Purple shamrock, or Oxalis triangularis, is also known as lucky shamrock. This 6-inch-tall plant grows in clumps and has four triangular-shaped purple leaves with rose markings. Funnel shaped flowers are white, with a pink tinge to them, and grow in clusters of 4 to 12. Purple shamrocks like moist, well-drained soil and full sun to filtered shade. Common diseases that attack this plant include fungal leaf spots, powdery mildew and rust. Botrytis and chlorotic ringspot virus can also be a problem.
Leaf Spot/Rust
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Leaf spot is a common term for several diseases that attack shamrocks and other plants. The main symptom of fungal leaf spot is the appearance of red, purplish-brown, tan or black spots on the leaves. The size of the spots varies, depending on the fungus. The spots may start running together to form blotches. Leaves turn yellow and fall off. Leaf rust is another fungal disease that causes yellow or yellowish-orange spots and bumps to appear on the upper surface of leaves. Leaf spots are mainly an aesthetic problem, which you can minimize by removing and destroying infected leaves, and by keeping the plant dry.
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Powdery Mildew
Shamrocks infected with powdery mildew have a white fungus growing on them that makes them look as if they've been sprinkled with powder. In addition, the leaves may become distorted, turn yellow and fall off the plant. Powdery mildew weakens plants because it reduces their ability to do photosynthesis and steals nutrients away from them. This fungus thrives in humid conditions. To reduce humidity, avoid crowding your plants and don't water them from overhead. Fungicides, sulfur and neem-based products that are labeled for powdery mildew are all effective ways to control this disease.
Chlorotic Ringspot
Shamrock chlorotic ringspot virus (SCRV) looks similar to yellowing caused by nutritional deficiencies. However, a diagnosis of SCRV can be made when noticing the appearance of yellowish rings on the leaves. The rings eventually fade into yellowish blotches and streaks. SCRV is believed to spread by aphids. Remove diseased plants and destroy them.
Botrytis
Botrytis blight, or gray mold rot is a fungal infection that attacks the flowers, stems and leaves of many plants, including purple shamrock. Symptoms can vary, but include the appearance of a fuzzy, whitish-gray mold that grows on the surface of leaves and other affected parts. Small, yellow or tan spots appear on the leaves, grow larger and blend with other spots, sometimes engulfing the entire leaf. Spots can later become whitish-gray or tan. This fungus usually attacks injured or aging tissues, and tender flower petals, rather than actively growing, healthy plants. Infected stems become water-soaked and rot at the soil line, causing the plant to collapse.