Thank you for reaching out to ask2.extension.

Rhubarb leaves can turn yellow for a number of reasons, including:  A lack of essential nutrients like nitrogen, magnesium, or iron can cause yellowing leaves, often with specific patterns;  Pests that attack the foliage or roots of plants, such as aphids, red spider mites, whiteflies, vine weevil, and cabbage root fly, can also cause leaf yellowing Overwatering can leave your plant's roots gasping for air, resulting in yellow leaves;  Poor drainage can also turn leaves yellow; Soil pH The level of acidity or alkalinity of the soil could be a problem. Vegetables generally prefer a fairly neutral soil pH;  Yellow leaves can appear when the plant receives too little sunlight, and with partial shade varieties, too much sunlight.

My suggestion would be to start over with new rhubarb plants.  Yours may have a disease that it can’t come back from.

This hasn’t been a great year for a lot of things.

I hope this helps.