Although 2018 is setting itself up to be a dry year, disease management should remain a priority for pea growers. In 2017, one third of surveyed pea fields had root rot symptoms that were severe enough to cause yield loss, despite relatively dry growing conditions.1 The survey found Fusarium avenaceum present in all 35 sampled fields and Fusarium wilt (F. oxysporum) present in 74% of fields.1 Most annual legume, cereal and oilseed crops are hosts for Fusarium, so crop rotation as a control option has limited value.2
Fusarium species are primarily soil-borne. Under cool and wet conditions that favour development, seed treatments can be used to control early season infection. One study found that metalaxyl-based fungicide seed treatment consistently increased seedling emergence, nodulation and seed yield, and reduced root rot severity in greenhouse and field trials inoculated with F. avenaceum.3 Conversely, under limited disease pressure on fields with no history of field peas, another study found that neither metalaxyl nor dithiocarbamate fungicide seed treatment had any positive effect on pea yields.4 Practices that improve overall plant vigour, such as early planting dates, well drained fields, disease-free seed and adequate crop nutrition used in combination can reduce the impact of root rot.2 Use of seed treatment is also recommended if Fusarium levels (in addition to Botrytis and Sclerotinia) exceed 10% on the seed source.5
Aphanomyces root rot was detected in 77% and 48% of fields surveyed in 2016 and 2017, respectively.1 Appropriately named a “water mould,” Aphanomyces favours wet or waterlogged soils, which explains the higher incidence of the disease in 2016 compared to 2017. To confirm the presence of Aphanomyces in your field, conduct a soil test ahead of planting.
Crop rotation is key for managing Aphanomyces euteiches root rot in peas. Long-term data from MASC (2000-2016) shows that at least four years between pea crops produced higher yields than tighter pea rotations (Figure 1). Dormant oospores can remain in soils for many years, so those infested with A. euteiches should be planted to a non-host crop for a minimum of six years to avoid buildup of disease inoculum. Lentils, alfalfa and dry beans are also susceptible, faba beans are tolerant and soybeans, other oilseeds and cereals are resistant to A. euteiches.
Labs with available soil testing services for Aphanomyces:
As in previous years, Mycosphaerella blight was the most widespread foliar disease, observed in all surveyed fields in 2017.1 Part of the Ascochyta complex, the host range is limited to peas and the primary source of inoculum is crop residue. Seed can be a minor source of inoculum and seed treatment should be used if >10% of the seed source is infected with Ascochyta.5 Crop rotation can reduce disease pressure, but spores travel long distances by wind. Thus, crop rotation, in addition to foliar fungicide application, is often necessary to effectively control this disease. Other foliar diseases that infect common annual crops, such as Sclerotinia and downy mildew, were infrequently observed in the 2017 pea disease survey and are generally not economically important.1
1 McLaren, D.L., T.L. Henderson, Y.M. Kim, K.F. Chang, S. Chatterton, T.J. Kerley, M.J. Thompson. 2018. Field Pea Diseases in Manitoba in 2017. Can. Plant Dis. Sur.98:188-191.
2 Gossen, B. D., R.L. Conner, K. Chang, J.S. Pasche, D. L. McLaren, M.A. Henriquez, S. Chatterton, S. Hwang. 2016. Identifying and Managing Root Rot of Pulses on the Northern Great Plains. Plant Disease. 100: 1965-1978.
3 Chang, K. F., Hwang, S. F., Ahmed, H. U., Gossen, B. D., Turnbull, G. D. and Strelkov, S. E. 2013. Management strategies to reduce losses caused by fusarium seedling blight of field pea. Can. J. Plant Sci. 93: 619625.
4 Kutcher, H. R., Lafond, G., Johnston, A. M., Miller, P. R., Gill, K. S., May, W. E., Hogg, T., Johnson, E., Biederbeck, V. O. and Nybo, B. 2002. Rhizobium inoculant and seed-applied fungicide effects on field pea production. Can. J. Plant Sci. 82: 645–651.
5 Saskatchewan Ministry of Agriculture. 2013. Guidelines for tolerances of seed-borne diseases in pulse seed intended for planting. http://publications.gov.sk.ca/documents/20/86762-64897dee-8dab-4df8-acaf-dc3deefeb5c9.pdf