Posted: 27/07/2012 at 10:00
Is it too late to take a photo of the symptoms, Dipadee?
As I've posted before, it's impossible to avoid fungal spores, the pesky little things that cause these problems. They're invisible, they travel in the air, and they are everywhere.
Preventive spraying is probably the best means of preventing infection. The spray coats the leaves, creating a barrier between the fungal spores and the leaf surface, preventing the spores getting a grip and doing their damage.
Traditionally, the most common spray has been copper sulfate-based and you'll find it under various brand names in any garden centre. Technically it's organic because copper is a naturally-occurring substance. On the other hand, it's a metal, and some growers worry about a build-up of metal in the soil when the sprays drips to the ground.
Spraying is preventive, meaning it has to be undertaken before the spores arrive. It's no use spraying after the spores have arrived - that is, once fungal (or bacterial) symptoms are showing. Usually you start spraying a couple of weeks after planting out and continue to spray every week or 10 days. If it rains within that timeframe, you have to respray to recoat the leaves. Importantly, you have to spray every leaf, and both sides of every leaf.
Preventive spraying doesn't guarantee 100% that you will be fungus (or bacteria) free, but it gives you a huge head start. The only alternative to spraying is very diligent housekeeping - at least 3' between plants to aid air circulation, judicious pruning of branches and leaves to avoid great clumps of leaves which hinder air circulation, and keeping the foliage as dry as possible. Damp foliage is a fungal spore's playground. Finally, removing the lowest branches to keep a gap of at least 1' between the lowest foliage and the soil will help against the spores - that fall from the leaves to the soil - splashing back up onto the leaves when watering.