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. 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, later showing on the undersurface. The spot turns yellow as it enlarges. In high humidity, a grayish-white mycelial growth is present on the leaf's underside.
Spots can be seen on the outsides of stalks. 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.
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 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.
Chemical control
- Copper products (Group M1) are not recommended as stand-alone materials.
- 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
- Phosphonates formulations (Group P7).
- Fosphite at 1 to 3 quarts/100 gal water/A on 14- to 21-day intervals. 4-hr reentry.
Biological control
- Stargus at 2 to 4 quarts/A plus a nonionic surfactant. Preharvest interval is 0 days. 4-hr reentry. O