Marin Master Gardeners
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Tackling tomato troubles

August 5, 2017
James Campbell

I’ve fertilized my tomato plants every six weeks. I reduced my watering once the fruit set; I have done everything the tomato experts recommend. What could possibly go wrong now — August?

HORNWORMS

Have you ever seen a naked tomato plant?

Pruning the suckers (the secondary leaves on a tomato plant) will expose existing leaves to more sunlight. More sun means more energy for fruit production and earlier ripening. Truth be told, I get overwhelmed when my tomato plants start to get huge and I can never keep up with the pruning.

One morning I found one of my tomato plants practically naked. Not only were the secondary leaves gone, but the primary leaves and even some of the blossoms had disappeared. I figured my mysterious helper was a family member who didn’t know the difference between primary and sucker leaves ... boy, was I wrong.

The culprit was a huge prehistoric-looking caterpillar that was preparing to become a moth. Even at 4 inches long and with “V” shaped markings the green-colored hornworm was not easy to spot on my plant. I found it hanging to the underside of a branch and the remedy was easy. I just picked it off and put it in a bucket of soapy water.

To prevent the hornworm caterpillars from feasting on your plants, check the undersides of your tomato leaves for green spherical eggs laid by the moth. If you find them, you can either remove the leaf or introduce beneficial insects like ladybugs that will attack the eggs.

TOMATO LATE BLIGHT

Our mild temperatures and the moist air from coastal fog create the perfect conditions for tomato late blight. The late blight pathogen, Phytophthora infestans, is related to the pathogen that causes sudden oak death, Phytophthora ramorum. ‘Phytophthora’ in Latin means plant destroyer. This is a serious and highly communicable water mold, the same one that caused the infamous Irish Potato Famine. Potatoes are in the same family as tomatoes.

Late blight starts with greenish-brown spots on leaves which may develop into white fuzz found mostly on the bottom of the leaves. The lesions start to spread all over, killing leaflets, whole leaves and then move onto the stems. If fruit has formed, it soon has greasy, brown shoulders. The fruit may ripen, but it will not taste good.

It’s important to note that the spores are not in the soil, but are blown in the wind from plant to plant, from as far as several miles away. To protect plants from this disease, I try to keep the leaves dry and I shelter my plants from spore-bearing breezes. Some Bay Area gardeners spray their tomato plants with preventative copper-based fungicides. Once infected, however, no fungicides will cure this disease, but some may control it. The problem with fungicide use is that spores will continue to be produced, increasing the risk of infection of nearby plants.

Removing blighted plants must be done with extreme caution since spores of this pathogen can become airborne and infect neighboring plants. To remove infected plants, it is best to pull them out in the middle of a sunny day when leaves are their driest and any dislodged spores will likely be exposed to UV radiation. Put infected plant tissue in a plastic bag and leave the bag in the sun. The heat from the sun will kill the water mold and the plant. Do not compost the plants since the spores will only survive to infect other nightshade plants.

Nothing is more disappointing to a gardener than to tend to a plant, watch it mature and then not enjoy the fruits of the labor.

To learn more about combating tomato diseases, go to the UC integrated pest management program at ipm.ucdavis.edu and type “Tomato” into the search box.

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