Diseases of Vegetable, Ornamental and Spice Crops

Exercise 6: LEAF SPOT DISEASES CAUSED BY SEPTORIA/ PHAEOISARIOPSIS/ CERCOSPORA/ PHOMOPSIS SPP.

a) Septoria leaf spot of tomato (Causal organism Septoria lycopersici Speg.)

Kingdom : Fungi
Phylum : Ascomycota
Class : Deuteromycetes
Order : Sphaeropsidales
Family : Sphaeropsidaceae
Genus : Septoria
Species : lycopersici

Symptoms: The initial symptoms appear on the under surface of lower leaves as water soaked spots. As the spots grow larger, these become more or less circular in outline and show definite brown coloured margins with grey centres. A few black glistening pinhead sized pycnidia may be seen in the center of spots. The disease on the stem appears as small, slightly elongated, dark spots containing numerous black pycnidia. Small-elongated specks appear on calyx. Black circular spots may also be seen on young as well as mature fruits.

Mycelium: Mycelium is hyaline, thin walled and sparingly septate. Older mycelium is brown, infrequently branched and septate.

Asexual reproduction: Pycnidia are subglobose and composed of 2-3 layers of brown cells. Pycnidiospores are filiform, slightly curved, hyaline and septate with pointed or rounded ends (Fig. 1).

Fig. 1: Microscopic details of Septoria lycopersici

b) Phomopsis blight of brinjal (Causal organism Phomopsis vexans (Sacc. & Syd.) Harter)

Kingdom : Fungi
Phylum : Ascomycota
Class : Deuteromycetes
Order : Sphaeropsidales
Family : Sphaeropsidaceae
Genus : Phomopsis
Species : vexans

Symptoms: In nursery bed, the pathogen causes damping off. In field, it appears on the leaves as circular grey spots with light colored centers. In late stages, the lighter portion is studded with numerous black pycnidia. Affected leaves turn yellow and fall down pre maturely. The disease also appears on the stem in the form of elongated, blackish-brown lesions. When there is girdling of the stem at the base due to infection, the shoots above the infected area shows wilting and soon dry up. In strong wind, such plants topple down due to breaking of the main stem. On the fruit, disease manifests as pale sunken spots, which later enlarge and produce concentric rings of yellowish and brown zones and cover the entire fruit. These spots are also marked by the presence of many black pycnidia. If infection enters the fruit through the calyx, the whole fruit becomes mummified due to dry rot.

Mycelium: The mycelium is hyaline and septate.

Asexual reproduction: The pycnidium may or may not contain beak. The conidiophores (phialides) in the pycnidium are hyaline, simple or branched, sometimes septate. The pathogen produces two types of conidia viz., alpha and beta. Alpha spores are hyaline, one celled, sub cylindrical. Beta form of conidia, the stylospores are filiform, curved, hyaline and septate (Fig. 2). However, the formation of conidia in pycnidia of P. vexans is temperature dependent.

Fig. 2: Microscopic details of Phomopsis vexans

c) Angular leaf spot of French bean (Causal organism Phaeoisariopsis griseola (Sacc.) Ferr. )

Kingdom : Fungi
Phylum : Ascomycota
Class : Deuteromycetes
Order : Moniliales
Family : Moniliaceae
Genus : Phaeoisariopsis
Species : griseola

Symptoms: On cotyledonary leaves, the symptoms appear as circular spots while on true leaves, 3-5 angled spots appear in between veins and veinlets. The spots are dark grayish on upper surface and light gray on the lower surface of the leaves. With the passage of time, the spots change to reddish brown and finally attain dark brown colour. On close observation, the spots reveal the presence of coremia bearing large number of spores. Such severely infected leaves show upward curling and defoliate prematurely. The fungus also attacks the pods causing superficial, smooth, usually circular spots with reddish brown center and ashy black borders. Elongated dark brown spots appear both on petioles and stems. Under severe conditions, complete defoliation occurs resulting in pre-mature death of the plants. In late infections i.e. at maturity stage, the seeds underneath the pod lesions show yellowish brown discolouration which is more prominent at hilum region but can occur any where on the seed surface.

Asexual reproduction: The fungus produces a stromatic structure in the substomatal cavity. Conidiophores are loosely joined together in the form of synnemata or coremia, which are darker at the base and lighter towards the tips (Fig. 3). Conidia (1-3 septate and rarely 4-5) are borne singly on the tips of columnar hyphae. These are light grey to hyaline, cylindrical to spindle shaped and straight to slightly curved.

Fig. 3: Microscopic details of Phaeoisariopsis griseola

d) Cercospora leaf spot of bell pepper (Causal organism Cercospora capsici Heald & Wolf )

Kingdom : Fungi
Phylum : Ascomycota
Class : Deuteromycetes
Order : Moniliales
Family : Dematiaceae
Genus : Cercospora
Species : capsici

Symptoms: Small necrotic lesions resembling frog eye appear on the leaf surface later coalesce giving an irregular and blighted appearance. The spots have brown borders with light coloured, faded or greyish central part. The leaves turn yellow and fall down prematurely. Symptoms also appear on the stems and petioles as elongated or irregular spots. In severe cases the fruits are also attacked and get decayed. Defoliation is the most damaging phase of the disease.

Mycelium: Septate, branched, hyaline, intercellular or intracellular.

Asexual reproduction: Conidiophore fasciculate, straight or slightly curved, continuous, simple yellowish brown. Conidia are filiform, hyaline and septate (Fig. 4).

Fig. 4: Conidiophore and conidia of Cercospora capsici