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.
Quite similar to the Euphorbia maculata (Spotted Spurge) which is not always spotted.
Note how the leaf shape of this species is a bit different, somewhat broader the base on the long side and broader at the apex on the short side of the blade. The apices are essentially acute. Perhaps the most significant characteristic is the presence of pilose hairs on the leaf blades.
Rough Cinquefoil – P. norvegica seems most likely although Rusian (or Downy) Cinquefoil – P.intermedia is closely related and hard to distinguish photographically. USDA range maps state native to Canada for both spp.
Another introduced species of wild plant from Eurasia that is now growing in our Montreal garden