Alternaria brassicicola (Pig.) Wiltshire

Cabbage alternaria (black spot, gray leaf spot, pod spot, etc.)

General

• World famous disease.
• Several species of Alternaria affect cultivated or wild brassicas, especially cabbages, turnips, etc. : A. brassicicola (Schwein.) Wiltshire which produces a multitude of shorter multicellular spores and A. brassicae (Berk.) Sacc. characterized by isolated, few and fairly long conidia. A. japonica Yoshii (= A. raphani Groves & Skolko) particularly affects radish.
• A. brassicicola seems to be the most frequent species in tropical to equatorial zones.
• Disease rather observed in the field.

Sensitive botanical family (s)

Affected production areas :

Organs attacked

Symptoms and signs  

• Symptoms :
  • Small dark spots of 1 to 3 mm. They enlarge quite rapidly and form circular lesions which rapidly take on a yellowish-brown to black hue (figures 1 to 4) while a fairly marked yellow halo surrounds them (figure 5). They are sometimes partially angular when bounded by the ribs.
  • The altered plant tissues located in the center of the spots lighten up, from time to time show concentric "patterns". They become thin, leathery and soon fall off (Figures 6-8).
  • Leaves more or less riddled at term.
  • Lesions sometimes form on petioles and stems.

Similar symptoms are observed on other Brassicaceae, for example on turnips (figures 13 and 14).

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• Signs : presence of a more or less dense brown to black spore-bearing velvety texture on the lesions (figures 8 to 10); it sometimes contributes to the concentric aspect of these. In the presence A. of brassicae , the shape of the conidia is quite different (fig 11 and 12).
• Possible confusion : Sigatoka

Biology

• Preservation : has saprophytic potential allowing it to be preserved in the soil on a very large number of plant debris, thanks to its mycelium, its spores, and even microsclerotia. Also maintains on various alternative hosts and weeds, as well as on seeds which also ensure its dissemination.
• Sources of inoculum : primary inoculum from seed or soil and dispersed by water spray and soil particles.
• Infection : penetration of the lower leaves mainly through the stomata but also directly through the cuticle. Progressive invasion and destruction of leaf tissue, and appearance of lesions in a few days.
• Sporulation : conidiophores bearing conidia are formed on the surface of the damaged tissues at the origin of the poriferous velvety (figures 8 to 10).
• Dissemination : spores dispersed frequently by the wind over long distances, or in a more limited way by splashing water associated with heavy rains or sprinkler irrigation. Seed transmission possible, more common than for A. brassicicae .
• Favorable conditions : the presence of humidity, wet periods. Conversely, there is a slowdown in damage during the dry season. The dew, by wetting the leaves for several consecutive nights, favorably influences the germination of the conidia, the penetration of the germinal filaments, and subsequently the sporulation.

Protection

• Carry out long crop rotations , they should not involve other crucifers that are also sensitive.
• Avoid the presence of cultivated crucifers or weeds in the growing environment.
• Use healthy seeds or disinfect them with either hot water or a fungicide .
• Use healthy plants.
• Ensure good drainage of cultivated plots.
• Avoid too high planting densities in order to favor the aeration of the foliage.
• Strip the lower parts of the plants in order to eliminate the first affected leaves and improve the ventilation of the plant cover.
• Ban irrigation , prefer drip irrigation. If they are essential, carry them out in the morning so that the vegetation drains quickly during the day.
• Do not allow workers to work while vegetation is wet.
• fairly quickly plant debris , during cultivation following the various cultivation operations, and at the end of cultivation after uprooting the plants. They will have to be destroyed or buried deep.
• If necessary, spray fungicides taking into account authorized uses. The applications will be carried out every 7 to 10 days, on dry plants, and must be renewed following significant rains exceeding 20 mm.