Cardinal flower troubles

(Question)

Last summer, over 30 Cardinal Flower plants (Lobelia cardinalis) were effected by some sort of disease or insect problem in the garden. The garden bed that they were growing in receives half a day of sunshine. The soil is mainly composed of triple mix.

The problem started with one plant and gradually moved to the surrounding plants. I put down black plastic to contain the infestation but it continued all the same. The problem began with the leaves curling and changing to a lighter colour(see photo). The disease moved down the stem. Eventually, the entire plant would develop a whitish stem and fall over(see photo). The roots would become swollen/whitish and disconnect from the soil.

I have transplanted all the remaining plants to other areas were the Cardinal Flowers will receive more moisture. Do you know what may have caused this problem ?

I would like to re-plant that part of the garden with Bergamot (Monarda fistulosa). Should I remove the existing soil to a depth of 12-18″ to reduce a chance of re-contamination or was the problem species specific ?

Any of your thoughts would be appreciated.

Thanks for your help !

(Answer)

Dear gardener,

I have to tell you that you have us intrigued with this question.

Your plants were definitely affected by something and we think it may be Northern Root Knot nematodes (Meloidogyne hapla). The nematodes produce galls on the roots (look like knots) thus interrupting the translocation of water and minerals to the rest of the plant. The plants may appear stunted, wilt easily as well as showing signs of nutrient deficiency. The following websites give some information on the possible cause:

From the American Phytopathological Society: https://www.apsnet.org/edcenter/intropp/lessons/Nematodes/Pages/RootknotNematode.aspx. Revised in 2011

It is impossible to be 100% certain but this seems to be the closest disease according to your description above.

If you think that you have M. hapla in your soil, here are some of the natural solutions:

  • Eliminate current plants affected and do not dispose of them in your home compost.
  • Dispose of soil.
  • Build the bed with compost. M. hapla seems to thrive in sandy and warm soils. Adding compost to enrich the soil will cool down the bed and create an environment they do not like.
  • Your idea of the soil barrier may proof to be very efficient as it is believe that they migrate within the top 12” of soil.
  • My search found that  nematodes seem to dislike certain plants such as French Marigolds (Tagetes sp)
  • Practice plant rotation with non-host species (please refer to the following study from the government of Connecticut on more resistant plants https://www.ct.gov/caes/cwp/view.asp?a=2815&q=376930 (rating of 1.0 (-) are the best choices; rating of 4.0 (+) are the more susceptible). Also, there was another study by NC State University listing plants by susceptibility: https://www.ces.ncsu.edu/depts/pp/notes/Ornamental/nematodes/odin31_nematodes.htm Revised Oct 2000
  • Reduce plant stress by planting the correct plant with the correct cultural growing conditions. Lobelia cardinalis, for example, grows naturally in moist areas near streams, swamps and at the edge of forests, where the soil is humus-rich. Therefore, it is important to plant it in a location where plenty of moisture is available. However, I would shy away from planting it in your garden until you can be sure that there is not a problem. (Also be aware that this beautiful native is a short-lived perennial anyway, which self-seeds only under optimal conditions).

I hope I have not caused you any concerns and that we are getting closer to identify your problem. If you think that the above links do not describe your problem, please do not hesitate to let us know so that we can further pin point to the root cause (no pun intended) of the mortality.

Wishing you best of luck.