Sunflower plants – scientific name Helianthus annuus – have large, daisy-like flowers that come in yellow, red, orange, brown and maroon. The most common variety is bright yellow-colored, with brown centers that have heads filled with seeds. These plants are heliotropic, turning their flowers to follow the sun’s movement across the sky from east to west, and returning to their original east-facing position again at night. As with other plants, sunflowers can also suffer from conditions that cause yellowing leaves.
Sunflower leaves turning yellow: Common reasons
Fungal rot or downy mildew
Fungal rot is a common reason for yellowing leaves, especially if the plants are sitting in very wet soil. This may be caused by chronic overwatering or poor drainage. The wet conditions make the plant susceptible to certain fungi, such as Fusarium, Pythium, Rhizoctonia and Phytophthora, which cause stem, root or crown rot, as well as vascular wilts.
If your sunflower plants are affected by fungal rot, the leaves will appear dull and yellow, while the roots may become black, brown or decayed. You need to remove and dispose of the affected plants at once to avoid any spread to other plants.
Downy mildew could also cause discoloration on sunflower plants. This fungus thrives in wet soil and needs high humidity to germinate. It is usually found in over-watered seedlings with insufficient space between one another. This overcrowding creates a microclimate ideal for the fungus. Fuzz may also be present on the underside of the leaves, which could be gray, purple or brownish.
Treatments include mefenoxam, mancozeb or copper sprays. However, these options will only work during the early stages of the fungal infection.
Nutrient deficiencies
Yellowing sunflower leaves could also be due to nutrient deficiencies. If they are nitrogen-deficient, the leaves will turn yellow but remain green near the veins. If the leaves turn yellow and also develop purple or red discoloration, the plants likely lack phosphorus.
To fix this issue, check your soil’s pH and fertility before you add fertilizers, as other problems may also exist. If the plants are extremely yellowed, provide a dose of 10-10-10 fertilizer but be sure to test the soil first and adjust accordingly.
Pests
Pests like spider mites and aphids could cause yellowing leaves in sunflower plants. Spider mites suck the juice from plant tissue, which weakens the plants and causes yellowing leaves. Some plants may also manifest red or brown discoloration. These pests are best terminated with the use of sulfur sprays, insecticidal soaps and horticultural oils.
Aphids are soft-bodied insects which, like spider mites, also suck the sap of plant tissue. They feed in clusters on the undersides of leaves, which can cause curled, distorted and yellowed leaves. These pests produce a sticky fluid called honeydew which attracts ants and sooty mold colonies. To eradicate them, spray them with jets of water, or use horticultural oils or insecticidal soaps.
Being rootbound
Sunflowers also tend to develop discoloration if they are rootbound. The roots may become matted and tangled, and growth may become stunted. In this case, you should transfer some of the plants to other planters so they will flourish and retain their green leaves.
Interesting facts about sunflowers
Sunflower plants comprise nearly 70 species of the Aster family. They are native to North and South America, and some species are cultivated as ornamentals. The Jerusalem artichoke variety is cultivated for its edible underground tubers. These annual herbs have hairy stems and can grow from three to 15 feet, with flower heads reaching from three to 12 inches in width.
The disk flowers are usually yellow, purple or brown, while the petal-like ray flowers are yellow. These plants have economic and ornamental uses; their leaves can be used as fodder while the flowers produce a yellow dye and the seeds contain oil and are used for food. Sunflower oil is widely used, while sunflower oil cake is ideal for stock and poultry feeding. Sunflower seeds can be eaten dried or roasted and may be ground into nut butter.
Conclusion
Sunflower plants are annual herbs that are heliotropic because they follow the sun’s movement. These well-known flowering plants may experience yellowing leaves due to fungal rot or downy mildew, as well as due to nutrient deficiencies, rootbound issues or pests.
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