What can I do about garlic white rot? (March 2018)
White rot is the most severe allium disease, it is caused by the fungus Sclerotica cepivorum, it is very serious since the fungus is nearly impossible to completely remove once an area has been infected. Prevention is the best approach to ensure white rot is not brought into your garden - avoid bringing in contaminated equipment/tools/footwear/plants/soil. Also use your own healthy bulbs for planting to avoid the risk of bringing in infected garlic and grow onions from seed rather than purchasing bunched onion plants or sets.
The spread of white rot is not by spores, but thought the sclerotia (hard black beads that live both in the soil and on infected plants). White rot sclerotic persist in the soil for decades and remain dormant in the soil until they come within less than a centimetre of an allium, wherein the exudate from the plant will encourage the fungus to germinate.
Early stages begin with yellowing leaf tips that continues to die back until the entire leaf dies. When the bulb is dug up, a soft rot is visible, starting at the base of the bulb and covered by a characteristic fluffy white fungal growth. The presence of masses of tiny sclerotia (look like tiny black poppy seeds) is definitive for diagnosing the disease.
Once infected there are a number of organic options for minimizing the spread of the fungus although these methods are not always successful. If the infection is in a small match, one method is to simply remove the affected plants and all the surrounding soil, then dispose of the material and soil (do not compost). If the infection is widespread, ceasing irrigation can dry the fungus out but will not cure the problem, only minimize it. Solarizing is another option - transparent polyethylene is laid over the soil during the hottest months increasing temperature and killing the white rot but the heat is not likely to penetrate deep enough in the cooler climate of coastal Pacific Northwest. Since the fungus is stimulated by allium exudate, a last organic method is to seed a field with allium (preferably garlic) juice solution or powder. The presence of allium material will cause the sclerotica to germinate, but because there is no garlic actually present the fungus will starve and die. To make garlic extract, take a bulb of clean, disease-free garlic and discard the papery wrappers and the basal plate. Crush the whole bulb into 10 litres of water and water onto areas of your garden that youíre not currently using for growing allium family crops. The whole 10 litres should be applied to two square meters and should be done when temperatures are between 15-18 degrees celsius as this is the optimum germination temperature for the sclerotia. Another option is to rake or water in garlic powder. This process can be repeated several times if necessary.
Sources
Garlic Diseases: White Rot https://www.motherearthnews.com/organic-gardening/garlic-diseases-white-rot-zbcz1412
Gilkeson, Linda A., West Coast Gardening: Natural Insect, Weed and Disease Control, 2013.
How to Control Onion White Rot https://www.growveg.com.au/guides/how-to-control-onion-white-rot/
