Fusarium Bulb Rot of Onions
• The disease is caused by the soilborne fungus Fusarium proliferatum
• Infections occur in the field but the plants remain symptomless until 4-6 weeks in storage
• Progression of the disease occurs more rapidly when bulbs are stored at ambient temperatures.
• May be mistaken for Botrytis neck rot
Introduction
Fusarium bulb rot of onions has become an issue to onion growers in Utah. The causal agent, Fusarium proliferatum, has a wide range of hosts but onions are the only host of interest in this area (Proctor et al. 2010). In 2021 and 2022, select growers in Utah lost around 40% of their yields. Growers in the western United States have also been affected at different times over the last 20 years. These outbreaks are difficult to predict, and they cause high percentage losses. Rot typically sets in post-harvest.
• Fusarium bulb rot is NOT the same disease as Fusarium basal rot
• The causal agent for Fusarium basal rot is Fusarium oxysporum
• Fusarium basal rot will infect onions near the roots and then move up through the bulb, while Fusarium bulb rot will infect onions in the neck and then move to the lower scales
Symptoms
Spores enter the onion through the neck and begin to rot the upper part of the bulb. When white onions are infected, the outer scales may turn a pinkish color (Toit et al. 2003). With yellow and red onions, there are no obvious indications of disease. When symptomatic bulbs are cut open, the inner scales are rotten. Figures 1 and 2 show infected bulbs at different stages of infection. The outer scales look and feel normal while the inner scales turn a dark color. Other fungi or bacteria also take advantage of the compromised plant tissue and may increase rot in the inner scales. Infections are latent, meaning the bulb will not show any symptoms until weeks later. Most growers report finding symptomatic bulbs 4-6 weeks after being placed in storage. Figure 3 shows tiny white mycelium that can frequently bee seen between the scales in the neck.
References