Monthly Archives: September 2020

Milkweed yellows phytoplasma

Last year I noticed some of my common milkweed (Asclepias syriaca) plants were showing signs of chlorosis – yellowing of the leaves along their veins and some stunted growth. As they were growing in an area of very poor soil which could have been contaminated when the concrete drive was laid I wasn’t too worried, just grateful anything would grow there. However this summer is a different story!

By late August the plants with yellowed leaves had proliferated with no sign of blooms and were also showing considerable malformation which looks to me like a form of fasciation so I decided it was time for them to be removed. I was horrified by what I found: huge root systems spreading through the bed of milkweeds with great bunches of fasciated shoots. Mercifully the A. tuberosa (butterfly milkweed) seem unaffected.

Googling suggested that milkweeds generally suffer from few pests and diseases although they are less studied than commercial plants species. A bacterial disease, milkweed yellows phytoplasma, which is spread by leaf hoppers, seems the most likely cause of the malformations I found. The advice is to remove all affected plants and put them in the garbage rather than composting them, to avoid spread of the bacteria.

It was also suggested that as the bacteria is species-specific, it is unlikely to affect any caterpillars feeding on the plants. Apart from looking unattractive, the plants showed no signs of blooming so loss of flowers appears to be the main problem. I shall have to watch the milkweed patch more carefully as the root systems of the malformed plants were very extensive and I’m sure I’ve missed some.

Affected Plants

Roots and Shoots