due diligence and you too can enjoy your very own delicious tomatoes. Something to be proud of.

Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.

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