Turnip and Rutabaga (Brassica spp.)-Downy Mildew

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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 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.

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 the storage root 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.
  • Eradicate cruciferous weeds (wild mustards, etc.) 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 Spraying for mildew requires completely covering the plant. In general, high-gallonage ground application has been more effective than aerial application.

  • CAA-fungicide formulations (Group 40) are registered for use on turnip greens only and it is recommended that these formulations be used as a tank-mix with a fungicide with a different mode of action. Studies by Raid (2010) of the University of Florida showed that these formulations have good to excellent control of downy mildew on broccoli. 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 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. 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.
    • Badge SC at 1 to 2.25 pints/A on 3- 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 is registered for turnip greens only. 48-hr reentry.
    • Cueva at 16.8 gal/A on 7- to 10-day intervals for turnip greens only. 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 for turnip greens only. 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 for turnip greens only. 48-hr reentry. O
    • Nu-Cop 50 DF at 1 lb/A on 7-day intervals for turnip greens only. 48-hr reentry. O
    • Previsto at 0.75 to 2 quarts/A on 7- to 10-day intervals. for turnip greens only. 48-hr reentry. O
  • MilStop SP (85% potassium bicarbonate) at 2 to 5 lb/A. 1-hr reentry.
  • Omega 500F (Group 29) at 15.35 fl oz/A on a 7-day interval for turnip greens only. Preharvest interval is 7 days. 48-hr reentry for workers conducting hand-set irrigation activities; 12-hr reentry for other activities.
  • Orondis Ultra (Group 49 + 40) at 5.5 to 8 fl oz/A on 7- to 10-day intervals for turnip greens only. Do not apply more than two (2) sequential applications. Preharvest interval is 1 day. 4-hr reentry.
  • Phosphonates (Group P7) can be very effective.
    • Alude at 2.5 to 5 pints/A on 1- to 3-week intervals for turnip greens only. 4-hr reentry.
    • Fosphite at 1 to 3 quart/A on 2- to 3-week intervals. Can be applied up to the day of harvest. 4-hr reentry.
    • K-Phite 7LP at 1 to 4 quart/A on 7- to 14-day intervals. Can be applied up to the day of harvest. 4-hr reentry.
    • Rampart at 1 to 3 quart/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 quart/A on 2- to 3-week intervals. 4-hr reentry.
  • Resist 57 at 1 to 3 quart/A on 2- to 3-week intervals. 4-hr reentry.
  • Ranman 400SC (Group 21) at 2.75 fl oz/A on 7- to 10-day intervals for turnip greens only. Studies by Raid of the University of Florida (2010) showed moderately good control of downy mildew on broccoli with 30% disease severity on average in the Ranman-treated plants vs. 67% in nontreated plants. Preharvest interval is 0 days. 12-hr reentry.
  • Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide (or 2 to 4 quarts/A if used alone) on 5- to 10-day intervals. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
  • 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.
    • Reason 500 SC at 5.5 to 8.2 fl oz/A for turnip greens only. 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 nontreated plants. Do not apply within 2 days of harvest. 12-hr reentry.

Biological control

  • 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.