Gardening in Hawaiʻi with Tom Timmons: Is my tomato a hypochondriac? Or is it really sick?

A friend asked what is the topic for this week’s gardening article. I said: “Tomatoes.”

He replied: “It is going to be hard to find something witty to say about tomatoes.”

I gave him my glare stare of astonishment: “Not so, Grasshopper. When you finish this article, you will know the difference between knowledge and wisdom.”

Tomatoes belong to the ominous plant group called nightshades (Solanaceae). Sounds like they could kill you before you finish your salad. Sadly, tomatoes are subject to so many diseases that I think they may have suicidal tendencies. But with a little TLC, they will survive nicely until you rip them from the plant and chop them up for salsa.

Where you live on the island can increase tomatoes’ susceptibility to diseases and death. Of course, you could move to Iowa to solve some tomato problems. No? Then you should buy disease resistant tomato seeds.

I can’t tell you which seed companies have resistance to common diseases. Look for the following abbreviations on the seed packet or website: A-anthracnose, EB-early blight, LB-late blight, F-fusarium wilt, PM-powdery mildew, TMV-Tobacco Mosaic Virus, TSWV-Tomato Spotted Wilt Virus and TYLCV-Tomato Yellow Leaf Curl Virus. For a complete list of tomato diseases from Cornell University, click here. 

Before going to bed, I close the windows in case it rains. These are gardening tips comparable to closing the windows:

• Buy seeds that are resistant to common diseases.
• Plant in well-drained soil.
• Space tomato plants about 3 feet apart.
• Trellis and prune the plant to improve good air circulation.
• When watering, do not get water on the leaves.
• Keep the growing area clean of leaf debris.
• At first appearance of disease, remove and destroy infected leaves being careful not to touch healthy leaves.
• Next planting, rotate a non-Solanaceae plant where tomatoes are growing this year.

Doing these things reduces the chances that you will have to deal with a tomato disease.

You did everything right, and your tomatoes are looking sickly. Do they just need a snack or are they victims of a nasty little virus, mold, fungus or mildew? Take a picture of the infected part of the plant and compare it with a picture of a documented diseased plant. This isn’t a 100% guarantee of a correct diagnosis, but usually it is fairly accurate. Caveat: some diseases look alike and are hard to tell apart. Don’t be reluctant to contact a local Extension Agent or Master Gardener for a second opinion.

Anthracnose

This is a soil-borne disease that doesn’t appear until the fruit begins to ripen. A small, round sunken spot that may be pinkish or white in color often darkens as it increases in size on the fruit. Once infected, the fruit is inedible unless you are a bird or rat. 

The entire plant, roots, leaves, blossoms and fruit need to be removed and destroyed. Before planting anything again, treat the soil with a fungicide following the product’s directions. Increasing air flow and reducing ambient moisture may help.

Late blight

This is almost always a fatal disease that usually occurs during the cool, rainy months. It appears suddenly and kills quickly. Once it is established there is little you can do but destroy the plant. This is the same species which caused the Irish Potato Famine.

Late Blight usually appears on the upper, newer leaves as brown lesions. The lesions quickly spread, causing leaf distortion, followed by yellowing, shriveling and death. It can kill a tomato plant in a few days.

Don’t let the infected plant remain in your garden. Late blight spreads quickly and via water, too. Remove the entire plant, bag it and throw it in the garbage.

Early blight

This is less serious than Late Blight and is not likely to kill the plant if identified and immediately treated. Early Blight appears first on the lower leaves as dark brown spots. If you look closely, you will notice the spot is a series of concentric circles. The spot will grow and kill the leaf. Remove the dead and dying leaves immediately. Be careful not to touch any healthy leaves with the infected leaf or your hands once you have touched an infected leaf. Dispose of the infected leaves in the garbage. 

According to the U.S. Department of Agriculture, mix a heaping teaspoon of baking soda, teaspoon of olive oil and teaspoon of mild soap mixed with a gallon of water and spray the entire plant. This helps slow the spread of the disease. The most effective treatment is to spray the plant with copper fungicide. Always follow the directions.

Powdery mildew

Because of the high humidity and frequent rain, Powdery Mildew is common in Hawaiʻi. The good news is that powdery mildew doesn’t kill the plant. It does, however, affect the plant’s health and growth.

When it appears, immediately remove the affected leaves, but never more than 1/3 of the total leaf growth. Treat with Neem Oil, potassium bicarbonate or sulfur. All are organic treatments. These products should be available locally. Apply at first sign of Powdery Mildew following the manufacturer’s directions. 

