Blackpatch of Forage Legumes and Slaframine Intoxication

By Mark Sulc and L. H. Rhodes et.al

Recent concerns and management questions have arisen from equine owners and managers about clover slobbers. One of the causes of excessive salivation in horses is a fungal disease of clover, commonly referred to as “blackpatch”. This pathogen infects legumes (especially clover species) when temperatures exceed 80F with humid/wet conditions, like we have been experiencing in Ohio. Here we are re-running an article about blackpatch  written a number of years ago by Lanny Rhodes, Emeritus Professor of Plant Pathology at OSU. Unfortunately, since that article was written we have not seen the marketing of any fungicides labelled for use in clovers that could alleviate this disease in our pastures and hay fields containing clovers. So management options continue to be limited to not feeding clover containing pastures or hay during periods of disease infestation, or killing the clover from the pasture and hay stands to reduce the occurrence of this disease.

Blackpatch is a disease of red clover and other forage legumes caused by the fungus Rhizoctonia leguminicola. When severe, the disease can cause death of leaves and stems, giving them a blackened or scorched appearance.  As the fungus moves from plant to plant by growth of its filaments (hyphae), the affected areas of plants may reach several feet in diameter.  These large patches of dead foliage give rise to the name, ‘blackpatch.’  However, the most important aspect of this disease is not its affect on plants, but rather the potential harmful effects that infested forage or hay can have on animals.

Effects on livestock.  In the infected plant Rhizoctonia leguminicola produces a mycotoxin known as slaframine.  Animals grazing on infected plants or being fed infested hay are subject to poisoning by this toxic compound.  This is known as saframine intoxication (or slaframine toxicosis or salivary syndrome).  The most common sign of slaframine intoxication is excessive salivation. Most animals are observed with copious amounts of saliva dripping or running out of their mouth, giving rise to the common name for the syndrome, ‘clover slobbers’.  Affected animals may have difficulty in swallowing all of the excessive saliva. In addition, animals may show excessive lacrimation, diarrhea, and frequent urination. Cattle, sheep, horses, llamas, alpacas, and goats are all affected after ingestion of this mycotoxin, with horses being the most sensitive and the most severely affected.  Excessive salivation can be observed as soon as 4 hours after exposure and will remain as long as 24 hours after removal of the contaminated forage in the pasture or after removal of the hay.  Usually animals will experience an uneventful recovery if the contaminated forage or hay is removed. Only in unusual cases is treatment required. When animals are affected and are in weather such as temperatures in the 90s and high humidity, it is even more important that adequate amounts of fresh water be available. Electrolyte replacement may be considered if excessive salivation has been observed for several days.    

Geographic range and plant hosts.  Blackpatch, first reported from Kentucky in 1933, occurs throughout the eastern and midwestern U.S.  The disease is more prevalent in the southeastern U.S., where prolonged periods of warm, humid weather are more likely to occur.  Red clover (Trifolium pratense) appears to be the most susceptible species.  Blackpatch also occurs on other Trifolium species such as white (and Ladino) clover (Trifolium repens), subterranean clover (T.  subterraneum), crimson clover (T.  incarnatum) , and alsike clover (Trifolium hybridum).  Other hosts include white sweetclover (Melilotus alba), yellow sweetclover (Melilotus officinalis), alfalfa (Medicago sativa), black medic (Medicago lupulina), Korean lespedeza (Kummerowia stipulacea), kudzu (Pueraria lobata), and soybean (Glycine max)  

Symptoms in plants.  Infestations of R. leguminicola in pastures are often difficult to detect.  In the advanced stages of the disease, circular or oval patches of affected plants can be seen in the field.  Within these patches, which may be a few inches to several feet in diameter, leaves and stems wilt and eventually become blackened.  Individual leaf lesions have concentric light and dark brown rings, giving them a ‘target’ appearance.  Lesions occur on both leaves and stems, with most stem lesions developing near the base.   Overall the plants have a blackened appearance.  This discoloration is due not only to death of plant tissue, but also to the growth of the causal fungus on stem and leaf surfaces.  Close inspection of affected plants reveals a weblike growth made up of tiny dark filaments.  When the infestation is severe, the intertwined filaments cover the entire stem and may also be found in blighted flower heads.  Seeds developing in these infected flower heads usually become contaminated with the fungus, and thus provide a means for introduction of the pathogen into newly planted fields.  Short-range dissemination of the fungus occurs by means of mycelial growth or by movement of infected plant parts carried on mechanical harvesting equipment or grazing animals.

Losses.  Although mature clover plants are usually not killed by the fungus, forage and seed yield can be significantly reduced.  Also, both pre- and post-emergence damping off may occur when the fungus is present on the seed coat. 

Control.

• identify blackpatch and exclude animals from affected areas
• remove affected animals from affected pastures – feed unaffected feeds
• exclude legumes from pastures
• remove legumes from pastures
• wait for cool and dry conditions.