By C. M. Ocamb
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 species (broccoli, Brussels sprouts, cabbage, collards, canola, kale, mizuna, mustards, oilseed rape, oilseed turnip rape, rutabaga, turnip, etc.), 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. Leaf spots on cotyledons or first true leaves can be a result of infested seed, but epidemics are thought to arise from infected plant debris if seed lots used for planting are Leptosphaeria-free. Spots on leaves, stems, or pods develop after ascospores infect, and then stem cankers develop after the fungus spreads via the vascular system from leaves to the stems. In fall-sown crops or winter weeds, new pseudothecia can form on infected plants prior to harvest and continue forming in the residues on the soil surface after harvest whereas on spring-sown crops, pseudothecia do not form until after harvest. When residues are on the soil surface, ascospores are released from the pseudothecia following wet or humid conditions and when mean temperatures are between 46°F and 59°F. It has been reported that ascospore release by P. lingam and P. biglobosus under controlled conditions occurs at temperatures between 41°F to 68°F. Ascospores germinate at temperatures between 41°F and 68°F, but ascospores of P. biglobosus may germinate more rapidly compared to P. lingam. Germination of ascospores by both of these fungal species occurs more quickly in the higher range of temperature compared to 41°F, with 59 to 68°F being the optimum but temperature, cultivar, and tissue type are all important factors modulating ascospore germination. Non-germinated ascospores can potentially survive days to months in the absence of a host, and ascospores of P. lingam exposed to dry air survived longer than those of P. biglobosus. Conditions conducive for ascospore production and release commonly occur during the fall, winter, and springtime throughout the Pacific Northwest. Trap plants deployed weekly in multiple sites around the Willamette Valley in western Oregon beginning in the fall 2014 and 2015, showed that the first infections from ascospores occurred around mid-October. Timing of ascospore release is dependent on temperature and rainfall. During 2015-2016, ascospore infections of trap plants occurred from mid-October into May with severe disease pressure during February and March. The periods of higher ascospore release and infections of plants will depend on the environmental conditions. Ascospores infect through natural openings in the plant surface or via wounds; wounding of plants by insects, herbicides, or other mechanical means can increase disease incidence and severity. After successful infection of a leaf, hyphae will grow between the mesophyll cells towards the petiole and then along the vascular tissues, in or alongside the xylem vessels. Plenodomus lingam growing from an initial leaf lesion can ultimately result in stem cankers subsequently developing as cortical tissues in the stem are colonized, and this fungus can eventually invade the tap root, causing rot of roots, especially of the tap root or storage roots.
Two types of pycnidia which produce asexual spores known as pycnidiospores (or conidia) can be found; one on infected, live plants (thinner-walled pycnidium with a neck) and the other on crop residues (thicker-walled pycnidium with a narrow ostiole). Conidia are dispersed by splashing rain or irrigation water, so conidial movement is limited to relatively short distances, but conidia can add to the overall incidence and severity of disease within a field. Disease also spreads by workers or equipment moving through infected plantings during wet conditions when pycnidia are present. The process by which conidia of P. lingam and P. biglobosus infect plants is similar to that of the ascospores. The germination of conidia occurs at temperatures ranging between 41°F and 77°F. Examination of conidial inoculations using a set of P. lingam and P. biglobosus isolates on a set of differential cultivars of canola showed that a higher relative humidity level can increase the aggressiveness of P. lingam as well as the virulence of P. biglobosus.
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, even half ounce or smaller packets, 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).
This disease can reduce seed crop yields, and low levels of seed infection coupled with weather favorable for disease spread can lead to stand die-out in a field. After a disease outbreak in a larger geographic area, the threat of seed yield loss in fall-sown crops is much greater due to the abundance of affected crop residues on the soil surface, especially if field-wide outbreaks occurred in no-till production. Replanting or allowing volunteers to grow in a field site after a black leg epidemic can also result in high numbers of infected seedlings by fall in western Oregon or Washington Brassica seed fields, perpetuating disease. Widespread leaf spot and subsequent black leg stem cankers can result from unprotected plants being exposed to ascospores released from infected crop residues. Planting adjacent to a previously-infected field can result in a high disease incidence (40 to 60%) in the new planting by December, while planting within a few miles of an infected residue site can result in generalized infections across new fields (1 to 5%) by December. Researchers at OSU have observed uncontrolled black leg infections in fall-planted seed fields in western Oregon to increase from 1 to 5 % disease incidence up to 40 to 90% of the plants in a stand being infected within a 6-week period. Infections of fall-planted oilseed fields can lead to stand losses through winter into the spring.
