We vegetable gardeners are dealing with plenty of disease challenges this time of year, and tomato diseases are at the top of that list. Fortunately for us, my guest this week is vegetable pathologist Inga Meadows, who joined me to discuss the best and newest ways to prevent, identify, and control diseases that affect tomatoes.
Inga is North Carolina State University’s extension vegetable pathologist for western North Carolina. Her program helps growers by recommending disease prevention and suppression strategies. Her research is aimed at improving tools for managing diseases through both cultural methods and chemical control.
Inga grew up in a rural area in western Illinois and she says that’s where her interest in plants started. She went on to earn a bachelor’s in botany from Oregon State University and a master’s in plant and environmental sciences from Clemson University in South Carolina. In addition to her work for the university extension, she also volunteers for the American Chestnut Foundation by screening blight-resistant chestnuts.
And be sure to download my free Tomato Care Checklist that will help you identify the 6 main steps you need to take to minimize common tomato issues and have your healthiest tomato plants ever.
Prevention & The Disease Triangle
Inga says tomato disease problems start with the environment. For instance, on the East Coast, warm temperatures, high humidity, frequent rainfall and heavy dew all create conditions that are ripe for fungi and bacteria to take hold.
A helpful way to understand why plant diseases arise is the Disease Triangle: The three points of the triangle are the host, the pathogen and the environmental conditions.
When all three are present, a disease problem is very likely. The good news is, we don’t have to solve all three of those conditions. All it takes is knocking out one.
Steps to Take to Prevent Plant Diseases
Inga says plant pathologists like herself will say there are several ways to prevent disease, and it’s best to use as many of them as you can.
Grow in a raised bed: When growing in a new raised bed, you can start out with clean soil or potting mix that is free of soil-borne pathogens. Of course, after a few seasons of planting into that same bed, the soil won’t be pathogen-free anymore. One way around this is to cover a bed with an impermeable barrier that roots can’t grow through, and then placing grow bags or a container on top of the bed with fresh soil. You will be able to use the same footprint of the existing bed without worrying about the pathogens beneath. After three years, use the raised bed again.
Start with healthy transplants: When a grower moves tomato plants that already have spots on them from a greenhouse to the garden, it’s already game over, Inga says. “If you’re at one of your garden centers and you’re about to buy a tomato plant that has spots on it, don’t do it — resist,” she adds. The same goes for the sale plants that don’t look so good.
Rotate crops: Crop rotation is the practice of changing the crop family that’s planted in a certain area from season to season. For example, refrain from planting cucurbits (cucumber, cantaloupe, watermelon, etc.) in the same bed each year to reduce occurrences of cucurbit diseases affecting plants. The same rule goes for the nightshade family (tomato, eggplant, potato). Give the soil a break from a certain crop family for two years or three years, at the least, so pathogens don’t build up in the soil. There are fertility and pest concerns as well that also make crop rotation a smart practice.
Use mulch: Inga notes that some plant pathogenic bacteria are active in moisture and inactive in dry conditions, so rainfall not only makes the bacteria happy, it also splashes the bacteria around, helping it spread. Applying mulch creates a barrier that keeps soil-borne pathogens from splashing up on plants in the rain or when watering.
Use drip irrigation: A drip irrigation system or a soaker hose will apply water where the plants really need it — at the roots. Overhead watering gets the foliage wet, which creates an ideal environment for bacteria and fungi, so always apply water at soil level.
Keep hands off wet plants: If you pull or snip diseased leaves off a wet plant, your hands or your tools will transfer pathogens from one plant to the next. Wait until the morning dew has evaporated before touching plants.
Select resistant varieties: If your garden is new, you won’t know what disease problems are prevalent yet. However, if you have been at it for even a little while, you’ll have an idea of what pathogens are present in your garden. To overcome them, you can pick disease-resistant varieties. Look on the plant tags for letters that indicate what diseases a plant is resistant to: F for fusarium, VE for verticillium, etc.
Bacteria and fungi change over time and can eventually overcome resistance in a plant. A newly developed strain, also called a race, can glide right past the plant’s genetic defenses. That’s why resistant plants are never entirely in the clear.
Plant clean seed: Purchase seed from a reputable source to ensure it is free of seed-borne disease.
How to Tell Tomato Diseases Apart
Tomatoes are susceptible to many pathogens as well as many insects, so diagnosing a tomato disease confidently can be a struggle. However, with some knowledge, the possibilities can be narrowed down.
Bacterial diseases tend to present as tiny little spots — a few millimeters wide. The center of the spot may fall out, creating a lesion with a yellow halo. On a wet plant or in humid conditions, the underside of a leaf may have a greasy look and a darker green appearance. This is called “water soaking.”
Fungal disease present in a variety of ways. Early blight (Alternaria) creates concentric rings. Septoria creates a lot of little spots with black dots at the center where spores come out.
