VIEW ARTICLE

Etiology

Relationships of Planting Density and Competition to Growth Characteristics and Internal Crown Breakdown in Arrowleaf Clover. R. G. Pratt, Research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Crop Science Research Laboratory, Forage Research Unit, and Departments of Plant Pathology and Weed Science, and Agronomy, Mississippi State University, Mississippi State 39762; W. E. Knight, research agronomist, U.S. Department of Agriculture, Agricultural Research Service, Crop Science Research Laboratory, Forage Research Unit, and Departments of Plant Pathology and Weed Science, and Agronomy, Mississippi State University, Mississippi State 39762. Phytopathology 73:980-983. Accepted for publication 20 January 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/Phyto-73-980.

Arrowleaf clover was grown in field plots at intervals of 5- 40 cm between plants in pure stands, and at 40 cm between plants overseeded with ryegrass, during two growing seasons. Growth characteristics and internal crown breakdown (ICB) were evaluated near the end of each season. Numbers of flowering stems, crown and root diameters, and the incidence and severity of ICB increased with greater planting intervals in pure stands and decreased in plants grown with ryegrass. ICB scores were positively and closely correlated with values for all of the plant characteristics and with various ratios between them. ICB also was more frequent and severe following a harsh winter growing season than after a mild one. Fifteen species of fungi were represented among 41 isolates obtained from crowns of arrowleaf clover before and after ICB symptoms developed. Fusarium acuminatum, F. oxysporum, and Chaetomium trilaterali were isolated most frequently and occasionally caused symptoms in crowns following artificial inoculations; all other fungi were nonpathogenic. Results suggest that ICB in arrowleaf clover is a noninfectious disease that is related to morphological characteristics of plants grown at low densities or with insufficient aboveground competition. However, the incidence and severity of ICB may be affected by winter injury and by invasion of crowns by weakly pathogenic fungi.