There are two ways of looking at the difference between young seedlings and adult plants for resistance. One is a general overall comparison between a whole seedling bed compared to a whole adult plant bed. The other is a comparison between the resistance of an individual seedling, and the resistance of that same seedling when it has become an adult plant. The interesting thing would be the latter, if there actually is a difference. Such comparisons can be totally confounded, though, because of the total dependence of rust on environmental conditions.
For example, say a seedling bed is overcrowded, gets more shade, gets irrigated more often and so forth compared to adult plant bed. In that case you may see a difference just because of that because environmental conditions influence the severity of the rust. The same may even apply to the individual seedling. So what you would need to do for the latter is rate it while it is still growing in the same spot. As soon as you change the environment you potentially also change the severity of the rust. You may line out the seedlings with more space than they had between them in the seedling bed and that could influence the amount of rust. There is also the difference from year to year. If the seedling's first year is a year where the weather or other conditions are conducive to rust, but the next year is not, it may appear that the seedling has become more resistant.
That's not to say there can't be a difference between seedling and adult plant resistance because it does occur in other plant rusts. But we also have to take into account other influences. The amount of rust is dependent on the environment and even the most susceptible cultivar will not get much if conditions are not conducive to rust. You could totally slather a highly susceptible cultivar with rust spores but if you keep it in a dry environment, for example, it won't get rust.