Your local land grant university or extension service likely offers resources for identifying the plant pathogens that are prevalent in your area. Some even have a plant disease clinic where a sample can be sent in for a diagnosis and management recommendations. A lab may look at the plant tissue using a low-power microscope known as a stereoscope, or just a hand lens to find spores. For viruses, there are pathogen-specific test strips.
If a pathogen appears to be a fungus but there are no spores present for a diagnosis, the plant tissue will be placed in a “moist chamber” (a humid environment like inside a zip-lock bag or a plastic box with water) that induces the fungus to produce spores, Inga says.
The moist chamber may fail as a diagnostic tool when it encourages saprophytic fungi (the beneficial fungi that feed on dead plant tissue) to produce spores.
Tomato Disease Control Options
Inga says the home gardener has fewer options than a conventional grower. Still, there are effective disease-control practices that anyone can employ.
Leaf removal: When dealing with a foliar pathogen, removing affected leaves and disposing of them away from the garden, like in a garbage can, can slow the spread of disease.
Plant removal: To prevent a plant from creating any more inoculum (the propagules of the fungus or bacteria) in your garden, remove it entirely. Inga says the longer a diseased plant remains, the more spores or bacteria will be produced.
Sprays: Many of the fungicide or bactericide sprays that are available to homeowners are preventative, Inga notes. They must be applied before disease appears to be effective. The sprays coat the leaves, inhibiting a pathogen from infecting the leaf. Copper fungicide is an all-purpose fungicide that works well but can be overused. It may leave flecks on fruit or a blue or orange residue on plants. If copper accumulates, it can become toxic to plants. Inga says to always follow the recommended rate.
Plants Cannot Be Cured
Plants don’t have an immune system like we humans do. They have defense responses, Inga says, but giving them medicine won’t cure a disease. Commercial growers have some options for systemic treatments that go into a plant and can slow down a fungus, but they won’t kill the fungus.
You may start with clean soil and quality seed but still run into disease issues. If that’s the case, the culprit may be airborne.
Late blight (Phytophthora infestans) is an example of an airborne pathogen that affects tomatoes and potatoes. It is an aggressive disease that survives winter in Florida, then spreads northward during the tomato growing season. It requires cool, wet weather, so it won’t show up in 90° heat. It tends to show up in western North Carolina in late July and August, Inga says.
The sprays that are available to home gardeners are not effective at preventing late blight, so Inga recommends choosing varieties that are resistant. Fortunately, breeders are working to develop tomatoes that are both tasty and disease resistant, according to Inga.
Why You Shouldn’t Compost Diseased Plant Material
Plant diseases will survive in a compost pile. When that compost is finished and then applied to a garden, it will introduce those pathogens to the garden bed.
If you practice hot composting, some pathogens will be killed during the composting process. However, Inga points out, it’s only the center of the pile that really gets hot. The exterior likely won’t reach the necessary temperature.
Diseased plant material should go out with the trash. Inga says you could also keep a compost pile specifically for diseased organic material, as long as it is away from your garden and your other compost. Never take compost from that pile to amend your soil.
To Reduce Disease Issues, Don’t Overfeed Plants
To maintain a healthy plant, give tomatoes what they need. Tomatoes need full sun to do well, and they also need the right amount of “food,” Inga says. Overfertilizing and underfertilizing can make plants more susceptible to disease.
Pith necrosis is a soil bacteria that is common but doesn’t cause a problem until a tomato plant is overfed with nitrogen. The pith — the spongy vascular tissue in the plant stems — becomes infected and the plant wilts. If you cut into the lower stem while it is still green, you’ll see that the vascular tissue that should be white will have brown streaking or will be missing in spots.
Diagnosing Bacterial Wilt
Bacterial wilt can cause a tomato plant to wilt while it is still entirely green. If you cut off the stem of an infected tomato plant and squeeze the tip or submerge it in water, an oozy slime will come out. This is called bacterial streaming or bacterial ooze, Inga says.
Why & How to Grow Grafted Tomatoes
Some tomato varieties are tasty but susceptible to diseases, while others are disease-resistant but not tasty at all. That’s why grafted tomatoes are growing in popularity.
Grafting tomatoes is the process of choosing a disease-resistant rootstock and attaching to it a different variety of tomato that is desirable for its taste or another trait. The top growth is cut off the rootstock and replaced with the top growth of the second variety, known as the scion. They heal together to form one plant.
When planting a grafted tomato, it’s important that the graft point is not buried. If the scion touches soil, it will grow its own roots and the grafted plant will lose the benefit of the chosen rootstock.
I hope you enjoyed my conversation with Inga Meadows on tomato disease prevention and control. If you haven’t listened yet, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
Be sure to download your free copy of my Tomato Care Checklist!
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Tomato expert Craig LeHoullier joins me in leading this course on how to grow healthier, productive tomato plants and how to overcome tomato-growing challenges.
@MeadowsLab on Twitter
Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.