Two pathogens are mainly responsible for damping off in seedlings - Rhizoctonia solani and various species of Pythium. Both are necotrophs - drawing on the nutrients of their host's dead cells for growth.
Both diseases kill seedlings from the inside, growing through plant tissue and breaking down cells and cell walls. This leads to the collapse of the plants, or "damping off".
Seedlings stressed by being grown too close together or over-watered are most at risk. Wet conditions also favour the development of Pythium because its swimming spores allow it to spread rapidly from plant to plant.
Pythium can spread rapidly around nurseries - the pathogen has been isolated from various surfaces and water sources. Hygiene practices such as cleaning and disinfecting benches, floors and trolleys and covering clean-water tanks are therefore crucial to improving disease control.
Sciarid flies, shore flies and plant parasitic nematodes have been implicated in the spread of Pythium, so hygiene strategies should include control of these pests.
Pythium has developed tolerance to active ingredients of some key fungicides, underlining the need to reduce the opportunities for infection by practising good hygiene. It is closely related to Phytophthora, a cause of root rot, so chemical fungicides active against one are generally effective on the other, but not necessarily against Rhizoctonia.
As well as affecting seedlings, both Pythium and Rhizoctonia solani can also attack cuttings, leading to a basal rot, known in pelargoniums as "blackleg".
How to recognise it
The most characteristic sign of damping off is toppled seedlings. In the case of Pythium, as the disease spreads, circular bare patches develop where seedlings have died. Seed can be attacked at emergence. This leads to a poor stand, which at first may be blamed on seed viability.
Pythium survives in unfavourable conditions in the soil or growing medium in the form of thick-walled resting spores, or oospores. Spread from plant to plant is achieved by zoospores that need a film of water in which to swim towards the seedling, attracted by chemicals exuded by plant roots. Spores germinate to produce a tube that penetrates the plant tissue.
In Rhizoctonia solani, the resting structure is a sporangium - a mass of hyphae protected by a hard coat. Chemicals from plant roots induce the sporangium to germinate. The hyphal strands are then attracted to grow towards the root.
Once the root has been breached, both Pythium and Rhizoctonia solani grow inside the plant, breaking down tissues. Pythium is favoured by cooler, wetter conditions than Rhizoctonia for disease development.
Before seedling collapse, Pythium may cause brown lesions to develop at the base of the seedling stem causing girdling. Cotyledons may wilt. Rhizoctonia infection can lead to "wire-stem" in plants such as brassicas, where stems turn brown and shrivel at the base. In some conditions, a fine web of the fungus may be noticeable growing over foliage, leading to it also being described as "cobweb fungus".
Treatment: biological control
Atheta coriaria, Macrocheles robustulus, Hypoaspis miles and H. aculeifer will control sciarid and shore fly larvae and pupal development under protection.
Treatment: cultural control
- Nursery hygiene plays a particularly important role in preventing damping off. Clean surfaces such as floors, benches, capillary matting and trolleys with an approved disinfectant. Organic matter can reduce the efficacy of disinfectants so clean off debris first.
- Cover growing-media bunkers and water-holding tanks. Analyse irrigation water for Pythium contamination.
- Avoid storing new pots and trays next to any used ones.
- Avoid over-watering growing media.
- Monitor sciarid and shore fly adults by using yellow sticky traps.
Treatment: chemical control
Active ingredient: Azoxystrobin
FRAC code: 11
Action(s): A systemic, translaminar fungicide with eradicant and protectant abilities.
Active ingredients Boscalid + pyraclostrobin
FRAC codes: 7 + 11
Formulation: Signum* (BASF)
Action(s): Systemic and translaminar mixture with broad-spectrum control; compatible with some biological controls.
Active ingredient: Copper oxychloride
FRAC code: M1
Formulations: Cuprokylt (Certis), Headland Inorganic Liquid Copper (Headland)
Action(s): Contact, protectant, broad-spectrum fungicide for use on Pythium.
Active ingredients Fenamidone + fosetyl-aluminium
FRAC code 11 + 33
Formulations Fenomenal* (Bayer)
Action(s) Systemic, translaminar, eradicant and protectant fungicide. Compatible with some biological controls.
FRAC code 33 + 28
Formulation Previcur Energy* (Bayer)
Action(s) Systemic and contact fungicide. Compatible with some biological controls.
Active ingredient: Gliocladium catenulatum strain J1446
Formulation: Prestop* (Fargro)
Action(s): Bio-fungicide that can be applied preand post-potting for Pythium and Rhizoctonia control. Acts by competition and hyperparasitism.
Active ingredient: Metalaxyl-M
FRAC code: 4
Formulation: Subdue (Fargro)
Action(s): Systemic fungicide with protective and curative action for use as drench or compost incorporation with on-label approval for outdoor and protected ornamental plant production. Controls Pythium.
Active ingredient: Propamocarb hydrochloride
FRAC code: 28
Formulations: Proplant* (Fargro)
Action(s): Translocated protectant carbamate fungicide.
Active ingredient: Tolclofos-methyl
FRAC code: 14
Formulation: Basilex (Everris)
Action(s): Organophosphorus fungicide with on-label approval for Rhizoctonia in seedling ornamentals. Controls mycelial growth of fungus. Apply as a drench before sowing. Do not use on heathers. Must not be applied by hand-held equipment.
Fully updated by Dove Associates.
Use plant protection products safely. Always read the label and product information before use.
* Extension of authorisation (EoA) required for use in protected and outdoor ornamental plant production. Download your hard copy from https://secure.pesticides.gov.uk/offlabels/search.asp. EoA applied at grower risk.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.