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If you notice any parts of the dahlias that are brown or shriveled, you can remove them with a pair of gardening shears to improve the appearance of your plants. Doing this regularly can also help to prevent diseases from spreading among your plants. Make sure to discard the diseased leaves, buds, and stems, such as by throwing them away or burning them. This is a sign of a fungus, and eventually the fungus will lead to a wooly gray growth on the dahlia’s petals. The fungus will continue to spread if you don't remove the damaged petals. This fungus favors cool, wet conditions. If you live in a climate where this weather is common, you may need to spray the plant with a fungicide, such as iprodione, moncozeb, or fenhexamid, which you should be able to find at a garden supply store. This is a sign of the root knot nematode, which lives in warm, sandy soil, and which can affect a variety of different plants. The knots will appear near the roots, and they will continue to worsen until the plant dies. If your soil is infested with root knot nematodes, you will need to treat it with a nematicide before you can grow healthy plants. The discoloration, combined with wilting leaves, is a sign of verticillium wilt, and it can stay in the soil for several years. This fungus will eventually kill your plant, and it can spread to other dahlias quickly if not treated.  If burning isn't allowed in your area, discard these plants in the landfill. You will also need to destroy the entire dahlia plant if you see mosaic mottling on the leaves, which is usually due to a virus spread by aphids. Smut causes yellowish spots to appear on the leaves, and eventually leads to the leaves becoming brown and dry. Watering from directly overhead can lead to water pooling, allowing the growth of smut. Instead, try to water close to the base of the plant, so the water will go directly to the roots.
Cut off any diseased parts of the dahlias. Remove flowers immediately if you see brown, water-soaked spots. Pull up the entire plant, including the roots, if you see knots forming. Burn dahlia plants and roots if you see discoloration of the veins. Avoid watering dahlias from overhead to prevent smut.