Posted: 09/07/2012 at 15:58
No, spraying for the deficiency won't hurt. I think it's about 20g ES per litre of water. Spray in the mornings to let the foliage dry out during the day.
Spraying against fungal problems has become a tad controversial these days. Most of the common fungicides are chemicals and many are reluctant to use them and with good reason. They poison foraging insects amongst other things.
The most common organic treatment is a copper sulphate spray. Very popular here in Italy. You see tom plants everywhere with bright blue leaves. But, while it's organic, it's also a metal, it can eventually build up in the soil, and doesn't do foraging insects much good either. I don't use it because I don't like metal building up in the soil.
The very best treatment I ever came across - and used when I was back in Australia - was a chlorothalonil-based spray. It comes under various brand names - Daconil, Bravo, etc. It's synthetic, a chemical, but harmless to foraging insects. It will kill fish if you pour it into a fish pond. So I didn't pour it into any fish ponds. It's very widely used in the US - the home of heirloom tomato growing - and even by many organic growers. They acknowledge that it's not organic, but (a) it doesn't harm the garden wildlife; and (b) it's stunningly effective.
Problem is, it's hard to come by outside the States for domestic use. In fact, in Australia, it was only available in bulk for farm use. So a group of us tomato growers all chipped in, bought a container, and divided it up. It worked brilliantly.
A week or so ago someone posted in this forum about spraying with milk against fungal problems. Some people do, and swear by it, but there's no scientific evidence that it works. At best, it's thought that the milk might amend the pH of the leaf surface to a figure less sympathetic to the spores.
The principle of spraying against fungal problems isn't that you kill the spores. You can't. You spray preventively - before the spores arrive - and coat the leaves (on both sides) efectively stopping the spores getting a grip. Normally you would start spraying just after planting out and about once a week thereafter. If it rains, obviously, you have to respray. It's not always failsafe - though I found the chlorothalonil was - but it's the best you can do in terms of taking positive action.
I don't spray here in Italy but only because I can't get chlorothalonil here. I just do all the basic housework and hope for the best. I get Early Blight and Septoria Leaf Spot - the two most common fungal problems in the home garden, and they're pretty inevitable - but nipping off affected leaves as soon as the symptoms appear keeps things under control.