Clover Root Curculio Injury and Vascular Decay of Alfalfa Roots1

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Abstract
In an attempt to evaluate the importance and possible Interrelationships of insect, fungus, and nematode injury to the deterioration of alfalfa roots under field conditions over a period of 5 years, an insecticide, a nematicide, and a general soil fumigant were applied singly and in combinations to the soil prior to seeding. Root injury caused by larvae of the clover root curculio, Sitona hispidula (F.), was significantly reduced in the insecticide-treated plots. and there was more vascular decay in larval-injured roots than in roots free of insect injury. The nematicide did not significantly reduce populations of Pratylenchus negletus. Rensch (= P. minyus Sher. & Allen) so the possible relationships between nematode injury and vascular decay could not be determined. Winter injury of the crowns occurred 1 year and possibly afforded entry for vascular decay pathogens. The fungi most frequently isolated from roots were: Fusarium oxysporum Schl. sensu Snyd. and Han., a species of Pyrenochaeta, Phoma medicaginis Malbr. and Roum., and a sphaeropsidaceous fungus. F. oxysporum and the sphaeropsidaceous fungus were more frequently isolated from roots showing symptoms of vascular decay than from symptomless roots. All species were pathogenic on alfalfa seedlings. A study of the vertical distribution of larval injury showed that roots were most severely injured in the upper few inches of the root, but that injury did occur to a soil depth of 28 inches. In spite of the pressure of the biotic agents on the roots of alfalfa during this study, no significant plant mortality in untreated plots, no significant difference between treatments, and no significant difference in yields of forage occurred. It was concluded that alfalfa is an extremely hardy plant capable of withstanding the biotic agents that can cause mortality to alsike clovers under similar conditions.

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