Those who support monarchs and pollinators by fostering monarch habitat know it is sometimes challenging. Several diseases and pests can affect milkweed, leaving habitat managers wondering what to do. Some symptoms of diseased or unhealthy milkweed plants include unusual spots on the leaves, misshapen or asymmetrical leaves or pods, or plants that appear wilted or weakened. Recent inquiries about yellowing or odd-looking milkweeds inspired us to dig into plant diseases, specifically milkweed yellows phytoplasma.
Phytoplasmas are bacteria; they do not have a cell wall and are enclosed by a single membrane. They cause diseases in plants and are spread by insect vectors, primarily leafhoppers. Leafhoppers aren't the only insect that can spread phytoplasma, but most known vectors are in the insect order Hemiptera. Their piercing/sucking mouthparts allow them to feed on the phloem of plants, where phytoplasmas live. These phloem-feeding insect vectors can transfer diseases, such as milkweed yellows phytoplasma, by feeding on an infected plant and spreading it to healthy plants. After feeding on a diseased plant, the phytoplasma cells replicate in the insect and are injected into a healthy plant's phytoplasma cells via the insect’s saliva during feeding.
Phyllody—development of leaf-like growths in place of normal flower parts
Virescence—development of green color in place of normal flower color
Witches Broom—abnormal, excessive proliferation of axillary shoots resulting in a broom-like growth
Yellowing—leaves lose their normal green color, becoming yellow
Little leaf—development of abnormally small leaves
Proliferation—abnormal growth of numerous stems
Necrosis—death of cells and/or tissues
Dieback—the death of branches
Stunting—overall reduction of plant height
Bunch top—shortening of internodes at and near the tip of a branch, resulting in bunched growth at the end of the branch
What should you do if you suspect phytoplasma in your milkweed patch? Since insect vectors spread the disease, one way to get phytoplasma under control is to quickly and effectively eliminate any milkweeds suspected of phytoplasma at the first sign of the disease. By digging out an infected plant, you reduce the chances of other insects feeding on that plant and becoming vectors of the disease.
For more information on phytoplasma, visit the Phytoplasma Resource Center: https://plantpathology.ba.ars.usda.gov/phytoplasma.html
For more information on milkweed diseases and control, look to the Xerces Society’s handbook (Milkweed: A conservation practitioner’s Guide): https://www.xerces.org/publications/guidelines/milkweeds-conservation-practitioners-guide