Onion Botrytis Leaf Spot/Leaf Blight

Onion leaf showing symptoms of Botrytis leaf spot
Note the small, whitish, oval-shaped spots on the leaf surrounded by a light green or silver halo. Photo credit: Lindsey du Toit, Washington State University, via Bugwood.org

Onion Botrytis leaf blight/leaf spot is a fungal disease of alliums caused by Botrytis squamosa. Symptoms first appear as small whitish spots on the leaf. These spots are oval-shaped, and sometimes surrounded by a light green or silver halo that often appears water-soaked. Leaf tips will begin to dry and wither as the disease progresses, sometimes until the whole leaf dies back. Progressed infection can stunt bulb growth and reduce yield. Heavily infected fields often appear yellowish and blighted. Severe infection can stunt bulb growth and reduce yield.

Infection

Primary infection occurs from B. squamosa spores that overwinter in infected in-field plant debris, cull piles, stored bulbs, volunteer bulbs in-field, and in infected soil. Secondary infection can occur when conidia spores spread from moist, infected leaves. Favorable conditions for disease development include high relative humidity and rainfall, prolonged leaf wetness, and warm temperatures.

Disease Cycle

Botrytis squamosa overwinters as sclerotia in infected in-field plant debris, cull piles, stored bulbs, volunteer bulbs in the field, and infested soil. These sclerotia produce airborne conidia spores and ascospores (sexual spores) that travel to and infect onion leaves during periods of high moisture and low air movement. These same favorable conditions allow for secondary cycles of infection, where infected leaves produce more conidia, which spread to further infect the same leaf or others. Sclerotia are once again formed at the end of the season, and the disease cycle will continue the following season.

Disease Modeling

To view the predicted onion botrytis risk on any given day, visit the Vegetable Disease and Insect Forecasting Network (VDIFN) website. From the Disease tab, select the “Botrytis leaf blight” model. This BOTCAST model uses a cumulative disease severity index (CDSI) computed from gridded NOAA weather data to calculate the risk of onion botrytis development, which is displayed as a colored map overlay.

  • Threshold 1: (21 ≤ CDSI < 31) Warning threshold of “no spray applied unless rain predicted or overhead irrigation applied”
  • Threshold 2: high risk of rapid disease development, apply initial spray as soon as possible
  • CDSI > 40: extremely elevated risk

The start point should be set to the date of crop emergence. Click any grid point in VDIFN to get more detailed weather and disease progression information for that location.

Cultural control

Cultural control strategies include scouting regularly to identify the presence of the disease early before it has had a chance to spread and cause significant damage. Disease spread can be limited by avoiding working in fields when plants are wet and disinfecting tools and machinery. The following practices can help mitigate the risk of this disease:

  • Maintain proper spacing between plants
  • Destroy cull piles
  • Rogue volunteer plants
  • Distance seed and commercial onion fields
  • Destroy infested plant debris
  • Rotate away from susceptible crops (Alliums) to reduce sclerotia in soil (3 years)

Chemical control

Use disease forecasting tools to properly time the most effective disease prevention sprays. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.

Additional References

 


Adapted from UW Extension publication A3803, written by Karen Delahaut and Walt Stevenson in 2004.