University of California Agriculture and Natural Resources announced on Apr. 1 that recent moderate rains combined with temperatures between 65 to 75 degrees Fahrenheit have created favorable conditions for outbreaks of downy mildew in blackberries. The organization said these weather patterns, typical in February and March along the Central Coast, increase the risk of infection by the fungal pathogen Peronospora sparsa.
Downy mildew affects nearly all proprietary blackberry varieties, as well as Ollalieberry and Boysenberry. However, varieties such as Chester, Arapahoe, Apache, Navaho, and other “Native American named varieties” show greater resistance even under conducive conditions. The disease first appears as yellow spots on upper leaf surfaces before turning red or purple with angular lesions framed by leaf veins. As it progresses, leaves may become brown and fall off the plant.
Fruit from infected flowers can become crumbly or shrivel at both green and mature stages. Downy mildew is known to overwinter systemically within roots, crowns, and canes; new shoots emerging in spring can quickly spread the pathogen if moisture persists.
The university recommends growers limit periods of moisture around susceptible plants to reduce disease risk. Practices such as removing weeds and excess suckers around fruiting canes are advised to improve air circulation. Covering hedgerows with macro-tunnels has proven effective in reducing disease incidence.
For chemical control measures, Aliette (fosetyl aluminum) is often used but requires a 60-day pre-harvest interval after application. Phosphonate fungicides or fertilizers applied preventively can also help limit downy mildew by boosting plant defenses rather than directly suppressing pathogens; phosphite is preferred over phosphorous acid due to easier handling requirements.
Growers are urged to consult local Agricultural Commissioner’s Offices for up-to-date product registration information before using any fungicide or fertilizer products mentioned for downy mildew control.
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