Calcium for rust-prevention in roses
I first read about calcium's role in preventing rust in Texas rosarian Field Roebuck's "Complete book on roses". I also found a government abstract on the role of calcium in rust prevention.
Here's an excerpt from the link below "Results indicated that plants growing in more stressful low-calcium soils experienced higher rates of rust infection, suggesting that soil calcium may modulate host susceptibility in a manner opposite to that predicted by the a priori hypothesis."
Here is a link that might be useful: Effect of soil calcium on fungal infection