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 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). 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 only fair control in studies conducted by Cornell.
    • Nevado 4F at 1.5 pints/A on 14-day intervals. Do not apply within 7 days of harvest. 24-hr reentry.
    • 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.
    • Dithane DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on Allium seed crops. Washington and Oregon only (SLN WA-020028, SLN OR-020030). 24-hr reentry.
    • 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-090020, 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 only, SLN OR-130003). 24-hr reentry..
  • Omega 500F (Group 29) at 1 pint/A on 7- to 10-day intervals. 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. 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 very effective in studies conducted by 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. 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 good control when evaluated 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 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 good control when evaluated 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 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. 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.

  • Amplitude at 2 to 4 quarts/A on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
  • 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 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. 0-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 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.