Cause Peronospora parasitica (= Hyaloperonospora parasitica), a fungus-like microorganism that overwinters in roots or infected plant debris, affects nearly all cultivated and weed plants in the Crucifer family. Strains of the microorganism are generally more aggressive on the crop plant from which they were obtained. For example, the radish strain causes little damage to cabbage. Spores may be spread as contaminants on seeds. Conidia can spread by wind or water droplets. High humidity, fog, drizzling rains, and heavy dew favor disease development and spread. Optimum conditions for disease development are night temperatures of 46°F to 61°F for 4 or more successive nights, and day temperature about 75°F or lower. In areas with mild, wet winters, such as western Oregon or Washington, downy mildew can continue infecting fall-planted seed crops during the winter months, although only small amount of the total leaf surface area may be colonized on individual leaves, but this allows for a build-up within a field, potentially leading to severe increase in infections of spring growth if spring conditions are wet and mild. Fall and spring downy-mildew management is important for fall-planted seed fields to avoid extensive staghead formation, where an inflorescence is abnormal in shape and development due to downy mildew or white rust. White rust and downy mildew are commonly found cohabitating together in leaf colonies on brassicas in western Oregon and elsewhere.
Resistance is being incorporated into commercial varieties, but at present no resistant commercial varieties are available.
Symptoms Young seedlings are more susceptible than adult plants and can die from downy mildew if infected when young. The first observable symptom is small, light green-yellow lesions on the upper leaf surface, later showing on the undersurface. The spots turn yellow or orangish as they enlarge. During periods of high humidity, a grayish white moldy growth appears on the leaf's underside. Later the leaf may become papery and die.
On fall-planted seed crops, downy mildew can create tiny leaf spots during the winter months or other times where small clusters of, even just a few, conidia and conidiophores are produced on the underside of the leaf surface; rarely in large enough quantity to be seen without magnification. During warmer rain breaks, individual lesions may rapidly enlarge and then quickly collapse with a secondary soft rot during rainy periods. This can be confused for cold-damaged tissues with subsequent secondary leaf rot, which can also be common in areas of Oregon and Washington seed production during certain winters.
Downy mildew on an inflorescence can lead to staghead formation, abnormal shape and development of the inflorescence due to downy mildew or white rust. White rust and downy mildew are commonly found together in leaf colonies on brassicas in western Oregon and elsewhere.
- Eradicate cruciferous weeds (wild mustards, etc.) that may harbor the fungus.
- Manage irrigation to reduce periods of high humidity.
- Spring-planted, summer-harvested crops have fewer problems than fall-harvested.
Chemical control Apply first spray as soon as seedlings appear; repeat three (3) times a week until plants are set out in field. Spraying for mildew requires completely covering the plant. In general, high-gallonage ground application has been more effective than aerial application.
- Actigard at 0.5 to 1 oz/A on 7-day intervals for up to four (4) applications can suppress the development of downy mildew by inducing host-resistance pathways. Begin applications 7 to 10 days after thinning, before disease is present. If disease is present in the area, tank-mix the first application with another fungicide registered for downy mildew. Preharvest interval is 7 days. 12-hr reentry.
- CAA-fungicide (Carboxylic Acid Amides) formulations (Group 40) in combination with another fungicide that has a different mode of action. Do not apply more than once before alternating to a different mode of action
- Forum at 6 fl oz/A on 7-day intervals. Do not apply within 7 days of harvest. 12-hr reentry.
- Revus at 8 fl oz/A on 7- to 10-day intervals. Preharvest interval is 1 day. 4-hr reentry.
- Copper products (Group M1) are not recommended as stand-alone materials.
- Badge SC at 0.5 to 1.8 pints/A on 7- to 10-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
- Champ Formula 2 at 0.33 to 0.66 pint/A. 48-hr reentry.
- Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. May be applied on the day of harvest. 4-hr reentry. O
- Kocide 2000 at 0.75 to 1.5 lb/A or Kocide 3000 at 0.5 to 0.75 lb/A on 7- to 10-day intervals. 48-hr reentry. O
- Nordox 75WG at 0.33 to 0.66 lb/A on 7- to 10-day intervals. 12-hr reentry.
- Previsto at 0.75 to 2 quarts/A on 7- to 10-day intervals. 48-hr reentry. O
- Dithane F45 Rainshield (Group M3) 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.
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
- Phosphonates formulations (Group P7) should be very effective.
- Aliette WDG at 2 to 5 lb/A on 7- to 21-day intervals. Do not apply within 3 days of harvest. 12-hr reentry.
- Alude at 2.5 to 5 pints/A on 1- to 3-week intervals. 4-hr reentry.
- Quadris Flowable (Group 11) at 6 to 15.5 fl oz/A on 7- to 14-day intervals. Do not apply more than one (1) application before alternating to a non-Group 11 fungicide. Preharvest interval is 0 day. 4-hr reentry.
- Quadris Top (Group 3 + 11) at 12 to 14 fl oz/A on 7- to 14-day intervals. Do not apply more than one (1) application before alternating to a non-Group 11 fungicide. Preharvest interval is 1 day. 12-hr reentry.
- Ranman 400SC (Group 21) at 2.75 fl oz/A on 7- to 10-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Reason 500 SC (Group 11) at 5.5 to 8.2 fl oz/A. Do not apply within 2 days of harvest. 12-hr reentry.
- Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 5- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
- Revus (Group 40) at 8 fl oz/A on a 7- to 10-day interval with no more than two (2) consecutive applications. Do not apply within 1 day of harvest. 4-hr reentry.
- Zampro (Group 40 + 45) at 14 fl oz/A for no more than three (3) application per season. Do not apply more than two (2) applications before alternating to a fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
Biological control Efficacy unknown in Oregon.
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 1-hr reentry. O
- Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
- Cease at 3 to 6 quarts in 100 gal water. For greenhouse plants only. Preharvest interval is 0 days. 4-hr reentry. O
- Double Nickel LC at 0.5 to 4.5 pints A on 3- to 10-day intervals. Can be applied 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