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 fungus 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. Resistance is being incorporated into commercial brassicas, and there are limited varieties of broccoli, cabbage, Chinese cabbage, arugula, and radish that can be found in seed catalogs which are described as having resistance to downy mildew, but the resistance will depend on the genetics of pathogen population in the vicinity of a planting of these resistant varieties. The systemic phase of the disease is the most destructive because it can lead to secondary infections that may lead to death of seed plants; radish and turnip are notable for the occurrence of systemic infection of the storage root.
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. Fall and spring downy-mildew management is important for fall-planted 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 on brassicas in western Oregon and elsewhere.
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 and heads does not become apparent until closer to flowering. 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. 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.
- 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. 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) as a tank-mix with a fungicide with a different mode of action. Do not make more than one (1) sequential application before alternating to a different mode of action.
- Forum at 6 fl oz/A on 7-day intervals. Preharvest interval is 0 days. 12-hr reentry.
- Revus at 8 fl oz/A on 7- to 10-day intervals. Preharvest interval is 1 day. 12-hr reentry.
- Chlorothalonil formulations (Group M5) are labeled.
- Bravo Ultrex at 1.4 lb/A with water to cover on 7- to 10-day intervals. Preharvest interval is 7 days.12-hr reentry.
- Echo 720 at 1.5 pints/A on 7- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
- Copper products (Group M1) are not recommended as stand-alone materials and at high rates can cause phytotoxicity on some cabbage varieties in the form of flecking on wrapper leaves.
- Champ Formula 2 at 0.33 to 0.66 pint/A. 48-hr reentry.
- Champ WG at 1.06 lb/A for cabbage and 1 lb/A at for other crucifers on 7- to 10-day intervals. 48-hr reentry. O
- C-O-C-S WDG at 0.5 to 1 lb/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
- Cuprofix Ultra 40 Disperss at 0.75 to 1.25 lb/A on 7- to 10-day intervals. 48-hr reentry.
- 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. 24-hr reentry. O
- Nu Cop 50 WP at 0.5 to 1 lb/A on 7-day intervals. Do not apply within 1 day of harvest. 48-hr reentry. O
- Curzate 60 DF (Group 27) at 3.2 oz/A is available for use on cabbage seed crops in Washington only (SLN WA-990021). Use only in combination with another registered protectant fungicide. 12-hr reentry.
- Mancozeb formulations (Group M3) are labeled.
- Dithane DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-020028, SLN OR-020030). 24-hr reentry.
- Dithane F45 Rainshield 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.
- Roper DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Oregon only (SLN OR-130003). 24-hr reentry.
- MilStop SP (85% potassium bicarbonate) at 2 to 5 lb/A or 1 Tbsp/gal water on 7- to 14-day intervals. 1-hr reentry. O
- Phosphonates (Group P7) can 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.
- Alude at 2.5 to 5 pints/A on 7- to 21-day intervals. 4-hr reentry.
- Presidio (Group 43) at 3 to 4 fl oz/A on 10-day intervals as a tank-mix. Preharvest interval is 2 days. 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
- Ridomil Gold Bravo SC at 1.5 pints/A or Ridomil Gold SL at 0.125 to 0.25 pint/A tank-mixed with other fungicides registered for control of downy mildew. Warning Resistance to Ridomil has been confirmed in the Pacific Northwest. To minimize further development of resistance, do not make foliar applications if Ridomil was used at planting, do not use as a curative spray once disease has developed, and use only as a preventive spray and only in combination with another registered fungicide. Do not apply within 7 days of harvest. 48-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a 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 6 to 15.5 fl oz/A or Quadris Top at 10 to 14 fl oz/A on 7- to 14-day intervals. Preharvest interval is 0 days for Quadris Flowable, 1 day for Quadris Top. 4-hr reentry for Quadris Flowable, 12-hr reentry for Quadris Top.
- Reason 500 SC at 5.5 to 8.2 fl oz/A. Do not apply within 2 days of harvest. 12-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) applications 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.
- Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 1-hr reentry. 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
- 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
- Sonata at 2 to 4 quarts/A on 7- to 14-day intervals. Can be applied up to and on the day of harvest. 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.
Raid, R. N. 2010. Evaluation of fungicides for control of downy mildew on broccoli, Spring 2009. Plant Disease Management Report: Report No. 4:V060.
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.