GAR,
I would imagine that this is a fungal issue. In a lot of nurseries, they bunch the plants together very close. This does not allow for good air circulation between the plants and often leads to fungal problems.
What I have found in practice that works the best to combat fungal issues is cornmeal tea (juice). This is very simple to make.
Day one: Get a 2-3 gallon bucket, fill it with 1 gallon of water and put it in a sunny location until the next day.
Day two: Put 1 cup of cornmeal into some old hose, tie it and drop that into your now chlorine free water. (That is why you let the water rest for a day, the chlorine evaporates off; use this for compost tea brewing and applications too).
Day Three: Check the solution, if it is milky in color, it is ready, if not, move to day four, odds are it will be ready though. If it is ready, put it into a spray bottle, full strength and spray all the surfaces of your plant in the morning only. Repeat this every few days until you see the fungus hit the road.
How does it work: the cornmeal tea feeds another type of fungus that preys on the fungus that is attacking your plants. Increasing the colonies of the beneficial microbes to the point that they are the predominant microbial life and not leaving much room for the others. Aerobic compost teas also helps these colonies grow, giving the plant a natural defense against disease.
