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Rhubarb plants can become quite tall and wide around. Maybe dividing and relocating the plant to another area of your garden would work for you.

https://www.gardeningknowhow.com/edible/vegetables/rhubarb/dividing-rhubarb-plants.htm
https://www.gardeningknowhow.com/edible/vegetables/rhubarb/harvesting-rhubarb.htm

There are many different varieties of Rhubarb. Some are green stemmed, some pink and some red!

https://www.gardeningknowhow.com/edible/vegetables/rhubarb/types-of-rhubarb.htm

Make sure that you have plenty of drainage holes in your container. This looks like a fungal infection which will be caused by the soil remaining too wet for too long. I would apply dolomitic lime and wettable sulfur to the top of the soil. This will kill off the infection, while conditioning the soil back to a more fertile condition for the plant.

Keep in mind that the old leaves may not recover completely, but the spread of the infection will stop.

This article will give you more information on the care of rhubarb: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm

The insect pests on your plants appear to be psyllids, as sucking insect similar to aphids. Insecticidal soap spray will knock them down and 70% neem oil spray every two weeks should prevent a resurgence.

The best time to plant plumeria seeds is in the spring. This article should help:

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm

Some rhubarb types do not change color. It has nothing to do with the manure.