Alternaria diseases are the most common and serious menace to the cultivation of crucifer vegetables world over. Infection on young leaves, stems and siliquae generally results in heavy yield losses as well as quality of seed. Symptoms:
- The leaf spots incited by A. brassicicola appear as minute dark brown to black spots, which may enlarge by forming concentric rings (Plate-2a) and each spot is surrounded by a yellow halo of chlorotic tissues.
- In humid weather, the fungus appears as a bluish growth in the centre of the spots.
- The spots caused by A. brassicae are similar to those described above but they tend to remain smaller i.e < 1 cm and light brown in colour.
- The spots caused by A . raphani are slightly yellowish.
- Browning of cauliflower/ broccoli heads occur which generally begin at the margin of the individual flower or flower cluster.
- Plants grown for seeds show dark necrotic elongated lesions on the main axis, inflorescence, branches and on the siliquae.
- Infected seeds remain small in size, shrunken, discoloured or covered with fungal growth and have low viability.
- Turnip and rutabaga foliage and roots may also become infected in field but root symptoms develop after being placed in storage.
- The leaf spots are nearly circular, often zonate and are of various shades of brown to black. Radish plants kept for seed purpose are severely affected by A. raphani.
- On leaf, the spots are raised, spherical to elliptical and 0.5-1 cm in diameter, black sporulation may also appear on the lesions in the humid weather, the center soon dries and may drop down.
Pathogen (s): Three species of Alternaria are responsible for this disease: A. brassicicola (Schw.) Wiltshire, A. brassicae (Berk.) Sacc. and A. raphani Groves & Skolko.
A. brassicae:
- Attack most of the crucifer vegetables and produce conidia of bigger size with larger beak.
- Mycelium is immersed, hyphae branched, septate, hyaline.
- Conidiophores are simple, erect, bearing one to several small but distinct conidial scars.
- Conidia are solitary or occasionally in chains of up to 4, obclavate, rostrate with 16-19 transverse septa and 0-8 longitudinal or oblique septa, pale or greyish olive, the beak about 1/3 to 1/2 the length of the conidium.
A. brassicicola:
- Produces smaller conidia with shorter beak.
- Mycelium immersed, hyphae branched septate, hyaline at first, later brown or olivaceous brown.
- Conidiophores arising singly or in groups of 2-12 or more, emerging through stomata, usually simple and erect Conidia mostly in chains of up to 20 or more, nearly cylindrical, usually tapering slightly towards the apex or obclavate, the basal cell rounded, the beak usually almost non-existent, mostly less than 6, transverse septa, often constricted at the septa, pale to dark olivaceous brown, smooth or becoming slightly warted with age.
A. raphani:
- Usually attacks radish
- Conidiophores are simple or occasionally branched, septate and olivaceous brown in colour.
- Conidia are formed commonly in chains of 2-3, straight or slightly curved, obclavate or ellipsoidal, generally with a short beak, mid to dark golden brown or olivaceous brown in colour, smooth or sometimes minutely verruculose, with 3-7 transverse and often a number of longitudinal or oblique septa.
Disease cycle and epidemiology:
- The pathogen perennates on infected plant debris, in or on infected seeds and cruciferous weeds.
- Moderate temperature (21- 27o C), high relative humidity (95-100 % at least for 18h) and splashing rains favour the disease development and spread.
- The optimum temperature for sporulation is 18-24 and 20-30oC for A. brassicae and A. brassicicola, respectively, at which both fungi produce spores in 12-14 h.
- At temperature >24oC, sporulation of A. brassicae is inhibited.
- A. raphani infection progresses rapidly at 22-26oC.
Management:
- Collect and destroy the infected plant debris. Use disease free seeds and treat them with captan (0.3%).
- Spray the crop with mancozeb (0.25%) or copper oxychloride (0.3%) and repeat at 10 to 14 days interval.
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