The causes are both weather and plant health.

The four biggest exterior sources for black spot to thrive are: crowding (air circulation), heat, humidity and wet weather. Excessive heat weakens a bush's resistance to black spot. Wet weather and excessive heat fluctuate from year to year causing black spot issues to respond accordingly. Therefore, every year is different in this regard.

The health of the rose bush is something we have direct control over. A vibrant, fertile, microorganism rich, nutrient and pH balanced soil with a strongly disease resistant rose variety growing in it, given good management (weed control, adequate watering, correct pruning, etc.,), are the ticket for success.

Inherently weak growers are usually black spot magnets.

An A.G.R.S. trial, A.R.T.S trial, or an ADR award winning rose should fight black spot more aggressively under bad weather, bad soil, and bad care conditions, than those bushes not having such awards or roses known to be disease wimps by means of common knowledge shared among rose gardeners.

Moses.

Moses gave a very thorough response. I would only add plant maturity. the

more established roses generally having better resistance. Enjoy them! Clean roses are a rarity for me at this time of year!

Yes, important point, Markay. First year black spot issues are not a reason to grab the shovel. Two or even three years of trialing a rose is recommended. Most organizations like ADR trial for three years.

Moses.