Prevalence, Severity, and Association of Fungal Crown and Root Rots with Injury by the Clover Root Curculio in New York Alfalfa. D. W. KALB, Research Support Specialist Department of Plant Pathology; Cornell University, Ithaca, NY 14853-5908. G. C. BERGSTROM, Associate Professor, Department of Plant Pathology, and E. J. SHIELDS, Associate Professor, Department of Entomology, Cornell University, Ithaca, NY 14853-5908. Plant Dis. 78:491-495. Accepted for publication 7 February 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0491.

A survey of 61 randomly selected alfalfa fields in four physiographic regions of New York was utilized to assess the incidence, severity, and fungal flora associated with crown and root rots, as well as the incidence and severity of injury by the clover root curculio (Silona hispidulus) and its possible association with root diseases. Five stratified random subsamples, each comprising four to nine alfalfa plants, were evaluated from each field. Crown and root rot occurred in every field surveyed, with average tissue necrosis estimated at 21, 27, and 37% in plants from I-, 2-, and 3-yr-old stands, respectively. Fusarium oxysporum, F. solani, Phoma sp., and F. avenaceum were the pathogenic fungi predominantly associated with necrotic roots, accounting for 25, 21, 12, and 4% of isolations, respectively. Inoculation of alfalfa plants with each of 28 randomly selected isolates of F. oxysporum and 18 of F. solani resulted in root and crown necrosis similar to that observed in the field, but no isolate induced rapid wilting and shoot death as did reference isolates of F. oxysporum f. sp. medicaginis, causal fungus of Fusarium wilt. Every field and 92% of all individual plants exhibited injury from clover root curculio. The number of wounds that breached the root cortex ranged from 0 to 45 per plant and averaged 2.6, 5.5, and 6.1 for I-, 2-, and 3-yr-old plants, respectively. Epidermal scarring and deep feeding wounds were highly correlated as measures of clover root curculio activity. There were highly significant, positive linear correlations between average number of deep wounds and average tissue necrosis of plants for fields of each age, suggesting that clover root curculio injury had predisposed alfalfa plants to more severe crown and root rot. Although injury levels were generally lower in the two northern counties sampled, plants from individual fields in each physiographic region showed pest injury levels that potentially could result in economically significant reductions in yield. Reduction of pest-induced losses in New York alfalfa will rely on concomitant control of clover root curculio and fungi that cause crown and root rot.

Keyword(s): Medicago sativa