Question
I am in zone 6. Last spring I planted a garden full of red double knockout roses (24 small plants), which bloomed fairly well and quickly increased in size by the end of the season. This year they bloomed profusely, but after a few weeks, all the roses became a strange color and the new buds rotted. I looked up the condition and found it was a fast spreading fungal infection called botrytis. As recommended at the garden center, I then removed all the infected flower heads and sprayed the new growth with an antifungal treatment. My question is how did this happen in the first place? My neighbor has had the same rose bushes for a few years with never any problems, so I doubt it's the weather extremes we've had this spring. Early in the budding stage, I put down a powdered fertilizer that was specially formulated for knockout roses. Could that have contaminated the soil? I just don't want this infection to happen again, so what should I do to prevent it?

Answer
Every garden, even if it situated next to another one, has a micro climate that is different in some areas. Cold nights and warmer days are ideal conditions for fungus to grow anywhere. Red is the worst colour for mildew and botrytis.
If the soil is damp, then the problem will spread quicker even though watering the roses would have slowed down the disease. Which sounds like a contradiction. Your roses could now be prone to this problem so spraying them when conditions are right would be a wisdom. Fertilizer wouldn't have any effect. Every garden at some point in time suffers from this disease and it really is nothing that you do and it is the weather that causes it. The fact that the garden next door was not effected may be because there was more air circulation or that the roses were not given as much fertilizer. Generous amounts of fertilizer will make for pulpy soft growth, which is far more prone to spreading the fungal spores.