Latin name: Peronospora sparsa
Reason for Concern:
1) Primary damage is a dry berry symptom after warm, wet weather during fruit development.
2) Especially problematic in Boysenberries.
3) Blackberry cultivars 'Black Butte' and 'Loch Ness' are susceptible.
4) Reduces crop quality and marketability.
Small red spots along the veins and severe leaf distortion appear on new growth. Patches of infection on upper leaf surfaces turn yellow and burgundy with a brown border. Mature lesions are often angular and restricted by veins. On the leaf underside, white to gray fungi and spores appear opposite the lesions. Primocanes systemically infected are often stunted with reddish terminal leaves and red streaks on the sun exposed side of canes. Infected fruit is dull, and dries out rapidly. Green fruit infection induces reddening, shriveling, and hardening. Fruit infected later in the season shrivels, dries, and splits, appearing as two berries on one pedicel. Infected stems are dry and red.
This fungus overwinters, infecting canes, crowns, roots, and buds. In spring diseased root, crown, and cane buds infect shoots. Unfolding leaves are infected during warm, wet conditions. Bloom to harvest, spores on shoots infect flowers and developing berries. Diseased berries become the inoculum source for new berry infection cycles. After harvest, infection continues in developing primocanes on the ground. Climatic conditions cause seemingly sudden flare-ups of "dry cell" on ripening fruit. Most prevalent during wet weather around 65°F (18°C), the pathogen overwinters inside roots, crowns, and canes. Sporulation (spore reproduction) is found in dense foliage near canes or at plant bases. Spores, disseminated by wind, are produced during cool, wet nights. The disease survives on rose and wild blackberry, which can infect cultivated blackberry. Symptoms develop 10 to 11 days after infection. Disease develops easily on suckers in weedy and dense canopied areas.
- Infected leaves on primocanes are the first sites for spore production. Look for systemic infections on the new growth in May. Spores are produced on the underside of leaves, under the red spots. Initially the spore masses appear white, but they turn grey as they age.
- Watch in May for small, conspicuous, irregularly shaped patches on upper leaf surfaces, starting near the petiole then following leaf veins.
- Look at 4 areas of the field at least 40 leaves on primocanes closer to the ground. Record presence or absence.
- Red streaking on stems and petioles indicates systemic infection.
- In Boysenberries the fruits, sepals, and pedicels can show symptoms, causing fruit to become dry and shriveled.
- Dryberry mite, sunburn or various other fungi can also cause these fruit symptoms.