Q.Toba Hawthorn tree
Assuming there is a disease like cedar rust, can I cut the branch now rather than waiting during the dormant season so it stops the spread? Our tree is matured just so you know.

Here are some links and articles that have information on care and common pests and disease of Hawthorn Trees.
https://www.gardeningknowhow.com/edible/fruits/apples/cedar-apple-rust-control.htm
https://www.ipm.iastate.edu/ipm/info/plant-diseases/cedar-hawthorn-rust
https://extension.psu.edu/pests/plant-diseases/all-fact-sheets/hawthorn-diseases
https://www.gardeningknowhow.com/ornamental/trees/hawthorn/growing-hawthorn-trees.htm