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 overwinters in roots 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 pathogen 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, however, are specific to certain groupings of cruciferous 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, 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. White rust and downy mildew are commonly found cohabitating together on crucifer in western Oregon and elsewhere.
Symptoms The first observable symptom is the appearance of small, dark brown to black specks on leaves which enlarge into tan spots. 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.
Cultural control
- Rotate out of crucifers for at least two years and avoid planting adjacent to a field infected the previous season.
- Manage for cruciferous weeds (wild mustards, etc.) and volunteers that may harbor this microorganism during rotations.
- Remove or bury infected crop debris in the vicinity of new plantings.
- Manage irrigation to reduce periods of high humidity.
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.
Chemical control
- Actigard 50WG (Group P1) at 0.75 to 1 oz/A every 7 days can suppress the development of downy mildew by inducing host-resistance pathways. Do not apply within 7 days of harvest. 12-hr reentry.
- CAA-fungicide 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 labeled but do not use as stand-alone materials.
- Badge SC at 0.75 to 1.5 pints/A on 5- to 10-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
- Dithane F45 Rainshield (Group M3) at 1.5 quart/A on 7- to 10-day intervals is labeled for use on seed crops in Oregon for Alternaria leaf spot and can used in Oregon for downy mildew control (SLN OR-240011). 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.
- OSO 5% SC (Group 19) at 6.5 to 13 fl oz/A on 7- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
- Phosphonates formulations (Group P7; formerly Group 33) 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.
- Fosphite at 1 to 3 quarts/A on 2- to 3-week intervals. Can be applied up to the day of harvest. 4-hr reentry.
- Fungi-phite at 1 to 2 quarts/A on 7- to 14-day intervals. 4-hr reentry.
- K-Phite 7LP at 1 to 4 quarts/A on 7- to 14-day intervals. Can be applied up to the day of harvest. 4-hr reentry.
- Rampart at 1 to 3 quarts/A on 2- to 3-week intervals. Can be applied up to the day of harvest. 4-hr reentry.
- Resist 57 at 1 to 3 quarts/A on 2- to 3-week intervals. 4-hr reentry.
- Presidio (Group 43) at 3 to 4 fl oz/A on 7- to 10-day intervals as a tank-mix. Preharvest interval is 2 days. 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.
- Regalia (Group P5) at 1 to 1.5 quarts/A plus another fungicide on 7- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
- Strobilurin formulations (Group 11) are labeled for use. Do not apply more than one (1) foliar application of any Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Cabrio EG at 12 to 16 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Quadris Flowable at 12 to 15.5 fl oz/A on 7- to 14-day intervals. Preharvest interval is 0 day. 4-hr reentry.
- Reason 500 SC at 5.5 to 8.2 fl oz/A. Studies by Raid (2010) of the University of Florida showed Reason to have good to excellent control of downy mildew on broccoli with 5% disease severity on average in the Reason-treated plants vs. 67% in non-treated plants. Do not apply within 2 days of harvest. 12-hr reentry.
- Satori at 12 to 15.5 fl oz/A on 5- to 7-day intervals. 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
- 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 0.5 to 1 teaspoon/gal water. H 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 0.5 to 4 quarts/A plus a nonionic surfactant on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
References Koike, S.T. 1998. Downy mildew of arugula, caused by Peronospora parasitica, in California. Plant Disease 82:1063. https://doi.org/10.1094/PDIS.1998.82.9.1063B
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.
Raid, R. N. 2010. Evaluation of fungicides for control of downy mildew on broccoli, Spring 2009. Plant Disease Management Report: Report No. 4:V060.