Arugula (Eruca sativa)-Phoma Stem Canker (Black Leg)

See:

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 also include Brassica crops (broccoli, Brussels sprout, cabbage, collard, canola, kale, mizuna, mustard greens, 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 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 (up to 5 years). 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 59-68°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 survive 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 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. 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 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).

Symptoms Infected seed can result in stem lesions and small dark spots on cotyledons; seedling death may occur and resembles damping-off. Plants that survive black leg at the seeding stage from seedborne inoculum exhibit stunting, dark, irregular leaf spots, and blackish stem lesions. Pale, irregular spots due to ascospore infections may develop on cotyledons, leaves, stems or petioles, later becoming somewhat circular to oval, ashy-gray colored with scattered tiny, black pycnidia. Sometimes the center of lesions will fall out, especially after heavy rainfall, but a narrow, ashy-gray band with pycnidia usually remains.

Sampling Seed stock and seed from each lot should be assayed for the causal organism. 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 (all crucifer seed must be accompanied by an official test stating that the untreated seed is free from black leg (Leptosphaeria species):

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 cruciferous family (which includes arugula) 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 plant residues are at high risk for developing black leg.
• Delay spring plantings until the weather is drier to avoid periods of ascospore release.
• Bury residues after harvest by deep plowing if possible, 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.
• Hot water seed treatment for 25 to 30 min. in water at 122°F. Note: wetted seed will become gelatinous and stick to the drying surface, use of sterile sand for drying seed can be considered
• 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 is essential if growing seed fields. 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 hot water (25 to 30 min. in water at 122°F) or with a fungicide:

• Dynasty (Group 11) at 0.1 to 0.38 fl oz/100 lb seed plus a dye is registered for control of seedborne fungi but studies by WSU have shown Dynasty to not be completely effective on seedborne Leptosphaeria. 4-hr reentry.
• Maxim 4FS (Group 12) at 0.08 to 0.16 fl oz/100 lb seed plus a dye is registered for control of seedborne fungi but studies by WSU have shown Maxim to not be very effective on seedborne Leptosphaeria. See label for reentry restrictions.

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 (2) applications per crop. 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 0 days. 12-hr reentry.
• Merivon Xemium (Group 7 + 11) at 4 to 11 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 11). First test on a small portion of crop for phytotoxicity. Preharvest interval is 1 day. 12-hr reentry.

Reference 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.