Seed Crop, Crucifers (Brassica, Eruca, and Raphanus spp.)-Black Leg

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 crops (broccoli, Brussels sproutss, cabbage, cauliflower, collards, canola, kale, mizuna, mustard greens, 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 plant debris as pseudothecia, which release ascospores that may be wind-borne 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 (up to 5 years). In fall-sown crops or winter weeds, pseudothecia can form on dead portions of infected plants prior to harvest and continue forming on infected plant residues remaining on the soil surface. In spring-sown crops, pseudothecia do not form on dead plant portions until after harvest, and their production on residues atop the soil typically starts the following fall after a couple of wet periods. 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 October 2014 and 2015, showed that ascospore infections began around mid-October. 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 infection 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 build-up within a field. Workers or equipment moving through infected plantings during wet conditions can spread disease. 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 fungus can also move with infected seed, Leptosphaeria can survive for years in association with seed. But regional epidemics are thought to arise from infected plant debris if seed lots used for planting are Leptosphaeria-free. 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 yields. After a disease outbreak in a larger geographic area, the threat of seed loss in fall-sown crops is much greater due to the abundance of affected crop residues on the soil surface, especially if field-wide epidemics occur in no-till fields. Replanting or allowing volunteers to grow in a field site after a black leg epidemic can result in high numbers of infected seedlings by fall in western Oregon or Washington seed fields, perpetuating the disease at that location. Planting adjacent to a previously-infected field can result in a high disease incidence (40 to 60%) by December in the new planting, while planting within a few miles of an infected residue site can result in generalized infections across new fields (1 to 5%). Researchers at OSU have observed uncontrolled black leg infections in fall-planted seed fields in western Oregon to increase from a low disease incidence (1 to 5%) to high (40 to 90% of the stand infected) in a 6-week period. Infections of fall-planted seed fields can lead to stand losses through winter into spring.

Symptoms Pale, irregular spots develop on cotyledons, leaves, stems or petioles, later becoming somewhat circular or oval, ashy-gray with scattered tiny, black dots (pycnidia) on the surface. Stem lesions that develop after the fungus moves systemically are elongated with purple borders, often near the soil line, and may girdle the stem or extend below the soil surface, causing a brown-to-gray to black rot in the lower stem and roots. Vascular tissues may turn black in color prior to external rot symptoms. Damping-off can occur if plants are infected at the seedling stage, usually through the use of infected seeds. Plants produced from infected seeds that survive the seedling stage are stunted and often develop a rot of the stem at or just below the soil line, which moves upwards and can cause plant death.

As seed plants mature, they may fall sideways from lack of root anchorage or stem cankers may cause the top of the plant to fall over. Symptoms on seed pods are rare and inconspicuous. Black leg stem lesions may be invaded by Sclerotinia sclerotiorum or other secondary pathogens, masking black leg symptoms on stems of affected seed plants.

Sampling The states of Oregon and Washington require that all crucifer seed planted in their respective states must be accompanied by an official test stating that the untreated seed is free from black leg (Leptosphaeria species). Plant clinics, including the OSU Plant Disease Clinic, can test seed lots suspected of having seedborne Leptosphaeria prior to stock seed samples being submitted for official tests. 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:

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 to be to be free of Leptosphaeria. This is required for all Brassica, Raphanus, and Sinapis seed planted within Oregon.
• Rotate out of crucifers for at least three years.
• Control susceptible weeds and manage volunteer crucifers during rotations.
• Treat infested seed in water at 122°F for the prescribed period of time. It is recommended that if new to this process, treat a sample of seeds and evaluate the germination rate after treating to ensure that the seed viability remains at desired levels.
• 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 an increased risk for black leg.
• Delay spring plantings to avoid periods of ascospore release.
• Bury residues as soon as possible after harvest by deep plowing, or remove plant debris. Buried turnip residues had increased rates of decomposition, compared to residues left on the soil surface, in OSU studies by Berry. Flailing residue reduced the population of Leptosphaeria spp. compared to no-till and shallow tillage treatments. 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.

