Pests & Problems

Keys to Keeping Your Plants & Tubers Healthy

Maintain a healthy garden by controlling and preventing the spread of disease and virus through vectors (aphids, thrips, and other insects) which are known to be one of the main transporters. Pest prevention programs can assist in reducing damage to you plants and help control the spread of disease and virus. Research, along with new products is helping to protect our environment. Please be sure to handle products properly and research the best option for your gardening practices.

We strongly recommend preventing issues from arising by spraying before they do, and continuing to spray throughout the season. In general, as with most plants, dahlias should be sprayed with an insecticide every two to three weeks (follow the manufacturer’s recommendations) during the growing season. A good spraying program is the best preventative to stop infestations, virus transport, and create overall healthiness of your plants. Begin spraying once your plants have reached 12" or more and continue throughout the growing season and into the fall. The sprays listed here are just a few of the name brands available. Ortho, BioAdvanced, and Monterey brands produce excellent all-around sprays. You may also check with your local garden center for other great products available in your area. When you are looking for a product to use on or around your dahlias, anything that is safe for vegetables, roses, or other flowering plants will work fabulously for your dahlias. We carry a wonderful line of products on our website or at our farm in our Pest & Disease Control section. Spraying is necessary to stay ahead of pest issues. The products we use on the farm vary throughout the season based on the needs of our plants. We have bee hives on the farm and have had all our products approved by our beekeeper, and there are no neonicotinoids used on our farm.  Bees are vital to us as farmers, and without our bees, we would not be able to introduce new dahlias each year.

General Insects

Some general insects that attack dahlias are leaf miners, stem borers, aphids, and thrips. Preventative measures are always better than trying to get rid of pests once they arrive. Recommended sprays: BioAdvanced 3-in-1, Monterey Garden Insect Spray, Orthene, Neem Oil, Sevin-5, Organocide, and Malathion. All organic sprays tend to work best for preventative care but are not as effective in treating a problem that already exists.

Snails & Slugs

Treating slugs and snails is critical during the first few weeks after planting in all areas until sprouts are through the soil. Slugs/Snails will eat the sprouts off nightly before you ever see them. Apply slug/snail bait after planting and continue applying throughout the season. Any slug/snail bait product at your local garden center will work. We recommend applying regardless of visible trails. We find many times there are no trails, as they can live up to 5' underground.

Check out this helpful PDF by Oregon State University.

Earwigs

These insects will crawl up your plants and most often are found hiding or chewing on your blooms. They can be a nuisance especially when you cut your dahlias to bring the beautiful blooms inside. We recommend treating your plants with Sevin-5 dust or spray, malathion, or sprays containing pyrethrins.

Cucumber Beetles (similar to a ladybug, but green instead of red)

These insects fly in daily and are hard to control. They especially enjoy chewing on light colored blooms. We do not treat for them as we do not want to harm ladybugs and other beneficial insects. We recommend hand-picking them off your blooms.

Spider Mites

Spider Mites are the biggest and most common predators of dahlias. These pests will cause your plants to look brown around the base with possible spots on the leaves. Spider mites feed on leaves during the summer and return to the soil for the winter and must be sprayed to prevent future issues. Typically, your dahlia plant will look like it needs more water. Most plants will begin with yellow spots on the leaves and then the leaves will begin browning completely, working their way up from the bottom, and eventually killing the entire plant.

Spider mites thrive in hot weather and can attack certain varieties, while not bothering others right next to it. If you take a leaf off the bottom of the plant and turn it over, it could look dusty/dirty and feel grainy. These are tiny pests that are hard to see with the naked eye. These pests will appear and thrive mostly when the temperature rises and you water more, as they love humidity and hot temperatures. We recommend preventively spraying beginning 60 days after planting and continuing through the entire growing season. We recommend spraying with a Miticide, Monterey Horticultural Oil, BioAdvanced 3-in-1, Malathion (good for prevention, but does not work as well on infested plants), or any spray that lists mites in its description. If your plant dies completely, it may try and grow again. Spider mites infest already stressed plants most often, so it is important to keep your plants happy and healthy with proper care.

Dahlia Diseases

Although dahlias can get different diseases, often the issue is pest damage that can be resolved by taking care of the pests by spraying with an insect spray related to the plant issue. Although virus can occur, virus is not common and your entire crop will not suddenly all have a virus. Many insects suck the juices out of the leaves creating crinkled leaves or lines. This is not a virus, but rather signs of pest damage that should be treated for. Please do not dispose of any plants or tubers until we have had a chance to look at your photos and diagnosis them. Reach out on our Contact Us page.

Powdery Mildew (white fungus on leaves)

Powdery mildew is an airborne spore that you either have in your location or you don’t. Mildew commonly shows up in the fall with damper weather conditions. The leaves will begin to get a white powdery film or spots on them. We do not believe that this is caused by overhead watering, but rather weather temperatures and humidity. We recommend preventively spraying before this issue arises. Begin spraying 60 days after planting and continue through the fall, depending on your location. Reapplying about every 10-14 days or per the manufacturer’s directions on the spray you are using. Recommended sprays: Daconil, BioAdvanced 3-in-1, Funginex, or Fung-onil, or any oil-based fungicide sprays. If you do not treat for powdery mildew, it will kill your plant and potentially get into the tubers, killing them as well, which is devastating.

Crown Gall

Crown gall is a disease caused by the bacterium Agrobacterium tumefaciens, which enters the plant through wounds in roots or stems, it stimulates the plant tissues to grow in a disorganized way, producing swollen galls or a cauliflower looking mass. This bacterium is found in soil. If you suspect that any of your dahlias have crown gall, please take photos of the tubers and contact us immediately. All dahlia tubers that we have diagnosed to have crown gall should be disposed of immediately. Crown gall can be found in woody plants including fruits, vegetables, shrubs, and ornamental plants. We recommend removing the soil in your planting area to prevent further contamination if gall is found. Please remember to disinfect all tools between clumps of tubers to prevent any cross contamination. We are working to have more research done with dahlias.

Leafy Gall

Leafy gall is caused by the bacteria Rhodococcus fascians which enters the plant through wounds of the stems and roots of many herbaceous plants. Infection with this disease causes dense clusters of distorted leafy shoots that are fused together. If you suspect that any of your dahlias have leafy gall, please take photos of the tubers and contact us immediately. All dahlia tubers that we have diagnosed to have leafy gall should be disposed of immediately. We recommend removing the soil in your planting area to prevent further contamination if gall is found. Please remember to disinfect all tools between clumps of tubers to prevent any cross contamination. We are working to have more research done with dahlias.

Mosaic Virus

Mosaic Virus in dahlias will cause large-scale disfigurement of the plant. It is found worldwide and is spread by inoculation of the sap, either through human intervention or by the 13 species of aphids that serve as its natural vectors. Dahlias with Mosaic Virus can exhibit a multitude of symptoms, such as variegated leaves, yellowing on the leaves that follows the veins, or stunted/gnarled plants. Please treat your plant for pests and watch the new growth before assuming it is a virus.

If you suspect any issue, please send photos of the plants along with close-up photos of the bottom and top new growth on your plants. If your new growth on the top of the plant looks normal, this is a very good sign that it is just pest damage and not a virus.