Timing of an Additional Fall Harvest Affects Yield and Fusarium Root Rot Development in Alfalfa

Timing of an Additional Fall Harvest Affects

Fusarium Root and Crown Rot Development in Alfalfa

C. Dhont^1,2, L. Couture¹, F.P. Chalifour², R. Drapeau¹,

G. Tremblay³, Y. Castonguay¹, G. Belanger¹, and P. Nadeau¹

1Agriculture and Agri-Food Canada, Sainte-Foy, Quebec, Canada G1V 2J3
^{2Laval University, Quebec, Canada G1K 7P4, ³CEROM Inc. Quebec, Canada}

Fusarium root and crown rot reduces the longevity of alfalfa stands in Quebec. Management practices that affect plant reserves are likely to modify the level of infection by such pathogens. To assess the effect of an additional fall harvest on the development of Fusarium root and crown rot, we analysed samples from plots initially established to study the effect of fall cutting management on C and N reserves and spring regrowth. The experiment was conducted as a completely randomized block design with four replicates. The experiment took place at three sites located in different climatic regions in Quebec over two growing seasons (1996-97 and 1997-98). A comparison was made between two cultivars, AC Caribou and WL-225, currently recommended in Quebec. Treatments were an additional fall harvest taken during the critical fall rest period after 400, 500 or 600 Degree-Days (DD; 5^oC) following the last summer harvest. Plots not harvested during the fall were used as controls. Samples for disease evaluation were randomly collected from each plot in late fall and in the following spring before growth had resumed. The severity of root and crown rot was rated on a visual scale from 0 to 5.

Disease severity was highest at the most northern site. Cv. AC Caribou was consistently less affected than Cv. WL-225 across years and sites. An additional harvest during the critical fall rest period increased the intensity of symptoms as compared to controls. Plants harvested at 400 DD were the most affected by Fusarium root rot at the end of the fall. Disease severity was ranked in the following order : 400 DD > 500 DD > 600 DD > controls (no fall harvest).

In conclusion, harvesting alfalfa during the critical fall rest period increased the severity of Fusarium root rot. Furthermore, disease severity increased with the earliness of this additional fall harvest. As winter survival of alfalfa is dependent on the root organic reserves, the level of tissue damage by Fusarium root rot could have a significant impact on persistence and spring regrowth.

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