See:
Seed Crop, Crucifers (Brassica, Eruca, and Raphanus spp.) - Downy Mildew (Staghead)
By C. M. Ocamb
Cause Peronospora parasitica (= Hyaloperonospora parasitica), a fungus-like microorganism that survives in storage roots of systemically-infected plants or on leaves or stems of infected, living plants. Infected plant residues are possibly a source of new infections via oospores present in debris. Spores may be spread as contaminants on seeds but there is no proof that the microbe enters the seed. Nearly all cultivated plants and weeds in the Brassicaceae family are susceptible to this pathogen and seedlings may be more susceptible compared to older plants. Several strains of the pathogen are specific to certain groupings of crops.
High humidity, fog, drizzle, and heavy dew favor disease development and spread. Once P. parasitica has infected a plant, conidia are produced on aerial plant portions and can be dispersed by wind or splashing water to cause secondary spread of the disease. Moderate temperatures during the day time (68ºF to 75ºF) and a high relative humidity, fog, drizzling rains, or heavy dew favor disease development and spread. In areas with mild, wet winters, such as western Oregon or Washington, the pathogen 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. White rust and downy mildew are commonly found cohabitating together on brassicas in western Oregon and elsewhere.
Resistance is being incorporated into commercial varieties, but at present no resistant commercial varieties are available.
Symptoms
The first observable symptom is small, light green-yellow lesions on the upper leaf surface, the leaf spots turn yellow on the upper leaf surface as lesions enlarge. During high humidity, a grayish-white mycelial growth appears on the underside of leaf spots as the pathogen produces spores. Leaf spots may become papery and die, especially if temperatures warm above 75ºF. When seedlings are attacked, sporulation may develop on both sides of the cotyledons, and death of the young plant can occur. Plants can become systemically colonized, especially if infected at the seedling stage, but the black streaking and discoloration inside stems does not become apparent until closer to harvest.
On fall-planted 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 western Oregon and Washington during certain winters.
Cultural control
- Rotate out of crucifers for at least two years and avoid planting adjacent to a field infected the previous season.
- Manage cruciferous weeds (wild mustards, etc.) that may harbor the fungus-like microorganism during rotations.
- Remove or bury infected crop debris in the vicinity of new plantings.
- 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.
- Orondis Ultra (Group 49 + 40) at 5.5 to 8 fl oz/A beginning prior to disease development and continue on 7- to 10-day intervals. Preharvest interval is 1 day. 4-hr reentry.
- 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. 24-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. Does not benefit from the addition of an adjuvant. 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. 4-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
- 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
- Romeo at 0.45 to 0.68 lb/A on 7- to 10-day intervals starting prior to infection. Preharvest interval is 0 days. 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
References Nashaat, N.I. 2007. Downy Mildew. In: Rimmer, S.R., Shattuck, V.I., and Buchwaldt, L., editors. Compendium of Brassica Diseases. St. Paul, (MN): APS Press; p. 28-30.
Singh, S., Sharma, S.R., Kalia, P., Sharma, P., Kumar, V., Kumar, R., Meena, B.L., Kumar, S., and Sharma, T.R. 2013. Screening of cauliflower (Brassica oleracea L. var. botrytis L.) germplasm for resistance to downy mildew [Hyaloperonospora parasitica Constant (Pers.:Fr) Fr.] and designing appropriate multiple resistance breeding strategies. J. Hort. Sci. Biotech. 88(1):103-109.