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I have a recipe that calls for 2 tablespoons baking soda, 5 tablespoons vegetable oil, and 1 teaspoon Castile soap. Mix with 2 gallons of water. Spray on tomato plants to control blight.
Can I use the same recipe with Dawn Liquid Detergent instead of Castile Soap?
I wouldn't, though I admit I do not know the function of Castile soap in your recipe. It might be that its active ingredients are an essential part of the treatment: (saponin and/or oleic acid).
Castile soap is a vegetable product (mostly) - was distinguished by use of olive oil - but detergent is full of some nasty chemicals, including those than can cause cancer. While the amount required is small it is to be applied too high up the food chain for comfort. 'Dawn' is specifically mentioned by David Wolfe.
An alternative solution (but not tried by me) is the three inch copper wire one, with a video about it here: How to organically control tomato blight (YouTube).
I gotta give you some very bad news. Once tomatoes get blight there is no treatment. Nope. Not at all. One spore, splashed onto the plant via water will be the end of that plant...doubt any other tomato plants will not get the same fungus. It is so sad. The only fungus that is treatable after infection is powdery mildew. I've learned a while back the hard way losing 75 feet of healthy, vigorous, 4' tall, tomato plants laden with tomatoes. One day I found a few blackened, shriveled leaves. 3 days later every single plant was dead. I had collected all the tomatoes I could that first day but every single tomato blackened.
The only treatment is fungicide before the plant is infected. Fungicide is analogous to a raincoat protecting the plant from blight, fungal spores splashed up off the soil onto the plant when it rains or with overhead watering. Once a plant is infected it is systemic and there is nothing you can do.
So one of my projects for master gardener classes was tomatoes and blight. The best way I found was to prune up leaves from below to keep them away from the soil and make a mini hoop house. Water them by drip lines. Those spores are everywhere especially where tomatoes were grown for earlier seasons. I had a virgin garden, far from any garden and my plants were still infected.
Tough lesson to learn. Seriously, once a plant has been infected with just one spore that plant is done. There are no other treatments. Crop rotation is important, adding manures and composts are a great infection vector, you have no idea what comes with them from other gardens.
To this day, I plant my tomatoes in pots with sterilized potting soil. I drag these 5 and 10 gallon pots in doors if they are outside when it rains. In a greenhouse I only water the soil no overhead watering. Major fans blowing 24/7. I've never had another blight since.
Go ahead and try these other hummmm, recipes. Can't hurt. If your plants continue to look worse the best thing is to pull them up and put them in a burn pile after allowing them to dry under plastic. Won't prevent spreading spores but there isn't a more responsible way to dispose. Rotate your crops. No peppers, potatoes, eggplant in the same soil tomatoes were grown and vise versa for at least 2 years. Even if there was no sign or problems with blight. Other vegetables as well, rotation is a hard and fast rule. I love planting in pots in sterile soil as it leaves my beds open for more plants and I can be fairly sure no blight spores will be in the potting soil. I am sorry.
