R. J. Howard

Clubroot of cruciferous crops – new perspectives on an old disease†

Abstract Clubroot, caused by the obligate parasite Plasmodiophora brassicae, is an economically important disease affecting plants in the family Brassicaceae worldwide. In Canada, it has mainly been a problem on cruciferous vegetables in the traditional production areas of Ontario, Quebec, British Columbia and the Atlantic Provinces. In the Prairie Provinces, clubroot has been reported sporadically in a few home gardens and commercial vegetable fields in Alberta and Manitoba over the past 80 years; however, this situation changed dramatically with the discovery of 12 infected canola (Brassica napus) fields near Edmonton, AB in 2003. Annual surveys carried out since 2003 have revealed that clubroot is a much more widespread and serious disease in Alberta than initially thought. By 2008, it had been detected in about 410 canola, mustard and vegetable fields in central and southern areas of the province. The Alberta Clubroot Management Plan was developed to guide farmers, agribusinesses, oil and gas companies, contractors, and the general public in adopting good growing practices or taking measures to prevent further spread of this disease. An unprecedented research effort is underway in western Canada to develop a better understanding of the biology and management of clubroot, especially in the canola production systems on the Prairies. These studies are broadly based and include activities such as the improvement of diagnostic tests, determination of P. brassicae pathotypes, investigation of modes of seed and soil transmission, evaluation of fungicide efficacy, soil amendments, biological control agents and equipment sanitation protocols, and modelling disease distribution and risk. Through these efforts, new information is being generated that will give the agricultural industry and other stakeholders some new perspectives on this old disease threat.

Virulence and spread of Plasmodiophora brassicae [clubroot] in Alberta, Canada

Clubroot, caused by Plasmodiophora brassicae, has emerged as an important disease of canola (Brassica napus) in Alberta, Canada. Annual surveys from 2005 to 2008 revealed 405 clubroot-infested fields, mainly in central Alberta, but also in the southern part of the province. Within-field surveys suggested that the main mechanism of clubroot spread is through the movement of infested soil on farm machinery, as the frequency of infestation was highest at the field entrances and decreased sharply at distances of 150 and 300 m from the entrance. Characterization of P. brassicae populations from Alberta on the differentials of the European Clubroot Differential (ECD) series, P.H. Williams, and A. Somé et al. revealed that pathotype ECD –/15/12, 3, or P2, respectively, is predominant in the central part of the province, a finding consistent with earlier reports. However, novel virulence phenotypes were also observed. Two populations from southern Alberta were classified as pathotype 5 or P3 on the hosts of P.H. Williams and A. Somé et al., respectively, or as pathotypes ECD –/6/8 and –/4/0 on the ECD set, suggesting independent introductions of P. brassicae to this region or greater diversity than previously thought. Pathogen populations collected from infected canola plants from Quebec and Manitoba also yielded unique virulence patterns but were still virulent on most B. napus hosts, and an Ontario population induced significant disease only on a few of the vegetable differentials. Virulence levels overall were highest for the P. brassicae populations from central Alberta, which may help to explain the rapid increase in clubroot observed in this province.

Effect of temperature, seeding date, fungicide seed treatment and inoculation with Fusarium avenaceum on seedling survival, root rot severity and yield of lentil.

Early seeding of lentil is necessary on the northern prairies to ensure that the crop has time to mature. However, planting into cold soils in spring results in slow germination, which may predispose seedlings to infection by soil-borne pathogens. In a controlled-environment study of the impact of temperature on infection of lentil seedlings (cv. Eston) byFusarium avenaceum, root rot symptoms were most severe at warm temperatures (20° to 27.5 °C) and declined in warmer or cooler soils. Field plots were seeded on three dates, which were spaced about 2 wk apart between early May and early June in four station years. Seedling emergence was most consistent and seed yield was highest in three of four sites at the second seeding date. Emergence and yield were substantially lower for the latest seeding date. Inoculation with F. avenaceum reduced establishment and seed yield. Seed treatment with Crown (thiabendazole and carbathiin) improved seedling survival, reduced root rot severity and increased seed yield rel...

