|roses - 10 plants , must prevent leaves from turning yellow and all falling off in august.|
|Based on your description I am not certain what is happening to your roses in August, although it may be black spot, a fungal infection that is quite common on roses in your area.
This type of problem can be fought first with good sanitation. It is helpful to strip off any overwintering foliage and dispose of it in the trash, rake away any fallen leaves and also replace the top layer of mulch at that time. Next, always remove prunings from the garden and put them in the trash. Cultural methods to control would include: make sure your plants are pruned to allow good air flow through the center of the bush, and that they are planted in a full sun location with good air circulation. Avoid watering in the evening, and avoid wetting the foliage when you water. Some gardeners will also use a dormant season spray containing sulfur. All of these steps can help reduce the reinfection rate.
Another important step is to select rose varieties with known resistance to black spot; some roses are simply far more prone to it than others.
I hope this helps!