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This plant has been with us for 3 years. In the last month I have notice this white powder on the leaves and is loosing a lot of stalks. Do you know what is it and how to treat this plant?

A little hard to tell from the pictures; I'm not even sure what your plant is.

I started to say white powdery stuff on plant leaves is frequently a fungal disease called powdery mildew. It thrives in conditions of high humidity and low air circulation.

However, when I looked at your pictures close up, especially the first one, it looked less like a thin powdery coating and more like discrete lumps. It looks to me like it might be insects, like aphids or whiteflies. Does it move if you touch it?

No, it doesn't move. Is just like a powder.

looks similar to the powder like substance I have on the lower leaves of my summer squash plant. The advice that I found was to remove the leaves with the powder on it or it will spread.

Squash are very prone to a fungal disease called powdery mildew, which looks greyish white and sort of powdery like this:


https://www.furneysnursery.com/wp-conten ... mildew.jpg

In that case you should remove the affected leaves and treat the rest with some kind of anti-fungal like milk solution, baking powder solution or hydrogen peroxide.

But the powdery mildew isn't an actual powder, it is more in the leaves than on them (although it can be wiped off).

OP's pictures (especially the middle one) look more like lumps, which is why I wasn't thinking powdery mildew, although it could perhaps be powdery mildew AND aphids or something.