Black spots with white ring around them on Zinnia leaves

Question

We have grown Zinnias in our garden since 3-4 years, In South India . The initial batches were very healthy and robust ,with stunning flowers.

but since the last 1 year or so I have noticed that the zinnias seem to have stunted growth and start flowering small flowers before even reaching a height 1 feet even though we provide enough manure.

And these spots appear only after the plants have grown for 1 month or so. Saplings are perfectly spotless. Never seen any grasshopper or other insects chewing on the leaves. But there are tiny red ants swarming on the unopened buds, I suspect they eat the buds. Tried Gammaxene and copious amounts of turmeric powder to drive them away.

There are these black spots with white rings around them ,on the leaves and flowers.

Black spots on the stem.

What are these spots? Are they some fungus? Are there any methods to get rid of them?

Eco friendly methods are appreciated.

Answer 1

The spotting on the leaves is a fungus, probably alternaria. It could also be a bacterial infection (Xanthomonas) - this second link has some information on how to tell them apart. Because sites disappear or move, I'm going to quote the relevent information from the second link so that it stays with this question:

[Fungal] diseases can be introduced to a crop through contaminated seed and spread from plant to plant via splashing water. Infections usually begin on lower leaves and move upward in the plant canopy. High humidity, overhead irrigation and close plant spacing promote long leaf wetness periods which favor disease development. Cultural practices to help minimize both diseases are similar – rogue out symptomatic plants, promote good air circulation and alter irrigation practices to keep foliage as dry as possible.

I would add to this that you should remove all traces of the infected plants from your garden to prevent them from possibly infecting next year's plants, and I would also practice crop rotation over a three-year period, if space allows. Because the fungus is transmitted via splashing water, mulching your beds heavily is also a good idea, although I've had mixed luck with mulching and tomatoes (they still get infected, but just later in the season).

This site also has a little more information and specifically recommends crop rotation, garden cleanup and changing the timing of any irrigation - although of course you have no control over the weather in your area.

Your plants are extremely stressed right now, which leaves them open to even more pest and fungal problems. The tiny red ants (are you sure they're not spider mites?) are interesting and may be attracted to the buds by aphids; these could also be the black spots on the stems.