Answer: It sounds as though your roses have both black spot and powdery mildew. A fungus causes black spot on roses. Some cultural practices may be helpful in controlling it. These include planting varieties resistant to it, ensuring your plants are in a location with good air circulation, avoiding wetting the leaves when watering, and cleaning up, removing and destroying any infected leaves. Do this especially well each fall to minimize reinfection from year to year. A clean layer of organic mulch (such as shredded bark or half finished compost or chopped leaves) applied before the plants leaf out again in spring should also help prevent reinfection. Chlorotic leaves (those with absence of color) indicate you need to feed more often. Fertilizing roses is a popular topic for endless discussion -- and one everyone has an opinion about! In general, roses do prefer a rich diet. The soil pH should be near 6.0 in order for the rose to be able to use the nutrients. Frequency will depend to some extent on what type of roses you are growing (and what type of fertilizer you use). Repeat bloomers such as hybrid teas seem to respond better to frequent light applications, while the older varieties of shrub roses are a bit less demanding and so once a year in early spring may be enough for them. You can use granular, powdered or timed release type fertilizer or, if you have the time and patience, the foliar spray type or even a combination; nitrogen is probably the most important nutrient, along with phosphorus and potassium. Some gardeners will also add Epsom salts, about a half a cup per plant per month, to provide magnesium. However, to really know what nutrients are needed in what quantity (too much is no better than too little) you should run some basic soil tests. The white powder is a fungus called powdery mildew. It usually attacks leaves when days are warm and nights are cool, and when there is inadequate air circulation and/or not enough sunshine. Prune away the affected plant parts to improve air circulation within the plant. Best wishes with your roses.
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