Seed Crop, Crucifers (Brassica, Eruca, and Raphanus spp.)-Alternaria Diseases (Black Spot, Gray Leaf Spot, Pod Spot)

Latest revision: 
March 2023

By C. M. Ocamb

Cause Alternaria brassicae and A. brassicicola can infect leaves, petioles, stems, flower parts, and seed pods of a wide range of Brassicaceae crops and weeds. These fungi survive on residues of infected crucifer crops and weeds, producing asexual spores (conidia) when conditions are conducive and the debris is on the soil surface. The conidia can be moved by wind and water. Flea beetles have also been reported to transmit A. brassicicola to cabbage.

Conidia produced on infected plant debris are considered the primary inoculum for disease outbreaks. Studies have shown that A. brassicae and A. brassicicola on infected plant debris can produce conidia for as long as the debris remains intact, with infected stem tissues releasing spores up to 23 weeks later and which can spread diseases across a field or to neighboring fields. Conidia are generally present throughout the cropping cycle, on dead or living host material, so the threat of disease build-up during conducive environmental conditions is fairly constant. Conidia produced by A. brassicicola infect plants through direct penetration and rarely infect though stomata. Wet weather when crops are mature can lead to a rapid build-up in disease levels but conidia are dispersed during warm, dry periods following wet conditions. Temperature and relative humidity can both modulate disease development but the genetics of the plant host as well as the respective Alternaria species appear to play a role in maximum and minimum temperature range that promotes disease development. Studies have shown that sporulation by A. brassicae and A. brassicicola requires a relative humidity level at or above 91.5% and 87% while optimum temperatures were reported to be 64 to 75ºF and 68 to 86ºF, respectively. In a separate study, growth and sporulation by A. brassicae occurred when relative humidity was above 50% and temperature was between 41ºF and 86ºF, but growth and sporulation increased as relative humidity increased and 73ºF was found to be the optimum temperature. A high relative humidity (>95%) or free moisture are required for spore germination. It has been reported that a minimum wetness period of four hours was necessary for infection of oilseed rape at 64ºF, and that disease severity increased with an increasing wetness period up to 12 hours. Germination of spores produced by A. brassicicola and subsequent development of disease occurs when ambient temperature is between 82ºF and 88ºF. Generally, A. brassicae can infect under cooler temperatures and more quickly than can A. brassicicola.

Alternaria brassicae and A. brassicicola can also be seedborne. Most of the seedborne presence is superficial, but studies have found A. brassicae and A. brassicicola occurring internally in seed. Alternaria brassicae gradually dies out during storage at room temperature, with Indian mustard seed being free of the pathogen after five months in storage. But internal infections by A. brassicicola may persist for much longer, up to 12 years in cabbage seed studies.

Symptoms Older leaves and older plants are more susceptible. Small, dark or yellow leaf spots first develop, enlarging to circular areas that are brown to gray in color with or without concentric rings and possibly with black or purple borders and/or surrounded by yellow halos. Sometimes the leaf spots are limited by leaf veins so the spots are angular in appearance rather than circular. The centers of leaf spots may be coated with sooty black spore masses, and can crack or may drop out, producing shot holes. Leaf spot size ranges from areas barely visible (0.1 inch or smaller diameter) to more than 2 inches in diameter. Leaf lesions can lead to premature defoliation. Cauliflower curds as well as heads of cabbage, Brussels sprouts, and broccoli may be infected, leading to dark brown spots on curds or heads. Stem and petiole lesions are elongated and dark brown to black in color. On infected seed pods, violet to tan to black spots develop, and large areas of the pods may be dark in coloration, and can lead to infected seeds that are smaller in size and shriveled in appearance. Planting of infested seed can lead to seedling death when soil temperatures are warm.

Cultural control

  • Rotate out of crucifers for at least three years. Chose varieties with resistance to Alternaria if available. Avoid planting spring and fall-sown crops in close vicinity to each other. If possible, time planting so that crops mature before conditions are favorable for disease.
  • Control susceptible weeds and volunteers in between cropping with crucifers.
  • Use pathogen-free seed. Seed not known to be clean may be treated in water at 122°F for 15 to 30 min. Treat a small sample of each seed lot, and check the germination after treatment to ensure seed will survive treatment.
  • Removal and destruction of symptomatic older leaves can be helpful early in the epidemic.
  • Bury or remove crucifer residues.

Chemical control Crops grown for seed can be sprayed at bloom and during the growing season.

