Damping off

General

• Syndrome, set of symptoms frequently observed in the nurseries of many vegetable species, but also in the field following direct sowing.

• Their causes are very often biotic or parasitic: fungi or Oomycetes subservient to the soil and/or transmitted by seeds; they occur in all the production regions of the world, and particularly in the tropics.

• Affected production areas :

• Sensitive botanical family (s)

• Organs attacked :

• Symptoms :
  • Seeds becoming soft, rotting and not germinating.
  • Initially moist lesions located on the hypocotyl, particularly near the surface of the substrate, browning and/or reddening as they evolve. They eventually surround the stem with soft tissues (figures 1 and 2).
  • Brown necrosis and rotting of roots and rootlets (figure 4).
  • Wilting and drying of cotyledons and young leaves, collapse of seedlings (Figures 3, 5 and 6).

• Possible causes : Oomycetes ( Pythium spp., Phytophthora spp.), Rhizoctonia solani , Sclerotium rolfsii , Sclerotinia sclerotiorum , Botrytis cinerea , several seed-borne fungi, etc.

• Signs : various structures depending on the fungus or oomycete responsible (mycelium - figures 7 and 8-, sclerotia, oospores - figure 9 -, sporangia, etc.).

• Possible confusion : some abiotic problems related for example to the poor quality of the seeds revealing an insufficient germination energy, to the nature of the soil and/or to its hydric state, etc.

• Main reflexes : recover and observe a few moribund seedlings in order to note the presence of root damage and lesions at the collar which will confirm damping off. Confirm that the affected seedlings are distributed in more or less extensive foci, and investigate in the nursery to ascertain the possible presence of other foci. Check whether these hotbeds do not correspond to the wettest and/or coldest areas of the nursery. Seedling samples may be sent to a laboratory to identify the causative pathogen. 

See also this  sheet