I have a small rhubarb plant and one of the stems has gotten some rust on it. What should I do? (there are only 3 small stems on the plant)
Q. Is there a reason your rhubarb is standing in water?
Rhubarb prefers even moisture during the growing season (but does require well-drained soil), it most definitely does not like standing in water...
If it has been standing in water for a while, I would hazard a guess you're seeing a form of fungi (disease) brought on by crown rot.
Unless you create a better growing environment for your rhubarb I believe you will be forever fighting a losing battle...
Q. Can you grow them an area that gets some shade, preferably somewhere that gets morning & early afternoon full sun, then shade for the remainder of the day?
Or set something up to provide that kind of environment eg
Also rhubarb prefers soil with good organic content, which by its very definition means the soil will have good drainage...
Even in your climate rhubarb should only need approximately 2inches (50mm) of water a week, assuming you take care of the other requirements...
If you really want to grow rhubarb I think you might be best building yourself a raised bed:
Do you have just spots, or do some of those spots have holes?
If there are holes, then according to "The Organic Gardener's Handbook of Natural Pest and Disease Control", another possibility is rhubarb curculios: yellowish gray, powder covered, 1/2-3/4 inch snout beetles. They eat into the stalks to lay eggs.
If there are no holes, then Mike Perry's diagnosis of fungus is probably correct -- you need to correct the culture.