Cause Plenodomus lingam (syn. Leptosphaeria maculans, anamorph: Phoma lingam) and Plenodomus biglobosus (syn. Leptosphaeria biglobosa) are fungi that can infect a range of cruciferous crops. Host plants include Brassica, Sinapis (white and yellow mustard), and Raphanus (daikon and radish). Cruciferous weeds common in the Pacific Northwest can be host to black leg, including birdsrape mustard (Brassica rapa), black mustard (B. nigra), western yellow cress or curvepod yellowcress (Rorippa curvisiliqua), tansymustard (Descurainia pinnata), hedge mustard (Sisymbrium officinale), tumble mustard (S. altissimum), small tumbleweed mustard (S. loeselii), pennycress (Thlaspi arvense), mouseear cress (Arabidopsis thaliana), annual wallrocket (Diplotaxis muralis), perennial wallrocket (D. tenuifolia), and wild radish. The fungus survives on living plants as pycnidia, which produce water-splashed pycnidiospores, or with plant debris as pycnidia and/or pseudothecia. Pseudothecia release ascospores that may be wind windborne long distances, at least several miles and perhaps further on storm fronts. The fungus survives in association with infected plant residues until the plant debris decomposes (1 to 5 years); it can survive for years as mycelia in infected seed.
The black leg fungi can also move via infected seed, Leptosphaeria can survive for years in association with seed. But if, as a general practice, seed lots used for planting are Leptosphaeria-free then the spread of regional epidemics is thought to arise from infected plant debris. Even low levels of seed infection, when coupled with weather favorable for disease, can lead to severe losses, including stand die-out within a field. The Oregon Department of Agriculture requires that all crucifer seed that is to be planted in Oregon must be accompanied by an official test showing that the untreated seed is free from black leg (Leptosphaeria species).
Symptoms Damping-off may occur if plants are infected at the seedling stage due to infected seed. Plants affected after the seedling stage may be stunted. Generalized leaf spots can become numerous across the field. Pale, irregular spots develop on cotyledons, leaves, stems or petioles, later becoming somewhat circular to oval, ashy-gray colored with scattered tiny, black pycnidia. Stem lesions that develop after the fungus moves systemically are elongated with purple borders, often near the soil line, and may girdle the stem as well as decay pith tissues, and extend below the soil surface, causing a brown-to-gray to black rot in the lower stem and roots.
Sampling Seed stock and seed from each lot should be assayed for the causal organism. Seed stock can be assayed for the causal organism in official tests conducted by a number of labs. The following labs are certified to officially test seed lots that require Leptosphaeria testing before distribution in the state of Oregon (all crucifer seed must be accompanied by an official test stating that the untreated seed is free from black leg (Leptosphaeria maculans):
Agri Seed Testing, Inc., 1930 Davcor Street SE, Salem, OR 97302; Tel: 503-585-1440; Fax: 503-588-0733; Email: office@agriseedtesting.com; http://www.agriseedtesting.com
Eurofins STA Laboratories (1821 Vista View Drive, Longmont, CO 80504; Tel: 303-651-6417; Fax: 303-772-4003; Customer Service: 800-426-9124; Email: stacoinfo@eurofinsus.com)
Iowa State University Seed Laboratory, 109 Seed Science Center, Iowa State University, Ames, Iowa 50011-3228; 515-294-6826; Email: seedlab@iastate.edu; http://www.seeds.iastate.edu/seedtest/
Oregon Department of Agriculture, Commodity Inspection Program, Salem, OR (Fax: 503-986-4737; Email: seedservices@oda.state.or.us)
Oregon State University Seed Laboratory (3291 SW Campus Way, Corvallis, OR 97331; Tel: 541-737-4799; Fax: 541-737-2126)
Cultural control
- Plant only seed certified by official testing to be free of Leptosphaeria.
- Rotate out of crucifers for at least three years.
- Control susceptible weeds and manage volunteer crucifers during rotations.
- Avoid planting in or adjacent to a site where disease has occurred within the last 3 to 4 years; plantings within a quarter of a mile of a field containing Leptosphaeria-infected crucifer residues are at high risk for developing black leg.
- Delay planting in the spring until the weather is drier to avoid periods of ascospore release.
- Bury residues after harvest by deep plowing if possible, or remove plant debris. Flailing residue reduced the population of Leptosphaeria spp. compared to no-till and shallow tillage treatments, although it didn't increase canola residue breakdown in OSU studies by Berry. Flailing the crop residues once, followed by a deeper incorporation (greater than 2-inches deep), may be sufficient as long as the residue remains buried for several years, depending on amount and size of plant debris as well as environmental conditions.
Chemical control Seed treatment and a foliar fungicide program will be essential if growing seed fields in any region where black leg epidemics have occurred across a larger geographic area. Infected seed can occur in seed lots, certified to be free of Leptosphaeria, at levels below the detection threshold by seed testing, and chemical or hot water seed treatments will aid in controlling disease introduction by infected seed.
Plant seed treated with a fungicide:
- Saltro (Group 7) at 1.23 fl oz/100 lb seed (commercial seed treaters only) plus a dye. 12-hr reentry.
If leaf spots are observed in a planting, foliar sprays will help to reduce the build-up within the field:
- Elatus (Group 7 + 11) at 7.3 oz/A as a single application at the rosette stage between 2nd true leaf and bolting. Preharvest interval is 30 days. 12-hr reentry.
- Miravis Neo (Group 7 + 3 + 11) at 13.7 fl oz/A during the rosette stage between 2nd true leaf and bolting. Preharvest interval is 30 days. 12-hr reentry.
References Berry, P.A. 2019. Decomposition of Brassicaceae Residue in the Willamette Valley. Ph.D. Dissertation. Oregon State University.
Rimmer, S.R., and van der Berg, C.G.J. 2007. Black leg (Phoma stem canker). p. 19-22 in Compendium of Brassica Diseases. S.R. Rimmer, S.R., Shattuck, V.I., and Buchwaldt, L. (eds.). St. Paul, MN: APS Press.
Williams, P.H. 1992. Biology of Leptosphaeria maculans. Can J Plant Pathology 14:30-35.