Diseases of Vegetable, Ornamental and Spice Crops

Exercise 3: DISEASES CAUSED BY DOWNY MILDEW AND WHITE RUST FUNGI

a) Downy mildew of crucifers (Causal organism Peronospora parasitica (Pers.) ex. Fr.)

Kingdom : Chromista
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Peronosporaceae
Genus : Peronospora
Species : parasitica

Symptoms: The symptoms are first visible in nursery plants. Translucid spots appear on the underside of leaves in the inter-veinal spaces, which are purplish brown while corresponding upper surface is tan to yellow in colour. Downy growth bearing conidia and conidiophores appear on the under surface of the leaves. Infection of cauliflower curd and cabbage heads has also been observed. The affected curds/ heads look brownish at the top that later turn dark brown to black.

Mycelium: Hyaline, well developed, branched, coenocytic, intercellular, haustoria are finger shaped.

Asexual reproduction: Sporangiophore hyaline, branched dichotomously at the tip (Fig. 1). Sporangium round to oval, hyaline, germinates by germ tube.

Sexual reproduction: Oogamous, by gametangial contact. Oospores round, thick walled (Fig. 1), dark brown and formed in senescent tissue.

Fig. 1. Microscopic details of Pernospora

b) Downy mildew of cucurbits (Causal organism Pseudoperonospora cubensis (Berk. and Curt.) Rostow.

Kingdom : Chromista
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Peronosporaceae
Genus : Pseudoperonospora
Species : cubensis

Symptoms: Appearance of yellow angular spots on the upper surface of the leaves while on the corresponding lower surface, a purplish downy growth of the pathogen appears in moist weather. Several such yellow spots appear in large number on the leaf surface, which led to the death of the leaves and whole vine.

Mycelium: Hyaline, well developed, branched, coenocytic, intercellular, ovate or branched haustoria.

Asexual reproduction: Sporangiophore hyaline, dichotomously branched at acute angles in the upper third of their growth (Fig. 2(A)). Sporangium oval to elliptical (Fig.2(B)), pale grey to purple, double walled and papillate, germinates indirectly by producing 3-7 zoospores.

Sexual reproduction: Oogamous, by gametangial contact but rare in cucumber. Oospores are thick, smooth walled, light yellow in colour and formed in senescent tissue.

Fig. 2. Sporangiospores (A) and sporangia (B) of Pseudoperonospora

c) Downy mildew of onion (Causal organism Peronospora destructor (Berk.) Casp.)

Kingdom : Chromista
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Peronosporaceae
Genus : Peronospora
Species : destructor

Symptoms: Systemic infection occurs when the plants are raised from diseased bulbs or infected seedlings are used for planting. Plants raised from such bulbs remain stunted become distorted and light green in colour. In humid weather, sporulation get developed on the leaves and covered with felty white to grayish fungal growth. In secondary and local infections, spots with greenish and chlorotic zones appear and at the point of infection, the plants fall off. In humid weather, the fungus develops as white to purplish downy growth on the spots. In such cases undersize bulbs are produced.

Mycelium: Hyaline, well developed, branched, coenocytic, intercellular with sac shaped haustoria.

Asexual reproduction: The sporangiophores are aseptate, hyaline swollen at the base, and at tips dichotomously branched. The sterigmata are acute to subacute and bear pyriform to fusiform sporangia.

Sexual reproduction: Oospores develop late in the season, measure 40 to 50 µm and germinate by germtube.



d) White rust of crucifers (Causal organism Albugo candida (Pers.ex Chev.) Kuntze.)

Kingdom : Chromista
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Albuginaceae
Genus : Albugo
Species : candida

Symptoms: White or creamy yellow, tiny and round pustules appear on lower surface of lower leaves. Two or more pustule coalesces to form bigger pustules. Infected leaves become thick and fleshy. Plants remain stunted. In systemic infection, various types of distortions appear in stem inflorescence and floral parts. The size of infected stem increases several folds while the floral organs become swollen, fleshy and violet coloured.

Mycelium: Hyaline, coenocytic, endophytic, intercellular with knob shaped haustoria.

Asexual reproduction: Sporangiophore club-shaped, unbranched, hyaline, clavate, bears sporangia in chain and in basipetal succession (Fig. 3). Sporangium spherical, thin walled, hyaline, germinate by zoospores. Zoospores hyaline, biflagellate and kidney shaped (Fig. 3).

Sexual reproduction: Oogamous, by gametangial contact. Oospore round, thick walled, dark brown in colour, outer wall tuberculate or warty (Fig 3).

Fig.3 Microscopic details of Albugo candida