Commercial Disease Controls
The Pennsylvania State University
Cooperative Extension

Onions

Seed Treatment

In areas where smut is a problem, use seed that has been treated with 10.67 ounces of either captan 50Wp or thiram 75Wp for every 10 pounds of seed. Where smut is not a problem, use either material at the rate of 0.67 teaspoon per pound for damping-off (4 ounces per 100 pounds).

Disease Control

Damping-Off
Field: Apply Ridomil Gold 4E at 0.5 to 1 pint per acre broadcast or banded immediately after seeding.

Downy Mildew
The disease organism can survive as oospores in soil, and as mold in bulbs, sets and occasional seed. Downy mildew is promoted by moist and cool to mild conditions. Start with disease-free seed and sets, provide at least 3 years without related crops, and avoid overwintering onions. Consider the following fungicides:

Mancozeb or maneb--3 lb 75WP/A applied at 7-day intervals, or alternated with:
Ridomil Gold Bravo--2 lb 81WP/A

If purple blotch or Botrytis blight are present, consider adding Bravo, 2 pt 6F/A (OLF), and if Botrytis blight is severe, also consider adding Rovral 1.5 lb 50W/A.

Purple Blotch (Alternaria)
The disease organism overwinters as mold in plant residue from onion related plants. Purple blotch is promoted by moist and warm conditions. Grow onions in well drained soil and rotate to provide a few years without onion-related plants. Sweet Spanish types are especially susceptible. Several of the most effective fungicides and mixtures of fungicides for purple blotch are listed below. One of these materials or mixtures may be needed every 7 days. However, rotate different classes of fungicides to slow development of resistance, and note that Rovral at the high rate, and Pristine, are labeled for use at 14-day intervals.

azoxystrobin (Quadris--6.2 - 15.4 oz 2.08F/A or Amistar--2-5 oz 80WDG/A) + mancozeb, 3 lb 75W/A + fixed copper, 2 lb 77W/a at 7- to 10-day intervals,or
chlorothalonil (Bravo, Echo, Equus)--l.5-3 pt 6F/A or OLF, + mancozeb, 3 lb 75W/A + fixed copper, 2 lb 77W/a at 7- to 10-day intervals (14-day preharvest interval for green bunching onions), or
chlorothalonil (Bravo, Echo, Equus)--l.5-3 pt 6F/A or OLF, + mancozeb + 3 lb 75W/A Rovral--1 lb 5OWP/A at 7- to 10-day intervals (14-day preharvest interval for green bunching onions), or
Pristine--10.5-18.5 oz 38WG/A at 14-day intervals (also will provide suppression of downy mildew), or
Rovral--1.5 lb 5OWP/A at 14-day intervals (For dry bulb onions only), or
Switch--11-14 oz 62.5WG/A at 7- to 10-day intervals,or
Switch--11-14 oz 62.5WG/A + mancozeb, 3 lb 75W/A plus fixed copper, 2 lb 77W/a at 7- to 10-day intervals

Botrytis Leaf Blight
The disease organism overwinters in cull piles, onion debris in soil, and as sclerotia where related plants recently were grown. Botrytis leaf blight is promoted by moist and cool to mild conditions. Eliminate sources of innoculum and provide 2 or 3 years between onion-related crops. Initiation of fungicides can be delayed until there is an average of 1 lesion on 10 leaves. Some of the most useful fungicides for Botrytis blight are listed below:

chlorothalonil (Bravo, Echo, Equus)--2-3 pt 6F/A or OLF alternated with chlorothalonil at 2 pt 6F/A plus fixed copper, 2 lb 77W/a at 7- to 10-day intervals (14-day preharvest interval for green bunching onions), or
Pristine--14.5-18.5 oz 38WG/A

White Rot
Use one of the following as a preplant soil fumigant and allow a 2- to 3-week waiting period after fumigation before seeding the fall crop.

Telone C-35--13-20.5 gal/A, or
Vapam HL--50-75gal/A

Neck Rot
Windrow plants thoroughly to ensure dry tops before topping operation.

Note: Where trade names are used, no discrimination is intended and no endorsement by the Cooperative Extension Service is implied. Information provided is intended for consideration by the user, but is not intended to be a recommendation. Production decisions should be based on consideration of many types of information (scientific, experiential, economic, legal, etc.) available to the user.