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 supression 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.
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.
Where trade names are used no discrimination is intended, and no endorsement by the Cooperative Extension Service is implied.
Dr. Alan A. MacNab, Professor, Plant Pathology
Penn State College of Agricultural Sciences
211 Buckhout Lab, University Park, PA 16802
Updated: Nov, 2005; EMH
Loaded: Nov, 2005; EMH