For seed production:

• Seedbeds should not have crucifer crops on or near them for at least 5 years.
• Transplants should not be dipped in water before transplanting.
• Inspect seedbeds and seed fields regularly for presence of the disease.

Seed Treatment Seed of all Brassica, Raphanus, and Sinapis crops planted within Oregon should be treated for Leptosphaeria after testing; use a hot water seed treatment for 15 to 25 min. in water at 122°F (50°C) or an effective fungicide. Infected seeds 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 introduced by infected seed.

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 in the vicinity.

Plant seed treated with a fungicide:

• Coronet fungicide seed treatment (Group 7 + 11) at 5.1 to 6.2 fl oz/100 lb seed (commercial seed treaters only, not registered for use on radish) 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.
• Mertect 340-F (Group 1) at 3.5 fl oz in an equal amount of water (1:1)/100 lb of seed. For seed production fields in Oregon only (SLN OR-100014). Mertect 340-F (SLN OR-100014) is labeled for use on Oregon brassicas grown for seed at 3.5 fl oz//100 lb seed. Studies conducted by OSU and WSU have shown Mertect to be an effective treatment for seedborne Leptosphaeria.
• Rovral 4F (Group 2) at 16 fl oz/100 lb seed as a slurry by commercial seed treaters only. Not to be used on farm. For seed production fields in Oregon (SLN OR-140013) and WA only (SLN WA-160007). Studies conducted by WSU have shown Rovral to not be completely effective for seedborne Leptosphaeria when used at 8 fl oz/100 lb seed.

If leaf spots are observed in a planting, foliar sprays will help to reduce the build-up within the field:

• Cabrio EG (Group 11) at 12 to 16 oz/A, maximum of two (two) applications per crop (not registered for use on canola/rapeseed). 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; only one (1) application is allowed in radish before an alternate fungicide is required. Tank-mix with another material if used in western Oregon. Preharvest interval is 0 days. 12-hr reentry.
• Inspire Super (Group 3 + 9) at 16 to 20 fl oz/A on 7- to 10-day intervals is labeled for other diseases on leafy brassicas and can be applied to aid in controlling black leg in Oregon only if applied in the fall as wet weather get under way. Make no more than two (2) sequential applications. Preharvest interval is 7 days. 12-hr reentry.
• Priaxor Xemium Brand (Group 7 + 11) at 6 to 8.2 fl oz/A. 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 3 days. 12-hr reentry.
• Proline 480 SC (Group 3) at 5.7 fl oz/A for Brassicaceae seed crops (brassicas, radish, mustards, and arugula) in Oregon only (SLN OR-150017); labeled for Brassica spp. in Washington only (SLN WA-160003). Do not windrow within 14 days of last application. Preharvest interval is 14 days. 12-hr reentry.
• Quadris Top (Group 11 + 3) at 12 to 14 fl oz /A is labeled for other fungal diseases on leafy brassicas and can be applied to help control Leptosphaeria in Oregon only. 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. Do not apply within one day of harvest. 12-hr reentry.
• Rovral 4 Flowable (Group 2) at 2 to 4 pints/A in 20 to 100 gal water is labeled for other diseases in Oregon only (SLN OR-130001) and will aid in black leg management. Not labeled for use on canola/rapeseed. Apply at full bloom, pod set, and before harvest if disease pressure is severe. 24-hr reentry.
• Tebuconazole (Onset 3.6L, Tebustar formulations, Monsoon, Tebu-Crop 3.6F, Toledo 3.6F, etc.) at 3 to 4 fl oz/A on 10-day intervals is labeled for other diseases of leafy brassicas and can be applied to aid in controlling Leptosphaeria in Oregon only. Do not apply within 7 days of harvest. 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.

Miller, S.A., and Lewis Ivey, M.L. Hot water treatment of vegetable seeds to eradicate bacterial plant pathogens in organic production systems. Ohio State University Extension Factsheet HYG-3086-05.

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. 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.