Michael Celetti, Plant Pathologist – Horticulture Program Lead, Ontario Ministry of Agriculture, Food and Rural Affairs, Guelph
The pathogen, Fusarium oxysporum f.sp. cepae, is not new to Ontario. It is a persistent soil-borne organism that can also contaminate onion sets and transplants or cloves of garlic used for seed. In fact it is most likely introduced into non-contaminated fields on infested garlic cloves or onion sets. The disease tends to occur more frequently in garlic than in onions and is more often a problem on Spanish onions varieties than on yellow cooking onion varieties.
Symptoms of this disease are often seen as early senescence of the infected plants. The tips of leaves of infected plants turn yellow then brown as symptoms progresses downward towards the bulb. Occasionally a reddish discolouration may appear on bulb sheathes of severely infected garlic plants early in the season. During very hot and dry conditions infected plants wilt and bulbs appear watery and brown. Often the roots rot off of the basal plate (Figure 1). Severely infected plants are easily removed from the soil when pulled, leaving the rotted basal plate and roots behind. On onions, a white mould is sometimes observed growing on the basal plate and frequently orange to salmon coloured spore masses appear around the rotted basal plate (Figure 1). Bulbs that appear to be free of symptoms at harvest but are infected can decay in storage, however, there is no evidence that the disease spreads from bulb to bulb during storage.
The pathogen infects when soils become very warm. Interestingly, even in heavily contaminated fields the disease rarely occurs when soil temperatures are below 15oC. However as the soil temperatures warms up and approach 25-28oC together with hot conditions like what was experienced in many regions of Ontario during 2012, the spores of the pathogen in soil germinate, infect developing bulbs and the disease becomes more prevalent and severe. Because this disease thrives under high soil temperatures this diseases usually shows up in mid to late summer.
The pathogen can infect onion or garlic bulbs directly at any stage of plant growth (even healthy plants!); however, a higher incidence of infected plants tends to occur when roots, bulbs or the basal plate are wounded by insects, nematodes or other pathogens. In garlic, the disease looks a lot like and is often associated with bulb and stem nematode injury (Figure 2) where as in onions it is sometimes associated with onion maggot damage.
The disease is managed effectively by crop rotation with non-host crops for 4 years and through planting vigorously growing onion and garlic varieties that are resistant to this disease.