Farmwind.........
Even disease resistant roses can blackspot if conditions are right. Bs spores are inactive after temps reach 85 degrees. If the temps stay anywhere below that, a rose is more susceptible to disease. In my climate, the norm is that once it starts to warm up, day temps start increasing by about ten degrees per week, then boom, the heat hits and temps are in the high nineties to low hundreds for the next several months. Sometimes, dropping down to the low nineties. Always with a forty to fifty degree difference between day temps and night temps.
If there are six hours of temps no higher than eighty-five degrees, the bs spores can become active and infect the foliage, but with the high temps during the day, those spores are fried and do not truly harm the foliage. If I have a long, wet spring, the roses that have a propensity to bs, will get it. If they do not re-foliate in time for the high summer temps, then they are the wrong rose for this garden. I am not going to spray.
This year, the day temps stayed in the high eighties and low-to-mid 90s most of the time, with a week or a little more than a week in the usual high 90s and low 100s, so I had more bs in my garden than usual.
I have heard for years that HTs are more disease prone, but in my garden they have had the most resistance to disease, even this year. So, for me, this class of roses is the best class for me to grow because many of them have the heavy, thick petal substance that can handle the heat and not fry as much as the roses in other classes.
I also prune to encourage denser foliage because I believe it helps the roses handle the heat better and serve to pull moisture up to the top of the plant, which in turn keeps the blooms from frying as easily.
So, since temps were lower this summer than the norm, I had more bs in the garden than I have seen in several years.
Right now, temps are in the usual high 90s and low 100s ... but not for the same number of hours per day as earlier in the year.
So even buying disease resistant roses, will not guarantee that you will have no bs in your garden every year, but it certainly helps.
There are other variables that can make a rose more disease prone, but that is for another post.
Smiles,
Lyn
