"White Haze" on Rosemary

Do you have any suggestions on growing rosemary, conditions / fertilizer etc. I had a plant that developed a "white haze" on the leaves – it was perfectly healthy for the longest time, but once this "powdery haze" appeared on the leaves and plant, it deteriorated to the point I threw it away. My boyfriend has grown some rosemary from seed and they were doing very well, but is now experiencing difficulty because the plants have noticeably weakened. Is there a secret to growing healthy rosemary? I've seen pictures of the bush growing to 6ft tall, but have never had success like that. Any suggestions you have would be appreciated.

The "powdery haze" is very likely a fungus disease called powdery mildew. It is treatable using a baking soda, oil and soap spray you can make at home. The recipe is: 3 tablespoons baking soda, 2-1/2 tablespoons horticultural oil, 1/2 teaspoon liquid soap mixed in 4 litres (1 gallon) of water. If horticultural oil (also known as summer spray oil) is not available use a light vegetable oil, but try it out on a few of your plants first. Spray twice a week until the problem clears up.
Your boyfriend's seedlings may not be getting enough light. When light is lacking, plants will become weak and spindly. They are elongating in an attempt to get closer to the light. Tell him to move the seedlings to a brighter window, preferably right in the windowsill of a south-facing window. When the danger of frost is past, consider moving the plants outdoors. Even if you keep them in pots or in a windowbox, take advantage of the stronger outdoor summer light. If your boyfriend does not have a brighter window supplementary artificial light may be necessary.