Question: Why is my celery turning yellow? Is it too hot or could it be a pest or disease? -Kenny F.
Answer: You might notice the foliage of your celery plants turning yellow due to one of the following reasons: disease, garden pests, nutritional deficiency, or overwatering. We’ll tell you how you can narrow down the cause of the yellowing plants and what to do to resolve the problem.
- Aphids: When aphids prey on a plant, the leaves won’t just turn yellow—they’ll also begin to wither, curl, and become deformed. This change in the leaves’ shape happens because the aphids suck the juices out of the foliage. Check the undersides of leaves for tiny insects that range in color from white to yellow or green to black. You may also see a sticky clear residue (referred to as honeydew) that eventually turns to a sooty mold-like substance, which the aphids leave behind. You can use the pressure nozzle of a garden hose to knock the insects off with a high-powered jet of water. Then follow up with a spray made of one quart of water and half a teaspoon of dish soap. You can find more remedies for an aphid infestation in our article All About Aphids, and How to Kill Them [https://www.gardeningchannel.com/all-about-aphid-control/].
- Celery mosaic virus: If the other causes of yellowing foliage don’t quite fit, the last possible cause is celery mosaic virus. The yellowed foliage will likely show a mottled or mosaic-like pattern, and leaves might be curled, wrinkled, or deformed in another way. Older leaves may eventually display necrotic spots, or plants may show stunted growth. Unfortunately, this virus has no cure, so you will need to remove and dispose of all infected plants if celery mosaic virus strikes your garden. Aphids and leafhoppers spread this virus, so you can reduce your risk by treating for these insects, keeping the garden clear of weeds that can host the virus, and choosing resistant varieties of plants. Taking two or three months off from growing celery and letting the ground remain unworked during this break will help reduce your risk of celery mosaic virus.
- Cercospora leaf blight: Cercospora leaf blight is a fungal disease that’s especially prevalent when the weather is warm and wet. Prevent Cercospora leaf blight by cleaning up plant debris and pulling weeds often, as weeds can play host to the fungus’ spores. Make sure to water plants at their base, avoiding the overhead watering that can splatter their leaves and foster fungal disease. The yellowing of this disease occurs in irregular elongated blotch shapes on the celery stalks, and the yellow hue can tend toward brown.
- Fusarium yellows: This soilborne disease is caused by the fungus Fusarium oxysporum. The fungus enters plants through their roots and is especially problematic in seasons that are warm, when the earth is heavy and wet, and spores proliferate in the soil. Plants afflicted with fusarium yellows will have red coloring to their stalks in addition to yellowed foliage. You can reduce your risk of fusarium yellows by choosing resistant or tolerant seeds and plants as well as ensuring proper soil drainage, maintaining a weed-free garden, cleaning up celery debris, and sanitizing your tools, gloves, and shoes.
- Nitrogen deficiency: When plants turn yellow due to a nutrient imbalance, the discoloration indicates that they aren’t getting enough nitrogen. Yellowing in the case of nitrogen deficiency will begin in the plant’s oldest leaves first, eventually taking over the foliage of the entire plant. Plants that are yellow due to a lack of nitrogen may also show stunted growth. To treat this issue, start feeding the plants with a fertilizer that contains plenty of nitrogen. You can tell how much nitrogen is in a fertilizer by checking the packaging for three numbers separated by hyphens. These numbers give the ratio of nitrogen, phosphate/phosphorus, and potash/potassium (in that order) that the fertilizer contains, so you’ll need a fertilizer with a high first number.
- Wireworms: Wireworms are the larva of click beetles, and they tend to strike seeds just before or after germination. They will continue feeding on plants via the underground root system, striking the small roots most heavily, and slowly damaging plants more as the season wears on. Their destruction underground can eventually result in plants that wither and die above ground. Celery that’s gone yellow as the result of a wireworm invasion will change color from the bottom up and will eventually exhibit stunted growth along with discoloration. You can always check the soil where affected plants are growing, keeping an eye out for wiry-jointed worms, to be certain your plants are suffering from wireworm damage. If you see worms in the soil, flood it with lots of water to dispel them. Pull up any affected plants, cleaning off their roots before moving them to a new location. Poorly drained soil is especially susceptible to wireworms, and improving drainage will lessen the prevalence of these pests. Once the worms cause visible damage, however, there’s no treatment that can save plants from what’s already been done. Moving them to a new location may allow for some salvageable growth, or you may wish to discard affected plants instead, depending on the extent of the damage and how far the growing season has progressed.