Grey leaf spot (Brassica)

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Alternaria
Grey leaf spot (Brassica)
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Scientific Classification
Kingdom: Fungi
Phylum: Ascomycota
Class: Dothideomycetes
Subclass: Pleosporomycetidae
Order: Pleosporales
Family: Pleosporaceae
Genus: Alternaria
Species: Alternaria brassicae
Synonyms
Black leaf spot

Alternaria alliariae-officinalis
Alternaria brassicae var. macrospora
Alternaria exitiosa
Alternaria herculea
Alternaria macrospora
Alternaria saccardoi
Cercospora lepidii
Cercospora moldavica
Macrosporium brassicae
Macrosporium brassicae var. macrosporum
Macrosporium herculeum
Macrosporium macrosporum
Polydesmus exitiosus
Puccinia brassicae
Rhopalidium brassicae
Sporidesmium brassicae
Sporidesmium exitiosum

Sporidesmium onnii

Grey leaf spot (Alternaria brassicae) is a plant pathogen causing leaf spots on most Brassica species including broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kohl rabi, kale, swede, and turnip.[1]

Symptoms[edit]

The pathogen can affect host species at all stages of growth, including seeds. On seedlings symptoms include dark stem lesions immediately after germination, that can result in damping off, or stunted seedlings.[2]

When older plants become infected, Alternaria symptoms often occur on the older leaves, since they are closer to the soil and are more readily infected as a consequence of rain splash or wind blown rain.[2] Spots caused by Alternaria brassicicola tend to be darker and have a more irregular margin than those cause by Alternaria brassicae. Brussels sprout yields can be seriously reduced due to a low tolerance for spots and blemishes.[3]

Fruit-bearing branches and seed pods show dark or blackened spots that result in yield loss due to premature pod ripening and shedding of the seeds. Infection can also occur on the fruit, before or after harvest. A common symptom of broccoli and cauliflower infection is a browning that occurs on the head.[2]

The disease may also be a means by which bacterial soft rot enters the stem, which may lead to plant death.[4]

In addition to destruction of a seed crop, the pathogens can live within the seed, spread the disease to other fields, and cause a loss of seedlings[5]

Treatment[edit]

Late infection, or infection of older leaves, does not characteristically reduce yields, and can be controlled through intensive removal of infected leaves.[4]

Non-Chemical[edit]

Hot-water treatment

Seed treatment with hot water is one method of controlling spores on the seed coat. However, this treatment sometimes depresses germination.

Crop rotations

Rotation with noncruciferous crops and eradication of cruciferous weed hosts can help control these pathogens. Since spores can survive on leaf tissue for 8 to 12 weeks and stem tissue for up to 23 weeks, fields that are replanted soon after harvest often coincide with a large amount of inoculum which is likely to effect the crop's emergence and early growth stages.[6]

Use of resistance

Cultivars of Brassica species differ in resistance to A. brassicae but differences are not large.[7]

Biological control

Preliminary studies with the Actinomycete fungus, Streptomyces arabicus, indicated an antifungal effect on A. brassicae and A. brassicicola in both laboratory and field studies.[8][9]

Chemical[edit]

The following fungicides can be used to control A. brassicae:

  • Benlate at 0.1 lb a.i./100 gal
  • Dithane M-45 at 0.2 lb a.i./100 gal
  • Dithane Z-78 at 0.2 lb a.i./100 gal
  • Ziram at 0.2 lb a.i./100 gal
  • Difolatan-80 at 0.2 lb a.i./100 gal
  • Thiram at 0.2 lb a.i./100 gal
  • Blitox-50 at 0.3 lb a.i./100 gal
  • Iprodione at 0.25 lb a.i./100 lb seed
  • Fenpropimorph at 0.25 lb a.i./100 lb seed
  • Aoxystrobin (Amistar) 2-5 oz/A. Apply prior to disease development and continue at 7-14 day intervals. Do not make more than one application of Amistar before alternating with a fungicide with a different mode of action.
  • Chlorothalonil (Bravo Ultrex 82WDG) 1.4 lb/A. Apply at the first sign of disease and repeat at 7-10 day intervals.
  • Cyprodinil plus fludioxonil 11-14 oz/A. Apply at the first sign of disease and repeat at 7-10 day intervals. Only turnip varieties harvested for their leaves may be treated.
  • Maneb (Maneb, Manex) Rates vary depending on the formulation.

See also[edit]

References[edit]

  1. Dicklow, M.B. (2005). Broccoli Alternaria - Alternaria diseases of Brassicas - The Plant Diagnostic Clinic. University of Massachusetts Amherst. Retrieved: 2010-08-08.
  2. a b c Ferreira, S.A. Boley, R.A. (1991). Blight, black leafspot, gray leafspot (Plant Disease Pathogen). Crop Knowledge Master. University of Hawaii, Manoa. Retrieved: 2010-08-08.
  3. Koike, S.T. Gladders, P. Paulus, A.O. (2007). "Vegetable diseases". Manson Publishing, London. ISBN 0123736757
  4. a b Chupp, C., and A.F. Sherf. 1960. Vegetable diseases and their control. Pp. 267-269. The Ronald Press Company. New York. 693 pp.
  5. Rangel, J.F. 1945. Two Alternaria diseases of cruciferous plants. Phytopathology 35:1002-1007.
  6. Humpherson-Jones, F.M. 1989. Survival of Alternaria brassicae and Alternaria brassicicola on crop debris of oilseed rape and cabbage. Ann. appl. Biol. 115:45-50.
  7. Bansal, V.K., G. Seguin-Swartz, G.F.W. Rakow, and G.A. Petrie. 1990. Reaction of Brassica species to infection by Alternaria brassicae. Can. J. Plant Sci. 70:1159-1162.
  8. Sharma, A.K., J.S. Gupta, and R.K. Maheshwari. 1984. The relationship of Streptomyces arabicus to Alternaria brassicae (Berk.) Sacc. and Alternaria brassicicola (Schew.) Wiltshire on the leaf surface of yellow sarson and taramira. Geobios New Reports 3:83-84.
  9. Sharma, A.K., J.S. Gupta, and S.P. Singh. 1985. Effect of temperature on the antifungal activity of Streptomyces arabicus against Alternaria brassicae (Berk) Sacc. and A. brassicicola (Schew.) Wiltshire. Geobios 12:168-169.