Bacterial Diseases of Onion

Bacterial diseases in most crops are very challenging to manage and onions are no exception. The three bacterial diseases that occasionally infect onion in Ontario include sour skin, slippery skin and soft rot. Onion plants tend to be tolerant to bacterial diseases during their early growth but become more susceptible later in the growing season after bulb initiation.

The three diseases are caused by three different bacteria with a common biology. They all live in soil and volunteer onions left in the field; they all require rain splashing on to the plants; and they all tend to infect wounded tissue in the field. Bacterial diseases of onion tend to be more severe during a very wet season such as the 2008 growing season or if a severe rain storm or prolonged wet period is followed by hot humid conditions in the field. Fields that are frequently over head irrigated particularly during periods when temperatures are hot (>30oC) are also more likely to become severely diseased if the bacteria is present in the soil. Heavy rain storms or overhead irrigation splash the bacteria contaminated soil onto leaves. The bacteria then infection through natural openings or wounds recently made by thrips, other diseases such as leaf blight or downy mildew or wounds caused by the damaging winds, pounding rain or hail accompanying the storm. Once the bacteria infect the leaves, they multiply inside the tissue and move downward into the bulb killing the tissue as they advance. Very warm temperatures favour the rapid multiplication of the bacteria once inside the tissue resulting in the rapid onset of symptoms. Unfortunately these pathogenic bacteria are capable of multiplying slowly at cool to moderate temperatures. Symptoms develop much slower during cool conditions and often go unnoticed during harvesting and prior to storage. Once in storage, the disease slowly invades the bulb tissue resulting in severe losses in quality.

Symptoms in the field often first appear as one or two wilted or yellow leaves in the center of the leaf cluster. The bulbs of infected plants may appear healthy except for a softening of the neck tissue. One difference observed between these diseases is that Slippery skin tends to move down one scale until it reaches the basal plate before spreading to adjacent scales. The infected scale appears watery or cooked and when pressure is applied to the bottom of the infected bulb the center scales slide out. Sour skin, on the other hand, often appears as a few rotted outer scales with adjacent and center scales remaining firm. Applying pressure to the severely infected bulbs with sour skin can also result in the center scales sliding out. A foul acidic odour usually accompanies this disease hence the name 'Sour skin'. Often, soft rot bacteria invade the diseased tissue resulting in further break down of the infected tissue which also produces a foul odour and complicates the diagnosis of these diseases. Regardless, slippery and sour skin can cause yield losses of up to 50%.

So what can growers do to avoid losses caused by these bacterial diseases? Unfortunately there are no silver bullet bactericide sprays to control these diseases. Disease management relies on the integration of cultural practices in the field to keep the disease at bay. Since the bacteria are soil-borne pathogens, crop rotation with non host crops such as cereals for at least 3 years will significantly reduce soil population levels. Above all, avoid planting near cull piles or in fields with a history of these diseases for at least 3 years, particularly if the disease was slippery skin. Although the susceptibility of different varieties to these diseses are not known, avoid planting varieties that have been severely affected by the diseases in the past and plant resistant varieties where known. Always provide adequate plant and row spacing so that air movement can dry the crop quickly and reduce conditions favourable for infection after a severe storm or heavy irrigation. Use a moderate fertilizer program and do not apply nitrogen after bulb initiation since this can exacerbate disease development and losses. Cultivate infested debris, volunteer onions and cull piles in the fall and spring. If possible, avoid over head irrigation in fields with a history of the disease particularly during very hot summer days. Often these diseases go unnoticed during harvest. Infected onions that are not allowed to cure in the field properly after a very wet season often rot in storage. Be sure that the bulbs are cured and the necks are closed before placing them in a cool dry storage facility.