Composted some infected leaves before I knew what it was. Can I use this compost in pots and containers without compromising the health of the plants, and/or spreading the disease
Your question is very timely, thank you, as composting plant matter is becoming increasingly popular. As a generally-accepted rule of green-thumb, when discarding plant material that has evidence of fungus, spores, disease, or an insect pest, or even contains seeds that you might consider weeds, and to avoid inadvertently serving dubious plant matter to other plants, it should simply be disposed of in a separate bag, in the garbage. This applies to your personal, as well as any municipal compost.
In your particular instance, and to re-cap for the readership: rheum rhubarbum, known as rhubard, is a perennial plant that propagates with big, fleshy rhizomes. It is of the Polygonaceae family, and produces large, somewhat triangular, poisonous leaves. However, the long, crunchy (celery-like) leaf stalks are very edible, with a strong, tart taste. Even though it is a vegetable, for culinary uses it is treated like a delectable fruit.
Due to the toxicity of its leaves, rhubarb has very few pests. However, rhubarb is prone to developing brown spots on its leaves, first seen as yellow patches that turn brown, or small red dots that enlarge to about 2 cm.in diameter, sometimes called ‘rust’. Both of these conditions are produced by spores. This can usually be prevented by good housekeeping, including keeping your growing area well drained, and clean of plant or weed debris, and above all ensure there is plenty of sun and good air circulation. Also, an older rhubarb patch can also start to develop brown spots once it is 8 to 10 years old, so it may be possible that all you need to do is divide your patch — and share with a friend! As a matter of further interest, here is a link to another gardener’s question regarding growing rhubarb: https://www.torontomastergardeners.ca/askagardener/rhubarb-not-growing-well/