Thank you Rob, Har, Warren, Sapote for your suggestions.
I went through the link and yes it looks similar like bacterial black spot.
All those mango plants were bought from the Regional Fruit Research Station, