Verticillium wilt increases with irrigation in wilt-susceptible
alfalfa
Paul G. Jefferson and Bruce D. Gossen
Agriculture and Agri-Food Canada, Swift Current, SK S9H 3X2
Agriculture and Agri-Food Canada, Saskatoon, SK S7N 0X2
Verticillium wilt (VW), caused by Verticillium albo-atrum, has occurred in
scattered irrigated alfalfa (Medicago sativa L.) fields in southwestern
Saskatchewan since the late 1970s, but does not cause detectable injury in
dryland fields. We were interested to examine the impact of irrigation on
the rate of spread of VW in this semiarid region. A line-source irrigation
system was used to produce an irrigation gradient on two adapted cultivars,
Beaver, a VW-susceptible cultivar commonly used for irrigated production in
the region, and Barrier (VW-resistant). Plots were seeded at Swift Current,
SK in 1994, in a commercial field where VW was observed in the previous year.
The irrigation gradient was sub-divided into six water level treatments
within each cultivar plot and each treatment combination was replicated four
times. Irrigation plus precipitation from April to August each year averaged
27, 29, 31, 33, 35 and 35 cm for gradient treatments one to six,
respectively, over 1995 to 1997. Stand density and counts of VW-infected
plants per subplot were recorded in spring, and forage yield was assessed
twice per season. In both 1995 and 1996, Beaver had more VW-infected plants
than Barrier, and the number of infected plants increased with increasing
irrigation water. As expected, the number of VW-infected plants in Barrier
showed no response to irrigation. However, water restrictions at this site
in 1996 and 1997 resulted in delayed and relatively low amounts of
irrigation. This resulted in a steady decline in the number of infected
plants in the trial from 1995 to 1997. Severe stand thinning associated with
winter injury was observed in spring of 1997. Stand density in Beaver was
negatively correlated to VW-infected plant counts from 1995 and 1996.
Barrier, however, exhibited no relationship between these variables. Forage
yield increased with increased irrigation in each year, except for Beaver in
1997. Forage yield of Beaver did not respond to irrigation in 1997 because
stand density had declined to 60% at water level six compared to 78% at
water level one. The stand density of Beaver at water level one was similar
to that of Barrier at all water levels. We concluded that impact of VW on
irrigated stands was correlated to the level of irrigation water applied, and
that VW does not affect susceptible alfalfa cultivars on dryland because its
rate of spread from plant to plant is very low in most years.
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