Fighting allium white rot and the UK’s waste problems with composted onions

Over 100,000 tonnes of waste vegetables and vegetable processing wastes are produced annually in the UK, of which around 30 per cent is onion waste. Disposal in landfill is no longer permitted and spreading of untreated wastes to land can result in infection in subsequent onion crops.

Fungal disease allium white rot (AWR) affects bulb and salad onions and can remain viable in the soil for over 20 years. Control currently relies on the fungicides Folicur (tebuconazle) and Signum (boscalid+ pyaclostrobin) and avoiding infested areas.

In field-scale trials, incorporation of composted onion waste gave better control of AWR than achieved with treatment of sets with Folicur (tebuconazole).

Sulphur-containing onion vola-tiles released from the compost stimulate premature germination of AWR sclerotia, which die in the absence of host onion plants, Dr Ralph Noble of Warwick HRI told last week’s ADAS/Syngenta Vegetable Industry Conference.

The onion waste must be composted at 50degC or higher for at least three days to ensure pathogens, pests and weed seeds present are destroyed and do not affect the land it is applied to.

Windrows of composting wastes should be turned at least twice to ensure all waste receives the necessary temperature treatment. Mixing wet and dry wastes reduces production of anaerobic odours and waste liquid leaching.

“The compost needs to be applied at least six months, ideally longer, before seed drilling or set planning to maximise the germination of sclerotia and avoid phytotoxicity to the onion crop. Applying the compost before an intervening wheat crop, which benefits from onion waste compost, is the best treatment,” said Dr Noble.

Wastes should be composted for no more than three weeks to prevent the onion volatiles being destroyed.

Composts prepared from other sulphur-containing wastes, such as brassica waste, poultry manure and sewage sludge, have not been as effective as onion waste in controlling AWR.

The composting approach has proved so effective for onion growers and processors that there is now insufficient composted onion waste to treat all AWR-infested land in the UK.

Excellent control of AWR has also been achieved in the field using composts colonised with biological control agent trichoderma viride isolate S17A, said Dr Noble.

Composts can also suppress fusarium basal rot, an increasing problem that is likely to become more so if climate change leads to warmer, wetter summers.