  • Badge SC (Group M1) at 0.5 to 1.8 pints/A on 7- to 10-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
  • Bravo Ultrex (Group M5) at 1.4 lb/A with water to cover. 12-hr reentry.
  • Champ WG (Group M1) at 1.06 lb/A for cabbage and at 1 lb/A for other crucifers on 7- to 10-day intervals. 48-hr reentry. O
  • Cuprofix Ultra 40 Disperss (Group M1) at 0.75 to 1.25 lb/A on 7- to 10-day intervals. 48-hr reentry.
  • Echo 720 (Group M5) at 1.5 pints/A at 7- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
  • Fontelis (Group 7) at 14 to 30 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action (non-Group 7). Preharvest interval is 0 days. 12-hr reentry.
  • JMS Stylet-Oil at 3 to 6 quarts/100 gal water. Do not spray if temperature is below 50°F, above 90°F or when plants are wet or under heat or moisture stress. 4-hr reentry.
  • Mancozeb formulations (Group M3) are labeled.
    • Dithane DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-020028, SLN OR-020030). 24-hr reentry.
    • Dithane F45 Rainshield at 1.5 quart/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-090020, SLN OR-090016). 24-hr reentry.
    • Roper DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only in Oregon (SLN OR-130003) and Washington (SLN WA-130003). 24-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.
  • Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 5- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
  • Rovral 4 Flowable (Group 2) at 2 to 4 pints/A in 20 to 100 gal water for seed production in Oregon (SLN OR-130001) only. Not labeled for use on canola/rapeseed. Apply at full bloom, pod set, and before harvest if disease pressure is severe. 24-hr reentry.
  • Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
    • Cabrio EG at 12 to 16 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
    • Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 14-day intervals. May be applied the day of harvest. 4-hr reentry.
  • Switch (Group 12 + 9) at 14 oz/A on 7- to 10-day intervals beginning at bloom for seed crops in Oregon (SLN OR-160001) and Washington (SLN WA-020016). Preharvest interval is 7 days. 12-hr reentry.

Biological control Efficacy unknown in Oregon.

  • Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 4-hr reentry. O
  • Cease at 3 to 6 quarts in 100 gal water. For greenhouse plants only. Preharvest interval is 0 days. 4-hr reentry. O

References Al-lami, H.F.D., You, M.P., and Barbetti, M.J. 2020. Temperature drives contrasting Alternaria leaf spot epidemic development in canola and mustard rape from Alternaria japonica and A. brassicae. Plant Dis. 104(6):1668-1674.

Ansari, N.A., Khan, M.W., and Muheet, A. 1989. Effect of some factors on growth and sporulation of Alternaria brassicae causing Alternaria blight of rapeseed and mustard. Acta Botanica Indica 17(1):49-53.

Chahal, A.S. 1981. Seed-borne infection of Alternaria brassicae in Indian mustard and its elimination during storage. Current Science 50(14):621-623.

Cucuzza, J. et al. 1994. Crucifer Diseases: A Practical Guide for Seedsmen, Growers and Agricultural Advisors. Saticoy, CA: Seminis Vegetable Seeds, Inc.

Dillard, H.R., Cobb, A.C., and Lamboy, J.S. 1998. Transmission of Alternaria brassicicola to cabbage by flea beetles (Phyllotreta cruciferae). Plant Dis. 82(2):153-157.

Hong, C.X., and Fitt, B.D. 1995. Effects of inoculum concentration, leaf age and wetness period on the development of dark leaf and pod spot (Alternaria brassicae) on oilseed rape (Brassica napus). Ann. Appl. Biol. 127(2):283-295.

Humpherson-Jones, F.N., and Maude, R.B. 1982. Studies on the epidemiology of Alternaria brassicicola in Brassica oleracea seed production crops. Annals of Applied Biology 100:61-71.

Humpherson-Jones, F.M., and Phelps, K. 1989. Climatic factors influencing spore production in Alternaria brassicae and Alternaria brassicicola. Ann. Appl. Biol. 114(3):449-458.

Maude, R.B., and Humpherson-Jones, F.M. 1980. Studies on the seed-borne phase of dark leaf spot (Alternaria brassicicola) and grey leaf spot (Alternaria brassicae) of brassicas. Ann. Appl. Biol. 95:311-319.

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.

Tewari, J.P., and Buchwald, L. 2007. Alternaria Diseases. In: Compendium of Brassica Diseases. Rimmer, S.R., Shattuck, V.I., and Buchwaldt, L. (eds.). St. Paul, MN: APS Press; p. 15-18.