Whether you have cared for your tomato plant from seed or you’ve hand-picked the right transplant from the garden center, a bit of panic may set in when you spot the leaves of your tomato plants turning from vibrant green to sickly yellow.
Especially as beginners, it’s hard to know what is normal and what isn’t. And if we find more and more leaves turning yellow, we rightly become alarmed. Still, sometimes it’s difficult to identify the problem and even harder to know what to do about it.
While yellow leaves on tomato plants can be caused by a dozen or more culprits, I’ve found these five among the most common. In this episode of the Beginner’s Garden Podcast, I discuss these five possible causes, along with identifying markers, prevention measures, and what to do about each of them. Click to listen to the episode or read the post below for the highlights.
When you first transplant your tomatoes into the ground, especially in the early spring when nights aren’t warm yet, tomatoes will go through a transplant adjustment period. In this week or two following transplant, you’ll notice your once-vibrant green leaves lightening in color. But, if you look at the newer leaves at the top, they are young, healthy, and growing.
What to do about it: Lower leaves should be snipped off anyway (see below), so as long as you see healthy, vibrant leaves at the top of the plant, cut off the yellow leaves at the stem. They won’t benefit the plant and will likely only serve as a gateway for disease.
Prevention: Some varieties of tomatoes are more susceptible than others, but waiting to plant the tomato plants until the optimum time will help to prevent transplant shock. Wait until the nighttime temperatures stay in the 50s.
Early blight is the culprit every year in my garden, and it’s easy to spot when you know what you’re looking for. Caused by a soil-borne fungus, early blight travels from the soil to the lower leaves. At the earliest stage of infection, these lower, older leaves will begin showing irregularly shaped yellow splotches that progress into brown spots with a yellow “halo” around them. The splotches appear almost like a target with a brown center. As the disease progresses unchecked, these entire stem and leaves turn yellow and then brown, and finally, they shrivel up completely.
What to do about it: At the earliest sign, cut off the affected leaves. The longer the leaves remain on the plant, the more likely the fungus will spread up the plant to the healthy leaves.
Prevention: Keep plants spaced out well (3 feet minimum) to allow airflow between the plants. Wet, humid conditions exacerbate early blight. Mulch heavily in the entire tomato area, creating a barrier between the soil and the tomato leaves. As the young plants grow, cut off lower leaves completely, especially if they are touching the ground (even if they’re healthy). Leave a 12-18″ gap between the ground and the lowest sets of leaves. When irrigating, chose a drip or soaker hose method to dispense water to the root zones of the plants; aim not to let the leaves get wet, which will allow the disease to spread more easily. If you must use overhead watering, do so at the beginning of the day so the water can evaporate quickly. At the end of the season, remove all plants and destroy them; do not compost. Rotate crops next season.
Septoria Leaf Spot
Another fungal disease, septoria leaf spot mimics early blight in many ways. Like early blight, spots on the lower, older leaves begin yellow but then darken into brown and then to tan or gray. These spots are smaller and more numerous than those in early blight, and they take more of a circular appearance. As the disease progresses and the spots grow, they will coalesce to larger brown area. Water-soaked lesions may also appear, sometimes on the underside of the leaf. Tiny black, pimple-like spots in the center can form a secondary infection, infecting the rest of the plant. Left unchecked, the leaves turn yellow, brown, and then die.
What to do about it: Use same measures as you would for early blight.
Prevention: Use the same prevention measures as you would for early blight.
With verticillium wilt, leaves will exhibit yellow and brown areas from the middle vein of a leaf to the edge, often in a V-shape. Plants wilt in the hot part of the day. Verticillium wilt is slow to progress and more uniform throughout the plant, but unfortunately, there is no cure and it will eventually overtake the plant. If you suspect verticillium wilt, scrape the stem at ground level, and if you see brown in the normally white vascular tissue, the infection has taken hold. Remove and destroy the plant, rotate tomatoes into a different area next year, and use seed labeled with a resistance to verticillium wilt, since the fungus can remain in the soil for many seasons.
Fusarium wilt normally doesn’t begin appearing until the fruit begins to mature on the plant. Lower leaves turn yellow, and sometimes this is limited to one stem or shoot, which is bright yellow and wilting. Initially the wilting will show recovery at night. The plant’s growth becomes stunted, but it’s possible if the progression is limited to one stem, some of the crop may reach maturity before the plant succumbs. If you suspect fusarium wilt, scraping the stem will reveal brown vascular tissue, and like with verticillium wilt, there is no cure. You’ll need to follow the same measures and destroy the plant, practice crop rotation, and grow resistant varieties in the future.
As I always recommend when encountering a problem with any plant, utilize your local cooperative extension service if you are in the US. The agents are always willing to help you identify problems in your garden. You can locate your closest extension service here.
Have you had any experience with any of these tomato plant conditions?
Verticillium Wilt — Ontario Crop IPM
Septoria Leaf Spot — University of Minnesota Extension
Septoria Leaf Spot — Dr. Sharon M. Douglas, The Connecticut Agricultural Experiment Station
Early Blight and Septoria Leaf Spot — University of Illinois Extension
Tomato Disorders: Early Blight and Septoria Leaf Spot, Karen Delahaut and Walt Stevenson, University of Wisconsin Extension
Early Blight and Septoria Leaf Spot — Cornell University Extension
Fusarium Wilt — University of Maryland Extension
Fusarium Wilt — Missouri Botanical Garden
Bacterial Wilt — Clemson Regulatory Services
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