Fungal infections can also cause black or brown spots on basil plants.
46+ Black Spots On Brassica Leaves. While both species are present, alternaria brassicicola is the most common. Three species of alternaria cause serious damage to brassicas:
Fact Sheet Cabbage Grey Leaf Spot 310 from www.pestnet.org
One of the most common culprits while growing basil is downy mildew. Though unattractive, these spots rarely cause serious harm to the plant and often go away when the weather warms up. What soil did you use for your 50/50 soil perlite, was it vermifire?
It usually occurs on the upper sides of leaves, but can also develop on the undersides.
Though unattractive, these spots rarely cause serious harm to the plant and often go away when the weather warms up. The spots grow and the leaves turn yellow and fall off. This disease can be caused by two species of the fungus alternaria. The black spots on mine seemed to be worse after using safer brand caterpillar killer, it left little white chunks that i think dried and cause even more spoting.
