Lange: Identification and Quantification of a New Canola Wilt in Western Canada

Date: January 31, 2002
Term: 2 years
Status: Completed
Researcher(s): Ralph Lange, M.Sc., Alberta Research Council; Dee Ann Benard, M.Sc., P.Ag. and Prem Kharbanda, Ph.D.
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: Canola Council of Canada, Alberta Agricultural Research Institute

Project Summary

A wilt disease previously unreported in North America was observed in the Peace River and north east regions of Alberta beginning in 1999. The disease induced chlorosis, stem necrosis, vascular discolouration and premature desiccation in Brassica napus and B. rapa. This disease could become a serious threat to canola production in Canada.

The objectives of this research were to identify the causal agent of wilt on canola; to determine its host range; to determine its race (if applicable, for breeding purposes); and to determine if any current canola cultivars are particularly resistant or susceptible.

Isolations from diseased tissue indicated that Fusarium avenaceum was the causal · agent, and these isolates showed a high degree of virulence. Host range experiments showed that F. avenaceum has a wide host range, and is capable of surviving on a wide range of crop, forage and weed species. The pathogen produced pectolytic enzymes, which may be implicated in the mode of action. Differences in cultivar susceptibility were demonstrated, with Nexera 705 being highly susceptible. Several attempts to develop a simple and reliable cultivar screening method were made, but the lack of corresponding field data limited this approach. In 2000, a province-wide disease survey was conducted, which documented the occurrence of Fusarium wilt and other canola diseases. A replicated field experiment also showed that Fusarium wilt can cause significant yield losses in canola.

In the fall of 1999, canola producers and Alberta Agriculture, Food and Rural Development Cereal and Oilseed Specialists began noticing that canola plants in some fields in the areas surrounding Fort Vermilion and Vegreville were exhibiting unusual symptoms. Affected plants were discoloured and pods had not filled properly. These symptoms differed from other canola diseases: roots of affected plants were intact, unlike in root rot; no Blackleg-like cankers were noticed; and the stem shredding, sderotia and lodging typical of Sderotinia stem rot were absent. The disease induced chlorosis, stem necrosis, vascular discolouration and premature desiccation in Brassica napus and B. rapa. Fields were found where up to 29% of all canola plants were diseased. Yield was severely reduced by the disease; fully-and partially-wilted plants yielded 0.2% and 19.3% of asymptomatic plants, respectively.

The symptoms of the disease strongly resembled another wilt disease caused by fungi of the genus Verticillium (Heale and Karapapa 1999). Verticillum dahliae is a serious pathogen of oilseed rape in Europe (Heale and Karapapa 1999; Zeise 1992), and V. albo-atrum has caused disease on rutabaga in California and on B. rapa in Kansas (Farr et al. 1989), but has not been recorded on canola plants (Brassica napus var. napus, and B. rapa subsp. oleifera) in North America. However, an unidentified Verticillium species has been isolated from rutabaga (B. napus var. napobrassica) in Quebec (Ginns 1986). Verticillium wilt also occurs on North American vegetable crops such as cauliflower (Ginns 1986).

Repeated attempts to recover Verticillium spp. from symptomatic plants using semi-selective I11-edia failed to result in the isolation of Verticillium spp, but repeatedly resulted in the isolation of Fusarium spp. For this reason, it was suspected that the causal agent might be Fusarium oxysporum, since this fungus causes most Fusarium wilt dieases worldwide. For example, Fusarium oxysporum has caused "yellows" or wilt of Brassica napus in North Carolina, and Fusarium oxysporum f. sp. conglutinans (FOC) has attacked B. rapa in Indiana, Misisissippi and Texas (Farr et al. 1989). Of particular interest to Canadian plant pathologists and plant breeders is a FOC race capable of infecting Brassica plants under the cool soil temperatures that occur in Canada (Bosland and Williams 1984; Bosland et al. 1988). Fusarium oxysporum has been isolated from rapeseed roots in Alberta (Berkenkamp and Vaartnou 1972), but has not been associated with wilt symptoms. FOC has not been isolated from oilseed Brassicas in Canada, however the fungus has caused wilt on cabbage and cauliflower in Ontario (Ginns 1986). Fusarium oxysporum has caused serious wilt of B. juncea in India; the fungus was also found to be highly pathogenic to B. rapa and Raphanus sativus (radish) (Gupta 1973; Rai and Singh 1973).

It is very possible that this disease could become a serious threat to canola production in Canada. Many important diseases can be traced back to a few scattered reports, only to later become major problems. For instance, Blackleg of canola was identified in a few fields in Saskatchewan in 1975, and since then has become an extremely serious disease across Canada (Gugel and Petrie 1992). Fusarium head blight of cereals, now a serious threat to western Canadian wheat and barley producers, was rare in Manitoba prior to 1985, when two infected fields were identified (Clear and Abramson 1986; Gordon 1952). This new wilt is a potential threat that requires an immediate response so that timely action can be taken.

Fusarium wilt of canola has not been previously described; therefore, this project was udertaken to begin some of this work. Even though Fusarial wilts are usually caused by F. oxysporum, the true causal agent needed to be identified and confirmed. The ability of other crop and weed species to act as alternative hosts needed to be determined as a prerequisite to the development of cultural control methods. An important source of control may exist in some level of natural resistance within the crop itself, so the possibility of susceptibility differences among canola cultivars required investigation. This project was undertaken to develop a clearer understanding o! this new disease of canola, in an effort to be proactive.

Full Report PDF: Identification and Quantification of a New Canola Wilt in Western Canada