Two pathogens are mainly responsible for damping off in seedlings — Rhizoctonia solani and various species of Pythium. Both are necrotrophs — they draw on the nutrients of their host’s dead cells
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 nursery sites — the pathogen has been isolated from various surfaces and water sources. Hygiene practices such as cleaning and disinfecting benches, floors and trolleys as well as 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 now developed tolerance to active ingredients of some key fungicides, which underlines the need to reduce the opportunities for infection by practising good hygiene.
Closely related to Phytophthora, Pythium is also 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 attack cuttings, leading to a basal rot, known in pelargoniums as "black leg".
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 might 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 and 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 any signs of Pythium contamination.
• Avoid storing new pots and trays next to any used ones.
• Avoid over-watering growing media — this is conducive to Pythium spread.
• Control sciarid and shore flies using predator-friendly insecticides and yellow sticky traps.
Treatment: chemical control
Active ingredient Azoxystrobin
FRAC code 11
Formulation Amistar* (Syngenta)
Action(s) A systemic, translaminar fungicide with eradicant and protectant abilities. Do not apply below 10°C or above 30°C. Do not use an adjuvant.
Active ingredient Bacillus subtilis
FRAC code 44
Formulations Serenade ASO* (Bayer CropScience)
Action(s) Bacterial fungicide. Controls Pythium, Phytophthora and Botrytis in bedding plants, nursery stock, pot plants and ornamental specimens with application restrictions.
Active ingredient Copper oxychloride
FRAC code M1
Formulations Cuprokylt* (Certis)
Action(s) Contact, protectant, broad-spectrum fungicide for use on Pythium. For use on outdoor crops only.
Active ingredient Fenamidone and Fosetyl-aluminium
FRAC codes 11 and 33
Formulation Avatar* (Everris), Fenomenal* (Bayer CropScience)
Action(s) Systemic fungicide. Check crop tolerance before application. Apply as drench to rooted hardy ornamental nursery stock cuttings after first potting and repeat monthly up to six applications. Controls Pythium.
Active ingredient Fosetyl-aluminium and Propamocarb hydrochloride
FRAC codes 33 & 28
Formulations Previcur Energy*
Action(s) Translocated protectant carbamate and systemic fungicide. Controls Pythium in bedding plants, nursery stock, pot plants and ornamental specimens with application restrictions.
Active ingredient Gliocladium catenulatum strain J1446
Formulation Prestop* (Everris)
Action(s) Biofungicide that can be applied pre- and 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. Avoid applications to Hedera, Cordyline, Viburnum and Prunus.
Active ingredient Propamocarb hydrochloride
FRAC code 28
Formulations Proplant* (Fargro)
Action(s) Translocated protectant carbamate fungicide. Controls Pythium in bedding plants, nursery stock, pot plants and ornamental specimens with application restrictions.
Active ingredient Tolclofos-methyl
FRAC code 14
Formulation Basilex (Everris)
Action(s) Organophosphorus fungicide with on-label approval for Rhizoctonia in seedling ornamentals.
mycelial growth of the fungus. Apply as a drench before sowing. Do not use on heathers. Must not be applied by handheld equipment.
Fully updated by Dove Associates
Use plant protection products safely. Always read the label and product information before use.
* Extension of Authorisation for Minor Use (EAMU) required for use in ornamental plant production outdoors and/or under protection.
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.