My roses are prone to blackspot. I have heard that spraying Jeyes Fluid into the soil helps to prevent this problem. Is this true and, if so, when would be the best time and method of doing this?
This practice used to be recommended but not any longer. Jeyes Fluid is not recommended for use on soil and the practice would have relatively little effect on black spot.
The best way to treat susceptible plants against blackspot is to spray from budbreak with a rose fungicide such as Rose clear.