is the trellis flush to the wall. It might be worth moving it out abit so the trellis sits at least 1" from the wall allowing air to circulate. this will reduce the incidence of mildew. Cant think of any specifically though Cathy. sure some one else will though.

The rose you can see in my picture here must have been climbing round the front door for at least fifty years. I do nothing to it at all and have never seen a speck of mildew on it. Only problem is, we have only been in the forty years so I have no idea what it is!

Powdery mildew is also caused by the roots being drier than the air so i wonder if the area these are planted in is either - a dry soil spot or planted too close to the wall.
They should be at least 2' away from a wall/solid area and then planted at an angle that tilits/slopes the plant towards it - this allows the roots to drink up as much moisture as possible.
Roses and clematis are best done like this because they'll both suffer powdery mildew otherwise.

I tried to grow a rose and a clematis in too dry soil and they mildewed very quickly, the roots were okay but the upper area wasn't happy at all. I mulched well and watered well in dry spells and both were fine after that.

Shade also encourages mildew, so if it doesn't get at least 6 hours of direct sun there, you may want to find something other than climbing roses to go there.

Very usefull,thanks all.