Just as other flowers lose their bloom, hardy garden chrysanthemums (Chrysanthemum morifolium) add a splash of color. Hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9, chrysanthemums grow up to 1 to 3 feet tall and wide. The showy flowers bloom in a variety of colors, including lavender, red and yellow. Chrysanthemums grow best in moderately moist soil in full sun to partial shade and make excellent borders or cut flowers. The last thing you want to see is brown leaves or flowers from pests and diseases.
1. Bacterial Diseases
Bacterial diseases can cause those cheery, colorful chrysanthemums to turn brown. For example, bacteria leaf spot starts with small, dark brown leaf spots that become large, irregular patches. You may notice symptoms on just one side of the plant. This disease spreads through splashing water, so you should avoid overhead watering. Chrysanthemum cuttings that have brown, decayed stems and brown leaf margins may have bacterial blight, even if the parent plant shows no symptoms. Destroy and dispose of infected cuttings and plants, and buy certified disease-free cuttings.
2. Fungus Diseases
Verticillium and fusarium wilts are fungus diseases that turn chrysanthemum leaves yellow or brown, beginning at the base of the plant. The best ways to control these soil-borne diseases are to destroy infected plants and buy certified disease-free chrysanthemums, states the Clemson University Cooperative Extension. Botrytis blight causes browning of chrysanthemum flowers, beginning with the lower petals. After the first sign of botrytis blight, spray the infected plant thoroughly with a solution of 3 teaspoons of mancozeb in 1 gallon of water, applying every seven to 10 days throughout the growing season.
3. Other Causes
Microscopic, worm-like foliar nematodes may be tiny, but they can kill a chrysanthemum plant. They live in fallen leaves and spread to chrysanthemums through rainwater. Symptoms include yellow-brown leaf spots that merge and lead to leaf and plant death. Remove infested plants and soil, and avoid wetting leaves when watering. If you see brown winding trails on chrysanthemum leaves, chrysanthemum leafminer larvae may be the cause. Prune infested plant parts and remove plant debris in the fall.
Chrysanthemums can cause skin irritation when touched and nausea and vomiting when eaten, so keep children and pets away from these plants. Wear long sleeves, long pants, shoes and socks, and chemical-proof gloves when you apply mancozeb to your chrysanthemums. If you are pregnant or nursing, you should ask someone else to apply mancozeb or any other pesticide and stay away from the treated area for at least 12 hours after treatment. Keep children and pets away from mancozeb-treated chrysanthemums for at least 12 hours after treatment, and keep pruning tools out of the reach of children.
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