An exceptionally wet summer weather pattern favors the growth of fungi on plants in the field and increases the risk for production of mycotoxins. Mycotoxins are naturally occurring compounds, some of which are toxic to cattle and horses, produced by fungi invading plant material. In the late 1940s and through the 1950s, beef and dairy producers and horse owners reported outbreaks of excessive salivation (slobbering) in cattle and horses associated with the consumption of second-cutting red clover (Trifolium pratense) and occasionally other legumes in the form of hay, pasture, or silage. It was not until the 1960s that “slobbers” was linked to a fungal pathogen, Rhizoctonia leguminicola, which is also associated with the forage disease black patch that occurs in pastures containing red clover. Slobbers primarily affects cattle and horses, although goats may also be affected. “Black patch” derives its name from the appearance of dark affected areas in the field and also the characteristic black or dark brown concentric or “target spot” lesions on the leaves of affected plants (See Figure 1). The dead, brown, diseased leaves and stems may be confused with normal maturation of red clover. Growth of the Rhizoctonia leguminicola fungus is most prevalent in second-cutting red clover hay or pasture associated with periods of wet weather and high humidity, temperatures between 75°F and 85°F, and a soil pH of 5.9-6.5. In addition to red clover, it has also been reported to infect white clover, soybean, kudzu, cowpea, blue lupine, alsike clover, alfalfa, Korean lespedeza, black medic, cicer milkvetch, and sainfoin; however, infected red clover plants are usually present in the same field. This fungus can overwinter on infected plants and survive at least two years on infected seed.

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This is part of the Forage-Related Cattle Disorders series.