© Copyright 2020 Hearst Communications, Inc. Thrips, tiny insects that are only 1/25-inch long, leave dark fecal pellets behind and tattered plant tissue from their feeding. Fungicides can also be used as a preventive measure. In addition, the larvae and adult fly are capable of spreading fungal diseases including Pythium, Thielaviopsis, and Botrytis. These may include: Stem rot – Stem rot occurs when dahlias are growing in heavy, poorly drained, wet soil. Identifying the diseases and pests is a relatively easy task, as is the process of treating the diseases or removing the insects that graze on the plant. Avoid mulching with shredded leaves or straw (both of which attract them). Start by knocking them from leaves with a spray of cold water in the morning (be sure it's not forceful enough to damage the plant). When treating infected dahlia tubers, you must sterilize your knives -- a procedure commonly done with bleach -- after cutting each tuber, or risk spreading the tuber rot further. Alternatively, spray down plants with insecticidal soap every three days for two weeks. There are thousands of dahlia cultivars, but in general, they can grow up to 5 feet tall. Other causes of leaf problems include myriad dahlia pests such as red spiders (which can more easily be seen under a magnifying glass), slugs, caterpillars and snails (which leave irregular holes). Dahlias, available in an incredible range of sizes, colors, and forms, grace your garden from midsummer to the first frost in autumn. One of the most common ailments to dahlias is also one of the hardest to see. However, the plants have numerous illnesses and pests that can afflict them and ruin their lovely leaves and flowers or, if left unchecked, kill them entirely. Some are caused by bacteria and fungi such as leaf smut (pale spots on leaves), powdery mildew, crumpled leaves with irregular and pale-yellow patterns (with no insects visible) and leaf wilt, which causes the leaves to (obviously) wilt and the stem interior to grow dark-brown or black streaks. They consume both flower and leaf and can burrow deeply into the plants. By: Mary H. Dyer, Credentialed Garden Writer. Should these fail, start by clipping off diseased parts of the plant and, if that fails, turn to commercial fungicides. Dahlias (Dahlia spp., USDA growing zones 8 through 11) are popular outdoor plants in warm climates and annuals or houseplants in colder regions of the world. Wear protective eye wear and rubber or latex gloves when using pesticides. There are around 40 different species in genus, all flowering plants usually native to Latin America. Insects Are Eating Foliage on My Garden's Flowering Perennials, National Dahlia Society: Key to Dahlia Pests and Diseases, Royal Horticultural Society: How to Grow Dahlias, The American Dahlia Society: Fungus Controls for Dahlias and the Garden. Identifying the diseases and pests is a relatively easy task, as is the process of treating the diseases or removing the insects that graze on the plant. Unfortunately, many diseases are fatal and the best recourse is to start fresh with new, disease-resistant tubers. Read on to learn more about a few of the most common diseases in dahlias. Repeat this three times. As far as dahlia disease treatment goes, some diseases, including powdery mildew and gray mold, can be treated by fungicides applied when the disease is first noticed. Water at the base of the plant and avoid wetting the foliage. The best way to control disease is to ensure soil is well drained and that plants aren’t crowded. Sign up for our newsletter. Follow the warnings and application instructions listed on the pesticide label. As far as dahlia disease treatment goes, some diseases, including powdery mildew and gray mold, can be treated by fungicides applied when the disease is first noticed. More sinister complications can arise in your dahlias which can sometimes kill the plant permanently. Mosaic also is spread in the field by aphids feeding on diseased plants and then working on healthy dahlias. After that time, a couple of deep waterings per week is usually adequate. Below you will find the most common diseases in dahlia plants: With the exception of viral diseases, which are transmitted by insects, most common dahlia diseases are the result of damp, humid conditions, overwatering or poorly drained soil. They also dislike cold, wet soil. Use discretion. Cuttings and root divisions from diseased plants will be diseased. Six of them have been bred specifically to be ornamental flowers and have found widespread use among gardeners and florists. Don’t water dahlia tubers until sprouts appear above the soil. Look for a white ring in the soil around the stem. Their flowers form interesting, almost psychedelic spherical patterns of jagged petals. Diseases Of Pumpkins: Learn About Pumpkin Diseases And Treatments, Dahlia Pests And Diseases – Common Problems With Dahlia Plants, Dahlia Wilt Disease: How To Treat Spotted Wilt Virus In Dahlias, Prickly Kale Leaves – Does Kale Have Thorns, Corn Husk Wreath Ideas: How To Make A Corn Husk Wreath, DIY Air Plant Wreaths: Wreath Making With Air Plants, Boxwood Bush Diseases: Learn About Diseases Affecting Boxwoods, Cucuzza Squash Plants: Tips On Growing Cucuzza Italian Squash, Are Railroad Ties Safe For Gardening: Using Railroad Ties For Garden Beds, Thanksgiving Tradition: Turning Homegrown Pumpkins Into Pie, Growing Thanksgiving Dinner – Must Have Turkey Side Dishes, Interesting Uses For Pecans: What To Do With Pecans, The Bountiful Garden: Bringing The Garden To Thanksgiving. They are perhaps best known for their flowers, which sport many petals in concentric rings and appear in a wide array of dazzling colors, from white and yellow to red and purple. As such, it's best to act quickly when a thrip infestation is suspected. Dahlias aren’t as difficult to grow as you may think, but proper care may prevent certain dahlia flower diseases. Fungicides can also be used as a preventive measure. The stems of taller plants may need to be tied to garden stakes for support, and the somewhat top-heavy plants can be harmed by strong winds. Find more gardening information on Gardening Know How: Keep up to date with all that's happening in and around the garden.