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 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, and arugula 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. All commercial-processing broccoli varieties are very susceptible. The systemic phase of the disease is the most destructive because it renders the heads useless for the freezing trade.
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.
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 harvest.
The stem of plants can also be infected and typically exhibit a dark brown to black lesion as a result. Spots can be seen on the outsides of stalks of broccoli heads. Streaks of brown to purple tissue, often in a net-necrosis pattern, are on the insides of stalks. These streaks may extend up to the small florets of the curd, stunting the curd. Streaking is difficult to detect without breaking the stalk for examination.
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.
- Rotate out of crucifers for at least two years and avoid planting adjacent to a field infected the previous season.
- Eradicate 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 ones.
- These disease-resistant, fresh-market varieties have performed well in the Willamette Valley of Oregon: 'Futura', 'Citation', 'Premium Crop', 'Green Comet', 'Green Valiant', and 'Crusade'.
- 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) are registered for use and it is recommended that these formulations be used as a tank-mix with a fungicide with a different mode of action. Do not make more than two (2) sequential applications if tank-mixed with another fungicide labeled for downy mildew control before alternating to a different mode of action, otherwise the next fungicide application needs to be a material with a different mode of action (non-Group 40) if applied as a stand-alone downy mildew treatment.
- Forum at 6 fl oz/A on 7-day intervals. Leafy Brassica greens and Brassica head and stem vegetables may be harvested on the day of the last application after sprays have dried. 12-hr reentry.
- Revus at 8 fl oz/A on 7- to 10-day intervals. Preharvest interval is 1 day. 4-hr reentry.
- Chlorothalonil formulations (Group M5) are labeled. Data from NCSU showed that this active ingredient contributed little to disease control in cabbage when combined or alternated with other products.
- Bonide Fung-onil is available for home gardens. H
- 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.
- Badge SC at 0.5 to 1.8 pints/A on 7-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. Older leaves may redden at the higher rate. 48-hr reentry.
- Champ WG at 1 lb/A on 7- to 14-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. 48-hr reentry. O
- Nu-Cop 50 DF at 1 lb/A on 7-day intervals. 48-hr reentry. O
- Previsto at 0.75 to 2 quarts/A on 7- to 10-day intervals. 48-hr reentry. O
- Dexter Max (Group M3 + 11) at 2 to 2.25 lb/A on 7- to 10-day intervals. Preharvest interval is 7 days. 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
- MilStop SP (85% potassium bicarbonate) at 2 to 5 lb/A. 1-hr reentry. O
- Orondis Opti (Group M5 + 49) at 1.75 to 2.5 pints/A on 7- to 10-day intervals. Do not apply more than two (2) sequential applications. Preharvest interval is 7 days. 12-hr reentry.
- Orondis Ultra (Group 49 + 40) at 5.5 to 8 fl oz/A on 7- to 10-day intervals. Do not apply more than two (2) sequential applications. 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.
- Alude at 2.5 to 5 pints/A on 1- to 3-week 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.
- 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 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. Do not apply within 7 days of harvest. 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. Data from NCSU showed that this active ingredient contributed little to disease control in cabbage when combined or alternated with other products.48-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) 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 day. 12-hr reentry.
- Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 14-day intervals or Quadris Top (Group 11 + 3) at 12 to 14 fl oz/A. Preharvest interval is 0 day for Quadris Flowable and 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) 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.
- 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
- Romeo at 0.23 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
- 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.