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Milkweed Yellows Phytoplasma

Posted on Tuesday, November 4, 2014 at 12:27 pm in Gardening for Monarchs

Milkweed Yellows Phytoplasma (Photo: Gary Stell)

By Wendy Caldwell

When it comes to milkweed, we still have a lot to learn, or at least I do! You may think that the only reason milkweed exists is to be a food source for monarch caterpillars. Well, I can tell you that there is a lot more going on in the milkweed patch than caterpillars munching away at your leaves; there are many interactions taking place.

The field guide Milkweed, Monarchs and More, by Ba Rea, Karen Oberhauser, and Mike Quinn, is a great resource for identifying organisms you may encounter while searching your milkweeds. If you don’t have a copy, they’re available for sale at the Monarch Store. While this field guide sheds light on numerous milkweed invertebrates, it doesn't get into too much detail about things such as plant disease. Recent inquiries about “funny looking milkweeds” inspired me to do some research into milkweed plant diseases; the one in particular that I explored is called 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 of the 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, allowing an incubation period during which the phytoplasma cells replicate and eventually reach the insect’s salivary glands, and then moving to a healthy plant and injecting phytoplasma cells into it during feeding.

How do you know if your milkweed has a phytoplasma disease?

There are a few symptoms to look for, according to the Phytoplasma Resource Center found on the USDA Agricultural Research Service website:

  • Phyllody: development of leaf-like growths in place of normal flower parts
  • Virescence: develoment of green color in place of normal flower color
  • Witches Broom: abnormal, excessive proliferation of axillary shoots resulting in broom-like growth
  • Yellowing: leave lose 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: 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 the disease is spread by insect vectors, one way to get phytoplasma under control is to quickly and effectively eliminate any milkweeds suspected of phytoplasma, at the first sign of 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:

For more information on milkweed diseases and control, look to the Xerces Society's handbook (Milkweed: A conservation practitioner's guide).

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