Progress towards the Sustainable Management of Clubroot (Plasmodiophora brassicae) of Canola on the Canadian Prairies

Progress towards the Sustainable Management of Clubroot [Plasmodiophora brassicae] of Canola on the Canadian Prairies Stephen E. Strelkov, Sheau-Fang Hwang, Ronald J. Howard, Murray Hartman and T. Kelly Turkington University of Alberta, Edmonton, AB; Alberta Agriculture and Rural Development, Edmonton, AB, Alberta Agriculture and Rural Development, Brooks, AB; Alberta Agriculture and Rural Development, Lacombe, AB; Agriculture and Agri-Food Canada, Lacombe, AB Corresponding author E-Mail: stephen.strelkov@ualberta.ca

Etiology, impact and control of rhizoctonia seedling blight and root rot of chickpea on the Canadian prairies

Studies were undertaken to assess the impact of seedling blight and root rot caused by Rhizoctonia solani AG-4 on nodulation and seed yield of chickpea. The effects of pathogen inoculum concentration and seed damage on disease severity were also measured. Chickpea seedlings were grown under controlled-environment conditions in sterilized soil amended with pathogenic isolates of R. solani and in field plot experiments where the inoculum was incorporated with the seed at planting. In greenhouse experiments, emergence and dry matter production declined and root rot severity increased with increasing inoculum concentration. Root rot reduced nodulation where lesions covered more than 25% of the root surface. For chickpea cultivar Sanford this level of infection was sufficient to reduce root mass, but for the cultivar Tyson, more than half of the root was covered with lesions before root mass began to decline. In a comparison of seed treatment fungicides, thiram + carbathiin (Vitaflo 280) and carbathiin + thiab...

Effects of soil temperature, seeding depth, and seeding date on rhizoctonia seedling blight and root rot of chickpea

Rhizoctonia solani Kuhn, a prevalent fungal pathogen in cultivated prairie soils, can be an important factor affecting seedling establishment of chickpea (Cicer arietinum L.). Studies were undertaken to determine the impact of different soil conditions on seedling establishment, development of root rot, and productivity of desi (Tyson) and kabuli (Sanford) chickpea plants sown in the presence of R. solani AG-4. The impact of soil and air temperatures and seeding depth was studied under controlled conditions, and the influence of seeding date was investigated at three field sites. The host responded to warm soils by increasing its growth rate, and the pathogen by increasing its virulence. The kabuli cultivar, Sanford, showed greater susceptibility to root rot caused by R. solani than the desi cultivar, Tyson. Root rot and shoot infection levels increased with soil temperature for both cultivars, but reached maximum values at substantially lower temperatures for Sanford. Shoot biomass of infected treatments...

Screening of Brassica germplasm for resistance to Plasmodiophora brassicae pathotypes prevalent in Canada for broadening diversity in clubroot resistance

Hasan, M. J., Strelkov, S. E., Howard, R. J. and Rahman, H. 2012. Screening ofBrassicagermplasm for resistance toPlasmodiophora brassicae pathotypes prevalent in Canada for broadening diversity in clubroot resistance. Can. J. Plant Sci. 92: 501–515. Clubroot disease of crucifers, caused by Plasmodiophora brassicae, poses a threat to the Canadian canola industry, and the development of resistant cultivars is urgently needed. Germplasm resistant to local pathotype(s) is the prime requirement for breeding clubroot-resistant cultivars. The objective of this study was to identify Brassica germplasm possessing resistance to P. brassicae pathotypes prevalent in Alberta. Pathotype-specific resistance was identified in the diploid species Brassica rapa (AA) and B. oleracea (CC), and in the amphidiploid B. napus (AACC). Among B. rapa genotypes, turnip was the most resistant, followed by winter type and spring type oilseed rape. The rutabaga group of B. napus, on the other hand, was homogeneous for resistance to Canadian P. brassicae pathotypes. The diploid species B. nigra (BB) also showed pathotype-specific resistance. However, the two amphidiploids carrying the B. nigra genome, B. juncea (AABB) and B. carinata (BBCC) were completely susceptible to clubroot.

