Field Response of Alfalfa to Harvest Frequency, Cultivar, Crown Pathogens, and Soil Fertility: II. Crown Rot

Abstract

Decline in stand density and productivity of alfalfa (Medicago sativa L.) over time in Canada and the USA has been attributed, in part, to crown and root diseases. In 1985, an experiment was initiated at three locations in Saskatchewan to examine the interaction of (i) harvest frequency, (ii) alfalfa cultivar, (iii) inoculation with fungal crown rot pathogens (Coprinus psychromorbidus, Phoma sclerotwides, or Fusarium spp.), and (iv) K nutrition on the etiology, lesion initiation, incidence, and severity of crown rot of alfalfa. Plant samples were collected from the plots in early spring and late fall each year, starting 1 yr after seeding. The pattern of symptom initiation and lesion expansion were similar at all sites. Lesions were observed initially at the base of senescing stems, and spread laterally into the crown and vertically into the tap root. Lesions were observed on plants at all sites on the first sampling date ( > 70% of plants affected at Loon Lake and Saskatoon and 32% at Melfort), but severity was low. By the fall of the following year (1987), incidence was near 100% at all sites, and severity had increased markedly. Harvest frequency and K fertility did not affect crown rot at any location. Inoculation with C. psychromorbidus resulted in small increases in crown rot severity at two sites, but inoculation With the other pathogens had no effect. 'Nordica' al-Agriculture Canada Res. Stn., Published in Agron. J. 86:88-93 (1994). falfa was more resistant to crown rot than 'Beaver'. These results indicate that resistance to crown rot may be advantageous if longterm persistence is required, but resistance is not required in shortterm rotations. Also, crown rot and crown bud rot, two alfalfa diseases that have previously been described as distinct entities, appear to be a single disease complex, based on similarities in symptom development and the organisms associated with affected plants.

Fusarium root and crown rot in alfalfa subjected to autumn harvests

“…2). In the drier environment of western Canada, the progress of fusarium root and crown rot in alfalfa was not affected by harvest frequency (Gossen 1994). However, in the current study, disease severity increased with the earliness of the additional autumn harvest.…”

Effects of low-temperature stress on development of winter crown rot in first-year alfalfa

“…The LTB, a ubiquitous winter pathogen of cereals and forages throughout Alberta and Saskatchewan (Gaudet et al 1989;Gaudet and Sholberg 1990;Gossen et al 1992;Hwang and Gaudet 1995), acted synergistically with sublethal low temperatures to increase mortality and reduce foliar regrowth in first-year alfalfa. Low-temperature stress reduced the ability of first-year alfalfa plants to resist attack by the LTB, and exposure of alfalfa plants to sub-lethal damage by the LTB reduced the capacity of the plants to survive and recover from brief exposure to low temperature.…”

“…Most alfalfa cultivars recommended for use in western Canada possess sufficient winter hardiness to survive in these regions provided that adequate snow is present (McKenzie and McLean 1984;Gossen et al 1992;Goplen and Gossen 1994). However, prolonged and persistent snow cover encourages the development of winter crown rot of alfalfa, which is caused by the cottony snow mold form of LTB (Cormack 1948;Gaudet and Bhalla 1988;Laroche et al 1995).…”

“…The LTB fungus was previously identified as Coprinus psychromorbidus "Redhead" and "Traquair" (Traquair 1980), but recent studies indicate that the C. psychromorbidus taxon includes several distinct species (Laroche et al 1995). This low-temperature parasitic fungus can attack dormant plants under the snow (Gaudet 1986;Gossen 1988;Gaudet and Sholberg 1990;Piening et al 1990;Hwang and Gaudet 1995), and can cause extensive winterkill to alfalfa in the central and northern Canadian prairies (Broadfoot and Cormack 1941;Cormack 1948;Smith 1975Smith , 1981. The disease is characterized by the occurrence of large dark brown lesions of the lower stems and upper crowns followed by progressive decay of the crown tissues (Cormack 1948(Cormack , 1952; the tap root usually remains relatively sound until natural decay sets in.…”

“…In addition to directly killing alfalfa, damage caused by the LTB fungus may stress plants by reducing cold hardiness, thereby predisposing plants to winterkill (Gossen et al 1992). Gaudet and Chen (1988) showed that cottony snow mold acts synergistically with low-temperature stress to decrease winter survival in winter wheat.…”