It has always cracked me up: ponies who get into clover and drool like leaky faucets. Until recently, I did not know the reason they drool is because of a toxic fungus that grows on the clover. (And I did not know its comical names: Saliva Syndrome! Slobbering Horse Syndrome!)
The Rhizoctonia leguminicola fungus, also called “black patch” because of the (sometimes microscopic) black spots that form on affected leaves, grows on white and red clover, and alfalfa. It produces the toxin slaframine, which can increase horses’ salivary gland output. In more extreme cases, it can also cause increased tear production and urination, difficulty breathing, and–rarely–even abortion. Some horses have have allergic reactions on facial skin that comes in contact with the fungus.
Horses that are more affected than others can either be especially sensitive to the toxin, or just prefer clover, so consume a larger quantity of slaframine.
“Black patch” prefers humid weather, so it is most prevalent in Spring and Summer. To control growth of the fungus, keep your pastures healthy. Clover thrives in stressful conditions, such as droughts, extended wet periods, poor soils, and over-grazing, so rotate your pastures to minimize stress. Use chemical treatments carefully to avoid harming your animals. Note that the fungus can also be present in hay containing affected clover or alfalfa.