Pacific Northwest 1998 An Online Guide to Plant Disease Control

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Blackberry -- Downy Mildew
See Also: Rose -- Downy Mildew
 
Cause: Peronospora sparsa, a fungus-lke organism that infects many caneberries (Rubus sp.), especially blackberry and 'Boysenberry'. The disease is not common on red raspberry. The blackberry cultivars 'Black Butte' and 'Loch Ness' are susceptible. It overwinters primarily as a systemic infection of canes, crowns, roots, and buds. The disease cycle starts each spring with the production of infected shoots from infected root, crown, and cane buds. Unfolding leaves are invaded during warm, wet conditions. Secondary infections of flowers and developing berries during wet, warm weather are initiated from flowering to harvest primarily by conidia produced on diseased shoots. Diseased berries then become an important source of inoculum for new cycles of berry infection. After harvest, infection of developing primocanes lying on the ground continues by internal mycelial growth and spore infection.
Symptoms: In summer to autumn, small, conspicuous, irregularly shaped patches appear on upper leaf surfaces, starting near the petiole then following leaf veins. Patches are yellow initially, becoming a deep wine red with a brownish edge and brownish discoloration near and alongside veins. Systemic infection is indicated by red streaking on stems and petioles. The organism attacks fruits, sepals, and pedicels of 'Boysenberry', causing fruit to become dry and shrivelled (dry cell syndrome). Berries also may split and appear to be two berries on one pedicel. The dryberry mite and sunburn can also cause a dry berry condition.

Note the deep-red coloration along the main veins of the leaves.

Some cultivars show numerous atypical lesions on leaves, but like typical symptoms, lesions are angular along leaf veins.

The receptacle of infected berries may split down the middle. These berries dry out and may look like doubles.

 
Cultural control:

  1. Early primocane control through chemical burning.
  2. Reduce or eliminate overhead irrigation.
  3. Practice good weed control.
  4. Eliminate wild blackberries and roses close to fields.
Chemical control: Recommended only for plantings confirmed as diseased.

  1. Aliette WDG at 5 lb/A sprayed over the trellis twice in fall before fall rains and twice in spring. Do not mix with surfactants or foliar fertilizers. Do not apply within 60 days of harvest. 12-hr reentry.
  2. Fosphite at 1 to 3 quarts/A. Do not use copper products within 20 days of treatment and do not use spray adjuvants. 4-hr reentry.
  3. Phostrol at 4.5 pt/A is registered for root rot control and may be effective for this disease. 4-hr reentry.
References:
Hall, G. 1989. Peronospora rubi. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 976. Surrey, England: Commonwealth Mycological Institute.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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