A new generation of novel pest and disease control solutions are being developed as we move our focus from crop protection chemicals. The latest new technique to come across our desk here at the Plant Enthusiast is a novel, but potentially very effective way to control White Rot in Onions.
Onion White Rot has been a huge problem for home gardeners and allotment growers as there is no effective chemical control.
Researchers are using the novel technique of composting onion waste and using it as a treatment to stop white rot in its tracks.
An estimated 30 000 tones of onion waste are produced in the UK each year and so this technique can help in waste management as well as crop protection.
Allium White Rot, which is caused by Sclerotium cepivorum enters the onion through its roots and affects bulb and salad onions alike. One of the key problems is that the fungus produces resting spores, that remain viable in the soil for over 20 years effectively writing off onion production on an effected allotment.
There are no chemical controls available for the amateur gardener and the commercially available chemicals are not giving total control of the disease. So a new control based on composted onion waste could be revolutionary.
In field-scale trials, where the composted onion waste was added to infected soil it has been shown to give better levels of control than some of the commercially available controls. The reason that this control is so effective is that the composted onion waste includes sulphur-containing onion volatiles, these when released from the compost tell the resting spores of onion white rust (sclerotia) that onions are present and ready to be infected. This encourages the spores to germinate and search for the crop, which of course is not there, the germinated spores therefore die, effectively clearing the ground of sclerotia that would otherwise stay active in the soil for 20 years or more.
The work has been carried out by Dr Ralph Noble of Warwick HRI. The onion waste is hot composted to ensure that pathogens, pests and weed seeds are killed. A hot garden compost heap should be able to achieve the required 50C. The compost is ready after three weeks of composting at this temperature, if it is left in the heap for too long then some of the volatile active ingredients are lost. If the control proves to be effective it may be that the compost could become available as a product in plant and garden centres, however there may also be shortages of composted onion waste as the technique is increasingly adopted by commercial growers.
Dr Noble explains “The compost needs to be applied at least six months, ideally longer, before seed drilling or set planting 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.”
On a garden or allotment scale, an application before a green manure crop is sown would make good use of the land while the control is given time to work.
Another new method of control of allium white rot can be achieved (by the commercial grower) with composts colonised with biological control agent Trichoderma viridae isolate S17A. These composts can also help to control fusarium basal rot, an increasing problem that is likely to get worse if we get warmer, wetter summers.