It is very difficult to tell if it is Alternaria or not unless we check for the spores. Generally, there are three kinds of leaf spots on basil caused by Cercospora spp., Alternaria spp., and Colletotrichum spp. But sometimes downy mildew infection may also cause this type of spots.
Downy mildew on basil foliage with initial symptoms of pale yellowing and small necrotic (dead) spots.
Leaf yellowing caused by advancing downy mildew disease
Basil with Cercospora leaf spot (Cercospora sp.)
Early downy mildew disease symptoms on young leaves
Posted by: Dr. Ravishankar Narayana
Posted: December 21, 2017