When I prepared my soil this year, I placed horse manure on it and after a month or so, I tilled it under. Then about another month or so, I planted my tomato plants. They were doing well but within the past couple of weeks, the new growth doesn't look well - after searching on the internet, the plants look as if they have leaf curl. I visited the office of our county's extension agent yesterday (he wasn't in) and left several cuttings from my plants for him to look at and advise me of my problem and if there is a remedy for them. I haven't heard from him yet so thought I would ask you. Also, while I was there, an office assistant said that it kind of looked like a chemical burn, which led me to think that the horse manure is my problem. Several websites I have looked at this morning suggest using cheap mulch or sawdust around the plants (not touching them) or doing some intense watering for 3-5 days. Any suggestions? Can applying a solution of vinegar and water (1 cup vinegar to one gallon of water) help to neutralize the nitrogen content?
Probable Leaf Curl and/or Chemical Burn
As I suspect you've been discovering, horse manure (or any manure) needs to be composted before being used on the garden. Here's an article on using horse manure: https://www.gardeningknowhow.com/composting/manures/horse-manure-compost.htm
There's no real "antidote" other than lots of water to wash out the extra nitrogen. I feel uncomfortable with using sawdust etc, because part of what you want to happen is to have air and water move freely through the soil.
Of course, if the agent gets back to you with a diagnosis of insect or pathogen, the manure becomes a moot point.