Damping off of seedlings

Host: Many fruits, flowers and vegetables including tomato
Pathogen: Many fungal pathogens including Pythium, Rhizoctonia and Fusarium

Distribution

Damping off is a very common disease of seedlings of a number of plants under various soils and climate conditions. The disease has been known to the Europe and America since 19^th century. Hesse (1874) and de Bary (1881) from Germany were among the pioneers to study this disease.

Symptoms

Damping off is used for "sudden death" (of seedlings). Infected seedling topple over suddenly within an hour. The disease is found at the seedling stage only. Most of the damping off causing pathogens are week parasite (facultative parasite). Hence, they can not infect the mature plantlets with developed secondary tissues.

Diagrammatic representation of 'damping off' of seedlings. a: healthy and erect seedling, b: infected seedling with progress of infection toward above ground stem from roots, c: infected seedling topple over and show "damping off ". Arrowheads mark the infected parts.

Pathogen enters the host through the roots and secretes cellulases and pectinases causing degradation of primary cell wall and middle lamella respectively. As this degradation progresses to the above ground, plant suddenly falls over because of lack of mechanical tissues and eventually collapsed. In large nursery, the disease progress in circular fashion.

Pathogens

Damping off of seedling is caused by variety of fungal pathogens. However, Pythium debaryanum Hesse is the most studied culprit. P. debaryanum is commonly found in most of the garden and field soils, where it survive saprophytically. On getting host, it enters to the roots and rapidly branches growing intercellularly as well intracellularly (thus, behave parasitically).
The mycelium of P. debaryanum is coenocytic and aseptate. It does not produce any haustorium. Terminal as well intercallary sporangia are borne on these hyphae. Under high humid condition, sporangium produces zoospores and behave like zoosporangium. In less humid condition, sporangium germinates directly and behave as conidium. This is why, sporangium is also referred as conidiosporangium. Conidia are reniform, biflagellate and naked (lack call wall). Sporangium of P. debaryanum lacks apical papilla.

Disease cycle

The moisture and temperature of nursery bed is ideal for the growth of damping off causing fungus.

Control measures

Once the disease appears in the seedlings, it is very hard to control. Hence, preventive measures should be adapted to minimize the loss. These are:

• Sterilized or tyndallized growing medium should be used for preparation of nurseries.
• Fumigation of nursery beds with formaldehyde eliminates any pathogen.
• Nursery beds should be with proper drainage and aeration.
• Sow the seeds thinly, as overcrowding induces humid and moist conditions.
• Watering should be done only on when needed.
• Fungicides such as copper oxychloride or captan (0.1 %) should be used either as foliar spray or soil drenching.
• Drenching of nursery beds with Bordeaux mixture is also recommended.

---

Content first created on 20-12-2020
last updated on 23-05-2021