Peronosporea are oomycetes, a group related to fungi, also known as water molds. They cause infectious diseases such as downy mildew.
Signs of damage:
Leaf curl and distortion. Leaf yellowing or stippling. Necrotic leaf spots. White sporulation of the pathogen on lower leaf surface. Defoliation.
How to prevent:
Use hygiene measures. Start clean, remove crop residues and prevent splashing water.