You may want to try watering with a solution of 1 part vinegar to 3-5 parts vinegar....not straight on the plant. Vinegar acting as a neutralising agent.
I found the following when I googled "neutralising raw horse manure" on the
www.tenthousandpercent.com website :
Understanding the Role of Manure
Manure can be a helpful and inexpensive addition to your compost batch. It’s important to understand the nature and role of manure before deploying it, however, to avoid potential serious problems.
-Manure is animal waste. As noted above, only cow (not steer or commercial) and horse manure are acceptable among mammal wastes. Chicken and other bird manure is also good for composting. Cow manure has properties as an antifungal compost additive and a unique complement of nutrients, and is therefore widely sought.
-Rule #1 : Never apply raw manure directly to your soil. This is a grave error made by many, from commercial farms to home gardeners. Direct application of raw manure to soil has the following negative effects :
-Raw manure contains numerous pathogens that will be passed on to your crop – potentially a deadly situation in the case of vegetables like leafy greens that grow low to the earth. One has only to recall the recent salmonella outbreaks to understand the gravity of this problem.
-Raw manure contains weed seeds that you likely do not want to introduce into your growing environment.
-Raw manure contains high concentrations of nitrogen that can cause ‘nitrogen burn,’ wherein the plant uptakes a harmful amount of the element and suffers what is in effect chemical burn, and ‘junkie plant syndrome,’ wherein a plant will get ‘high’ after the first application of raw manure, growing rapidly, and then ‘crash,’ wearing out and withering rapidly before producing a substantial harvest. Both of these effects substantially reduce your yield and biologically damage your plants.