Diseases of Vegetable, Ornamental and Spice Crops

Exercise 5: DISEASES CAUSED BY ALTERNARIA SPP.

a) Early blight of potato, tomato and geranium (Causal organism Alternaria solani (Ell. & Mart.) Sor.)

Kingdom : Fungi
Phylum : Ascomycota
Class : Deuteromycetes
Order : Moniliales
Family : Dematiaceae
Genus : Alternaria
Species : solani

Symptoms: Lowest leaves are attacked first and then the disease progresses upwards. Dark brown spots with concentric rings develop on the leaves, which give target board effect, the most characteristic symptom of the disease. 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. Streaks also develop on stem and branches. However, tuber infection is rare.

Mycelium: Septate, branched, dark brown and intercellular.

Asexual reproduction: Conidiophores short, septate, dark coloured, bears conidia in chains. Conida obclavate, septate transversely and longitudinally, dark brown, smooth walled and beaked (Fig. 1).

Fig.1: Conidiopore and conidia of Alternaria solani

b) Alternaria leaf spot of crucifers (Causal organism Alternaria brassicae (Berk.) Sacc. and Alternaria brassicicola (Schw.) Wiltshire)

Kingdom : Fungi
Phylum : Ascomycota
Class : Deutromycetes
Order : Moniliales
Family : Dematiaceae
Genus : Alternaria
Species : brassicae/ brassicicola

Symptoms: On leaves, symptoms appear as minute dark brown to black spots, which may enlarge by forming concentric rings (Fig. 2). Each spot is surrounded by a yellow halo of chlorotic tissues. Older lesions are circular, often zonated with a papery thin center which in humid weather may be covered with a mat of spores which are light brownish in case of A. brassicae and dark olive brown in case of A. brassisicola. Severe defoliation by A. brassicae has been reported in some cultivars of stubble turnips. The lesions are linear on stem, petiole and siliquae. Browning of cauliflower/ broccoli heads occur. Minute dark brown spots are visible on pods or siliqua.

Fig. 2: Symptoms of Alternaria leaf spot

A. brassicae

Mycelium: Mycelium well developed, hyphae branched, septate and hyaline.

Asexual reproduction: Conidiophores arising in groups of 2-10 from the hyphae, emerging through stomata, usually simple, erect, frequently geniculate, more or less cylindrical and septate. Conidia solitary or occasionally in chains of up to 4, straight or singly curved, obclavate, rostrate with 16-19 (usually 11-15) transverse septa and 0-8 (usually 0-3) longitudinal or oblique septa, pale or greyish olive and beaked (Fig. 3).

Fig. 3: Conidia of Alternaria brassicae

A. brassicicola

Mycelium: Mycelium well developed; hyphae branched, septate, hyaline at first, later brown or olivaceous brown, inter- and intra-cellular and smooth.

Asexual reproduction: Conidiophores arising singly or in groups of 2-12 or more, emerging through stomata, usually simple, erect occasionally geniculate, but often slightly swollen at the base, septate, pale to mild olivaceous brown and smooth. Conidia mostly in chains of up to 20 or more, sometimes branched, arising through small pores in the conidiophore wall, straight, nearly cylindrical, usually tapering slightly towards the apex or obclavate, the basal cell rounded, the beak usually almost non-existent with 1-11, mostly less than 6, transverse septa, often constricted at the septa, pale to dark olivaceous brown, smooth or becoming slightly warted with age.