Blossom end rot

This may appear to be a disease, but it is not. I include it because it is often mistaken as a disease. Blossom End Rot is the result of calcium deficiency. The tomato fruit develops decayed spots at the end away from where the fruit is attached to the stem.

End Rot is easy to prevent with mulch, compost and proper fertilization with a balanced fertilizer (10-10-10). I also use crushed eggshells to add calcium to the soil. However, these can attract slugs and snails. It is advisable to wrap the bottom 4 inches of the stem with a copper foil first to repel slugs and snails.

Tomato yellow leaf curl

The plants with tomato yellow leaf curl have a stunted appearance. The new leaves are smaller in size, wrinkled with yellow between the veins. The leaf edge curls upward. Flowers may appear but usually will drop before fruit is set.

Whiteflies transmit the TYLC. Whiteflies are found on the underside of the leaves. They can be washed off with a garden hose or sprayed with an insecticidal soap or Neem Oil. Remove any infected leaves immediately and put them in the garbage. If the entire plant becomes infected, remove it, bag it and put it in the garbage.

Tomato spotted wilt virus

Small brownish ringspots appear on the leaves. As they become more numerous the leaves may appear to be bronzed. The leaves may also have a purple color and roll up. The disease causes the plant to be stunted.

The tomato fruit is the clearest indicator with its yellow ringspots. TSWV is transmitted by thrips (insects), which are small and difficult to see clearly without a magnifying glass. They can be controlled with Neem Oil or Insecticidal Soap. A silver reflective mulch effectively repel thrips. 

Bacterial wilt

This disease is distinguished by the foliage of the entire plant suddenly wilting, but the leaves stay green. A pathogen in the soil enters through the roots and moves quickly up the stem of the plant.

The result is a blockage that prevents water from reaching the leaves. Poor soil drainage and high pH, alkaline soil are the main causes. To verify, cut a stem near the ground. Split it. The interior is dark, and water soaked, or in some cases the stem is hollow. Unfortunately, there is no cure for Bacterial Wilt, although there are a handful of varieties which are resistant. Remove the plant and dispose of it.

There are many more tomato diseases that I didn’t cover. If your plant has symptoms that you can’t identify, take a photo and send it to your local Master Gardener Help Desk or Extension Agent. 

Now, Grasshopper, the answer: knowledge is knowing that a tomato is a fruit; wisdom is not putting it in a fruit salad.

I do not use AI to write my articles. I don’t need a computer’s help to make errors; I manage to do that on my own. The information in this article has been reviewed for accuracy before publication.

References:

• Ebesu, Richard. 2004. Home Garden Tomato HGV-5, CTAHR, University of Hawaii.
• Tomato Spotted Wilt Virus. 2023. Clemson University
• Home & Garden Information Center. 2023. Powdery Mildew. Clemson Cooperative Extension, Clemson University.
• McGrath, Margaret. 2023. Early Blight on Tomatoes. College of Agriculture and Life Sciences.
• Cornell University.
• McGrath, Margaret. 2023. Late Blight on Tomatoes. College of Agriculture and Life Sciences. Cornell University.
• Tomato Plant Diseases and How to Stop Them. USDA. 2016.
• Schuh, Marissa and Grabowski, Michelle. 2023. Early Blight in Tomato and Potato. University of Minnesota.
• Gabor, Brad. 1997. Tomato Disease Guide. Siminis Vegetable Seed Co. 
• Radford, Joanna. 2020. Tomato Blossom-End Rot. North Carolina Cooperative Extension, North Carolina University.
• Melzer, Michael, et. al. 2020. Tomato Yellow Leaf Curl. University of Hawaii.
• Nelson, Scot. 2013. Powdery Mildew of Garden Vegetables in Hawai‘i.
• Baessler. Liz. 2023. Tomato Anthracnose Information: Learn About Anthracnose of Tomato Plants. Gardening Know How.

Editor’s Note: Every other week, Big Island Now will feature a guest gardening column by Tom Timmons. He is a certified Master Gardener respected for his gardening experience, but his views are not necessarily those of the University of Hawai‘i.

Other gardening columns by Tom Timmons:

• When planting papaya seeds, best to read the directions
• Are you smarter than a rose beetle? Can you outrun a snail?
• The evil web of an avocado lace bug
• Getting under the skin of bulb onions
• Choosing a fertilizer of your lawn and plants
• The KISS method of composting
• Why should you play in the dirt?
• Basil – the royal herb