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, becoming numerous across the field, have been observed in fall-planted crops after initial windblown spore (ascospore) infections. 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. Stem lesion may be narrow (<1/16 inch) longitudinal dark streaks on canola until harvest. Brown-to-black rot can be found inside affected stems. Vascular tissues may turn black in color prior to external rot symptoms.
Leaf spots may appear on over-wintered leaves at initially low levels but increase as pycnidiospores spread the leaf spot to new growth in the spring. Symptoms on seed pods are rare and inconspicuous. Black leg stem cankers in seed crops may be invaded by Sclerotinia sclerotiorum, masking black leg symptoms on the stems of affected plants.
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.
- Hot water seed treatment for 25 to 30 min. in water at 122°F.
- For organic production, removal of affected leaves shortly after leaf spots develop can reduce secondary spread within a planting as well as the incidence of stem cankers.
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:
- Coronet fungicide seed treatment (Group 7 + 11) at 3.1 to 6.2 fl oz/100 lb seed (commercial seed treaters only) plus a dye. Studies conducted by OSU and WSU have shown Coronet at the higher rate to be a very effective treatment for seedborne Leptosphaeria and other fungi.
- Difenoconazole 3L ST (Group 3) at 1 fl oz/100 lb seed plus a dye is registered for control of seedborne Leptosphaeria and it should be very effective.
- Helix Vibrance (Group 7 + 4 + 12 + 3 + an insecticide) at 23 fl oz/100 lb (commercial seed treaters only).
- 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:
- Aproach (Group 11) at 6 to 12 fl oz/A prior to disease development on 7- to 14-day intervals. Do not make more than one (1) application of any Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 28 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.
- Proline 480 SC (Group 3) at 4.3 to 5.7 fl oz/A is labeled for use on canola only to control Sclerotinia and can be applied in Oregon only to control black leg; a study out of OK showed Proline to give good control of black leg. Do not apply within 36 days of harvest. 12-hr reentry.
- Premixes of Group 7 + 11 fungicides are available for use.
- Elatus 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.
- Priaxor Xemium Brand at 4 to 8 fl oz/A. Do not make more than two (2) applications per season. Preharvest interval is 21 days. 12-hr reentry.
References Berry, P.A. 2019. Decomposition of Brassicaceae Residue in the Willamette Valley. Ph.D. Dissertation. Oregon State University.
du Toit, L.J., Derie, M. L., and Morrison, R.H. 2005. Evaluation of fungicide seed treatments for control of black leg of cauliflower, 2004. Fungicide & Nematicide Tests 60:ST011.
El Hadrami, A., Fernando, W.G.D., and Daayf, F. 2010. Variations in relative humidity modulate Leptosphaeria spp. pathogenicity and interfere with canola mechanisms of defence. Eur. J. Plant Path. 126(2):187-202.
Huang, Y.J., Fitt, B.D.L., Jedryczka, M. Dakowska, S., West, J.S., Gladders, P., Steed, J.M., and Li, Z.-Q. 2005. Patterns of ascospore release in relation to phoma stem canker epidemiology in England (Leptosphaeria maculans) and Poland (Leptosphaeria biglobosa). Eur. J. Plant Pathol. 111:263-277.
Huang, Y.J., Toscano-Underwood, C., Fitt, B.D.L., Todd, A.D., West, J.S., Koopmann, B., and Balesdent, M.H. 2001. Effects of temperature on germination and hyphal growth from ascospores of A-group and B-group Leptosphaeria maculans (phoma stem canker of oilseed rape). Ann. Appl. Biol. 139(2):193-207.
Naseri, B., Davidson, J.A., and Scott, E.S. 2008. Effect of temperature, cultivar and plant tissue on the germination of, and hyphal growth from, ascospores of Leptosphaeria maculans. Austral. Plant Pathol. 37:365-372.
Ocamb, C. M., Schneider, M., and C. Mallory-Smith. 2017. Evaluation of seed treatments for control of black leg on radish, 2016. Plant Disease Management Report: Report No. 11:V109.
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.