I guess Texas never gets cool enough at night for powdery mildew to become a problem on roses. It felt like that when I lived there, especially the year without air conditioning.
It's an interesting point about blackspot. My understanding was that it did not set in unless the temperature was pretty close to 90F. Much below about 80F, I don't think it strikes at all. My recollection of Texas weather is that springs tend to be damp in comparison to summers. In Austin, where I lived, the RH hovered around 50% through the daytime for most of the summer. And rain was pretty scarce. Of course, if humidity rises far from 50%, things can be very different. Not sure what the threshold is, though. I guess black spot has a high temperature limit - a temperature that kills it dead or at least keeps it from spreading. Perhaps that is near 100F.
In NJ I had downy mildew in the spring when the weather was cool and damp. It cleared up over the summer. So I think it's more a cool season fungus. I confused black spot an downy mildew with each other and am still not sure I could always tell them apart.
All three fungal diseases need high humidity. Not sure what that means, exactly; but certainly if the RH remains below 50% through day and night that's plenty dry enough. The common factor is humidity, but the three fungal diseases I've dealt with strike under different temperature conditions. Downy mildew is really a problem in cool, damp weather. In NJ April showers brought... downy mildew in early spring. It was generally gone - or at least it had stopped spreading by mid May. Powdery mildew is a problem in slightly warmer weather so long as the high temperature does not exceed 90F for long. It seems to strike more where there are larger temperature swings between day and night. Finally, black spot becomes the king of rose maladies when the temperature at night does not dip below, say 80F. In NJ that was all summer long. Interestingly, I don't remember having any problems with disease when I grew roses in Austin. Perhaps it was just dry enough. Or perhaps it was beginner's luck.
Another factor that influences which fungal disease prevails is the amount of residual spores. My guess is that the amount of residual spores just sitting there waiting to infect roses given the right conditions depends on how frequently favorable conditions prevail. Since it always dips below 80F at night here, and since for 10 months of the year the RH is never above 40%, my guess is that there's not many black spot spores floating around here. By contrast, every year in my garden I see powdery mildew on clover during the monsoon. Last year it struck some crowded dahlias but not the roses. So in my garden here powdery mildew is just sitting there waiting for rose vulnerability and the right conditions.