Management strategies to reduce losses caused by rhizoctonia seedling blight of field pea

Seedling blight can cause substantial reductions in stand density of field pea on the Canadian prairies. Ninety-four isolates of Rhizoctonia solani were obtained from soil samples collected from 37 pea fields in 1999 in Alberta, Canada. Sixty isolates were characterized as anastomosis group (AG)-4 and 12 isolates as AG-2-1. Some of these isolates caused severe pre-emergence damping-off and were classified as highly pathogenic; 41 of the 44 highly pathogenic isolates were AG-4 and three were AG-2-1. Two highly pathogenic AG-4 isolates were used to assess the effect of inoculum density on survival and growth of field pea seedlings, and the impact of seeding date, seeding depth, soil temperature, seed damage, seed treatments and seeding density on seedling blight and root rot injury. As inoculum density increased, so did root rot severity, while seedling establishment, shoot dry weight and root dry weight declined. Under controlled conditions, seedling establishment in the noninoculated control increased as ...

Effect of seed damage and metalaxyl seed treatment on pythium seedling blight and seed yield of field pea

When cool, wet conditions persist after planting, Pythium spp. can be an important constraint to stand establishment in field pea. Laboratory studies and field trials were conducted over 3 yr to assess the impact and evaluate the interactions of Pythium spp., metalaxyl seed treatment and damage to seed on seedling establishment, root rot severity and seed yield of field pea. Seedling emergence, seedling size, and seed yield were reduced by inoculation with Pythium spp. and by mechanical damage to the seed. Fungicide seed treatment reduced the impact of seed damage, but did not always restore seedling emergence and seed yield to the same level as from undamaged seed. Undamaged seed treated with metalaxyl was not affected by inoculation with Pythium spp. Differences among cultivars, although often significant, were small relative to the effect of seed injury. Laboratory studies showed a negative linear relationship between inoculum concentration and emergence from untreated seed. They also showed that Pythi...

Seeding date, temperature, and seed treatment affect pythium seedling blight of field pea

Pea root rot and seedling blight caused by Pythium spp. often result in poor seedling establishment and patchy stands. Laboratory and field trials were conducted to determine the role of soil temperature and seeding date in the severity of these diseases, since weather records indicate a gradual increase in mean soil temperature from 4.5 to 17.3°C over the April 15 to June 15 planting season. In a gradient plate test to assess the impact of temperature on infection of field pea seedlings, the optimum temperature for infection was 15-22.5°C for a strain of Pythium ultimum and 17.5-27.5°C for Pythium irregulare. Field trials, conducted from 1994 to 1996 at Westlock, Alta., and Saskatoon, Sask. using a mixed inoculum of Pythium irregulare and Pythium ultimum applied in the seed row at seeding, showed that inoculation consistently reduced emergence and yield relative to the untreated control. There were few differences in emergence or seed yield between the seeding dates in late April to mid-May, but emergence was 10-15% lower and seed yield was 20-50% lower when the crop was seeded in late May to early June. There were no seeding date x inoculation interactions at any site. We conclude that low soil temperatures do not increase the severity of pythium root rot and seedling blight in field peas and that the crop should be seeded early on the northern prairies to maximize yield, even where soils are infected with Pythium spp. Seed treatment fungicides generally improved seedling emergence and decreased root rot severity relative to the inoculated control, especially with metalaxyl. Metalaxyl and thiram + carbathiin produced 15-50% higher seed yield than the inoculated control.