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by Kyle
(Corpus Christi, TX)


I have 3 Mendevilla plants growing on a Trellis on my deck that the older leaves are yellowing with brown spots, quickly. Need help on how to treat this.

Comments for Mandevilla Problem

Sep 02, 2021
by: Jacki Cammidge, Certified Horticulturist

This is not exactly a leaf problem, but a weather issue. The yellowing is most likely a soil problem, specifically the pH of the soil.

So, we’ll talk about the leaf spots first. This looks like scorch, from really intense sun. If you’ve had weather anything like what I’ve experience this summer, you could have had a period of intense, hot sun. This is the result. I’m not worried about the health of the plant, as there are lots of other undamaged leaves.

Which brings me to the yellowing leaves; this could be caused by the plant being unable to access the Nitrogen in the soil. But don’t go applying more!

What I think has happened is that the pH is too low, and to enable the plant to get the nutrition it needs, you need to raise the pH. This is done, slowly, by adding small amounts of Dolomite lime to the soil in the root zone. As it’s getting to be fall now, you don’t really want the plant to go into an active growth spurt, just to make the leaves green again, right?

In the spring, you could start using a water soluble fertilizer with micro nutrients in it, weekly, weakly, or mulch with compost. This will set it up to rebound nicely.

Keep in mind that it may lose the damaged leaves (the ones with scorch) but it will replace them when it recovers from the nutrient problem.