Onion (Allium cepa)-Botrytis Leaf Blight

Cause The fungus, Botrytis squamosa (syn. Botryotinia squamosa), overwinters in cull piles, in field debris, or in soil. Wind disperses the spores. Cool temperatures (55°F to 75°F) and long periods of leaf wetness favor infection. Older leaves are more susceptible. There are no resistant varieties.

Symptoms Leaf infection initially results in small (0.25 inch), oval white spots with a green halo surrounding the lesion. Spots eventually may become tan and coalesce, blighting leaves.

Cultural control

• Get rid of cull piles and debris, two overwintering sources.
• Rotate field out of allium crops at least 2 to 3 years.
• Avoid extended overhead irrigation.

Chemical control Repeatedly applying protective fungicides may give some control.

• Carboxamide (Group 7) formulations are registered for use and are fairly effective in studies by Cornell. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action.
  • Endura at 6.8 oz/A on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry.
  • Fontelis at 16 to 24 fl oz/A on 7- to 14-day intervals. Preharvest interval is 3 days. 12-hr reentry.
• Chlorothalonil formulations (Group M5) are registered for use and appear moderately to very effective in studies conducted by Cornell.
  • Bonide Fung-onil is available for home gardens. H
  • Bravo Ultrex at 0.9 to 2.7 lb/A on 7- to 10-day intervals. Do not apply within 7 days of harvest for dry bulb onions and within 14 days of harvest for green bunching onions. 12-hr reentry.
• Copper formulations (Group M1) offer limited control.
  • Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. Poor control as a stand-alone product. May be applied on the day of harvest. 4-hr reentry. O
• Demethylation-inhibiting (DMI) fungicides (Group 3). Formulations in this fungicide group gave fair disease control based on studies by Cornell. Do not apply more than two (2) sequential applications before alternating to a different mode of action.
  • PropiMax EC at 4 fl oz/A on 7-to 10-day intervals. Do not apply within 14 days of harvest for dry bulbs. No preharvest interval for green onion types. 12-hr reentry.
• Iprodione formulations (Group 2) are registered but appear to give poor to fair control in studies conducted by Cornell and the University of Georgia.
  • Rovral 4 Flowable at 1.5 pints/A on 14-day intervals. For dry bulb onions only. Do not apply within 7 days of harvest. 24-hr reentry.
• Mancozeb formulations (Group M3) are labeled. Formulations in this group gave only fair control based on studies conducted by Cornell.
  • Dithane F45 Rainshield at 1.5 quart/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-220008, SLN OR-090016). 24-hr reentry.
  • ManKocide at 2.5 lb/A on 7-day intervals. Do not apply within 7 days of harvest. 48-hr reentry.
  • Roper DF Rainshield at 3 lb/A on 7-day intervals can be used on dry bulb onions and at 2 lb/A on 7- to 10-day intervals for Allium seed crops. Oregon and Washington seed crops only (SLN OR-130003, SLN WA-1300003). 24-hr reentry.
• Omega 500F (Group 29) at 1 pint/A on 7- to 10-day intervals. Moderate disease control based on studies conducted by Cornell. Do not use an adjuvant. Preharvest interval is 7 days. 12-hr reentry.
• OSO 5% SC (Group 19) at 3.75 to 13 fl oz/A on 7- to 14-day intervals. Poor to fair disease control based on studies conducted by Cornell. Can be applied the day of harvest. 4-hr reentry. O
• Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 14-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
• Scala SC (Group 9) at 9 or 18 fl oz/A on 7- to 14-day intervals. Appears poor to moderately effective in studies conducted by the University of Georgia and Cornell. When applying alone, do not make more than two (2) consecutive applications of Scala SC without alternating to an equal number of applications of a fungicide from a different resistance management group. Do not apply within 7 days of harvest. 12-hr reentry.
• Strobilurin fungicides (Group 11) are labeled for use but were not effective is studies conducted by Cornell. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
  • Quadris Flowable at 9 to 15.5 fl oz/A. Preharvest interval is 0 days. 4-hr reentry.
• Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand-alone product. 4-hr reentry. O
• Premixed fungicides are available for use. Do not make more than one (1) application of any Group 11 fungicide before alter-nating to a labeled fungicide with a different mode of action.
  • Dexter Max (Group M3 + 11) at 3.2 lb/A on 7-day intervals for dry bulb onion. Do not apply to exposed bulbs. Pre-harvest interval is 7 days. 24-hr reentry.
  • Dexter Xcel (Group M3 + 11 + 3) at 56 to 72 fl oz/A on 7- to 10-day intervals for dry bulb onion. Do not apply to exposed bulbs. Preharvest interval is 7 days. 24-hr reentry.
  • Gavel 75DF (Group 22 + M3) at 1.5 to 2 lb/A on 7-day intervals for dry bulb onions. Do not apply within 7 days of harvest. 48-hr reentry.
  • Inspire Super (Group 3 + 9) at 16 to 20 fl oz/A on 7- to 10-day intervals with no more than two (2) sequential |applications. Gave only fair control when used alone in studies conducted by Cornell. Preharvest interval is 7 days for bulb onions; 14 days for green onions. 12-hr reentry.
  • Luna Experience (Group 7 + 3) at 8 to 12.8 fl oz/A on 10- to 14-day intervals. Applications at the top rate showed fair to good disease control in studies conducted by Cornell. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 3). Preharvest interval is 7 days. 12-hr reentry.
  • Luna Tranquility (Group 7 + 9) at 16 to 27 fl oz/A on 7- to 14-day intervals resulted in fair to good control when evaluated by Cornell and the University of Georgia. Studies by the University of Idaho showed a significant reduction in Botrytis leaf blight relative to nontreated plants when this formulation was used on 14-day intervals at 24 oz/A, but there was no effect on bulb infections. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 9). Preharvest interval is 7 days. 12-hr reentry.
  • Merivon Xemium (Group 7 + 11) at 8 to 11 fl oz/A on 7- to 14-day intervals resulted in fair control when evaluated by Cornell and the University of Georgia. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 11). Preharvest interval is 7 days. 12-hr reentry.
  • Orondis Opti (Group M5 + 49) at 1.75 to 2.5 pints/A on 7- to 10-day intervals. Do not apply more than two (2) sequential applications. Preharvest interval is 14 days for green onions, 7 days for dry onions. 12-hr reentry.
  • Pristine (Group 7 + 11) at 14.5 to 18.5 oz/A. Studies by the University of Idaho showed Pristine application on 14-day intervals at 17.5 oz/A resulted in a significant reduction in Botrytis leaf blight relative to nontreated plants, but there was no effect on bulb infections. Preharvest interval is 7 days. 12-hr reentry.
  • Quadris Opti (Group 11 + M5) at 1.6 to 3.2 pints/A or Quadris Top (Group 3 + 11) at 12 to 14 fl oz/A. Quadris Top resulted in poor control in studies conducted by Cornell. Do not apply within 7 days of harvest. 12-hr reentry.
  • Quilt Xcel (Group 3 + 11) at 17.5 to 26 fl oz/A. May be applied the day of harvest for green onion types; 14-day pre-harvest interval for dry bulb onions. 12-hr reentry.
  • Switch 62.5 WG (Group 12 + 9) at 11 to 14 oz/A on 7- to 10-day intervals resulted in only fair control in studies conducted by Cornell. Do not apply within 7 days of harvest. 12-hr reentry.

Biological control Efficacy unknown in Oregon.

• Double Nickel LC at 0.5 to to 4.5 pints/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
• Prestop WG at 3.5 oz/5 gal water (0.5% suspension) on 3- to 6- week intervals. Apply only when no aboveground harvestable food commodities are present. 4-hr reentry. O
• Serenade Opti at 14 to 20 oz/A on 7- to 10-day intervals. Applications can be made up to and the day of harvest. 4-hr reentry. O
• Stargus at 2 to 4 quarts/A plus a nonionic surfactant on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O

Reference Dutta, B., Foster, M.J., and Donahoo, W.M. 2022. Evaluation of fungicides to manage Botrytis leaf blight on onion in Georgia, 2021. Plant Disease Management Reports 16: V043.

Hoepting, C.A. 2017. Efficacy of fungicide treatments for control of Botrytis leaf blight and Stemphylium leaf blight on onion, 2016. Plant Disease Management Reports 11:V127.

Hoepting, C.A., Caldwell, S.K., and van der Heide, E.R. 2021. Efficacy of fungicide treatments for control of Botrytis leaf blight and Stemphylium leaf blight in onion, Oswego, 2020. Plant Disease Management Reports 15:V123.

Hoepting, C.A., Caldwell, S.K., and van der Heide, E.R. 2022. Efficacy of FRAC 3 and 7 mixes for control of Botrytis leaf blight and Stemphylium leaf blight in onion, 2021. Plant Disease Management Reports 16: V139.

Hoepting, C.A., Caldwell, S.K., and van der Heide, E.R. 2022. Efficacy of fungicide products for control of Botrytis leaf blight and Stemphylium leaf blight in onion in Oswego, 2021. Plant Disease Management Reports 16: V140.

Hoepting, C.A., van der Heide, E.R., and Caldwell, S.K. 2022. Efficacy of fungicide tank mixes for control of Botrytis leaf blight and Stemphylium leaf blight in onion, Elba, 2021. Plant Disease Management Reports 16: V138.

Murdock, M.R., Pizolotto, C.A., and Woodhall, J.W. 2022. Evaluating Pristine and Luna Tranquility against Botrytis leaf spot, Stemphylium leaf blight and other foliar diseases on drip-irrigated onions in Idaho, 2020. Plant Disease Management Reports 16:V015.