My rosemary plant has developed a white powder on about 30% of the leaves. Does anyone know what it is, and what I should do to remove it?
White powder on rosemary, what should I do?
Most probably this would be mildew. Give your plant a good "washing" and wait till it gets dry. Then spray it with a solution of 1 part milk on 9 parts water. Repeat this every time it has rained. After a couple of weeks the mildew should be gone. Of course you can also get a deterrant at the garden center.
To make sure the fungus doesn't strike again, you can feed your rosemary some nettle or comfrey tea every couple of weeks. This way it will become stronger, healthier and it will be better able to fight off diseases. Also you should prune out some branches to get some air into the plant. Mildew often is the result of not enough air circulation.
The powdery mildew seems to be caused by excess humidity. I don't know anything about your growing conditions. I've got my rosemary in extremely sandy, thus very well drained, soil in full sun down here in Savannah and it thrives. Is it possible that you're being "too kind" to your rosemary by giving it a soil that's too rich and holds too much moisture?
Knocking on wood, this is not a problem I've had. Odd though, I just read an article on this. They did say it will develop if the air is humid and not enough air circulation. Not to overcrowd plants. Allow the soil to dry out and keep it in sunlight. Mildew won't kill it but can weaken it. Mine is planted where the soil stays quite dry and in our humid climate, I've not run into this. I suspect water ~ have you had lots of rain?
Podster, we've been in drought conditions until just the last week when we've had 3 huge thunderstorms, complete with a few small tornadoes. Even if we get drenching rains the primarily sand that my rosemary is growing in drains instantly. Also, these plants are in FULL sun all the time. There's no tree anywhere near them, and the shadow of the house moves to the back and the rosemary is in the front, trying to take over the front walkway.
I'm sorry ~ I was asking Widgetdog if they've had much rain, possibly the cause. Mgpaquin ~ your conditions sound much like this area of TX with the exception of the tornadoes ~ thankfully! I do agree tho, my soil is sandy and spare. Widgetdogs' soil may be too good...
Never heard of mildew on rosemary but UMass reported this week it is showing up on the herb on Cape Cod:
http://www.umassgreeninfo.org/landscape_message/landscape_message.html
2007 was a great growing year in CT. Bright sunny days. Tomato crops were twice that of wet and rainy 2006. But mildew (for me) in 2007 was terrible. Lost cukes, squash and zinnias to it. Wet conditions may make it worse but it can strike hard in the best of weather too.