Diseases of Vegetable, Ornamental and Spice Crops

DISEASES CAUSED BY PYTHIUM SPP

Exercise 1: DISEASES CAUSED BY PYTHIUM SPP.

a) Damping-off of vegetables (Causal organism: Pythium aphanidermatum (Edson) Fitz.)

Kingdom : Chromista
Division : Eumycota
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Pythiaceae
Genus : Pythium
Species : aphanidermatum

Symptoms: The disease manifests at two phases:

Pre-emergence damping-off: Seed rots or young seedlings are killed before their emergence from the soil resulting in patchy appearance of seedlings stand in the nursery .

Post-emergence damping-off: The infected tissue of stem at soil level initially appears to be water soaked and soft, subsequently, the stem at the infection point gets constricted resulting in toppling over and mortality of the seedlings.


b) Soft rot of ginger (Causal organism: Pythium aphanidermatum (Edson) Fitz.)

Symptoms: Leaves turn yellow. The yellowing starts from leaf tip and spreads down the leaf blade. The dead leaves hang down. Tillers at soil line become soft and later topples down. The rhizomes become soft and mushy. Roots also rot.

Mycelium: Well-developed, branched, coenocytic, hyaline, intracellular, without haustoria.

Asexual reproduction: Sporangium much lobed, toruloid, branched, produced only in liquid culture, formed terminally or intercalary on somatic hypha. Zoospores produced in vesicle (Fig. 1), which emerged out of sporangium.

Sexual reproduction: Oogamous, by gametangial contact, oospores smooth, thick walled, round (Fig. 1) and light brown.

Fig 1. Microscopic details of Pythium