Diseases of Vegetable, Ornamental and Spice Crops (2+1)

ALTERNARIA LEAF SPOTS

The disease is caused by three different species of Alternaria.

Symptoms

Alternaria solani:

• Dark brown spots with concentric rings develop on the leaves, which give target board effect, the most characteristic symptom of the disease (Plate-3a).
• In humid weather, the affected areas coalesce and form dark brown patches. In severe attacks, affected leaves shrivel and fall down prematurely resulting in early defoliation.

Alternaria alternata:

 

• Spots are small, circular scattered, dark brown spots (Plate-3b).
• Older spots are surrounded by yellow halo.
• The affected leaves dry prematurely.

 

A. alternata f.sp. lycopersici:

 

• Spots are small, angular, scattered and light brown in colour (Plate-3c).
• Spots are not surrounded by yellow halo.
• The symptoms also appear on stems and branches as light to dark brown spots(Plate 3d).

 

Pathogen (s)

Alternaria solani (Ell. and Mart) Jones and Grout:

• The mycelium consists of septate, branched, light brown hyphae, which become darker with age. Conidiophores emerge through the stomata from the dead centers of the spot.
• Conidia are beaked, muriform, dark and borne singly or in chains of two.
• Five to ten transverse septa and a few longitudinal septa are present in each conidium.

A. alternate (Fr.) Keissler:

• The conidia occurring on leaflet lesions under field conditions are 1-6 septate.
• The size of beak range from 3.0 - 10.0 µm with an average length of 7.9 µm.

A. alternata f.sp. lycopersici Grogan Kimble and Misaghi:

• The conidia occurring on leaflet and stem lesions under field conditions are 1-5 septate measuring 17.0 - 50.0 x 10.0 - 13.0 µm (av. 34.40 x 12.30 µm) and beak length ranged from 3.0 - 7.0 µm with an average of 6.13 µm.

Disease cycle and epidemiology

• Alternaria species survive in diseased plants debris and can persist for one to two years.
• Seed borne nature of A. solani has also been reported.
• Primary infection of lower leaves first takes place through conidia formed on crop debris in soil. Secondary spread of the disease occurs through conidia developed on primary spots.
• These conidia are blown by wind, water and insects to the neighbouring leaves\ plants.
• The optimum temperature for infection of A. solani is 28 to 30^oC while for A. alternata f. sp. lycopersici and A. alternata, it ranges between 25-30 and 20-25^oC, respectively.
• Maximum dispersal of conidia occurs in advanced stages of disease development and in between 9 am and 12 noon.

Management

• Collect and destroy the infected plant debris. Follow at least two years crop rotation.
• Select healthy seed and treat it with captan (0.3%).
• Remove the foliage particularly in indeterminate type of cvs/hybrids up to 15-20 cm to avoid moist and stagnant air conditions.
• Spray the crop with chlorothalonil (0.2%) or mancozeb (0.25%) and repeat at 10 to 14 days interval.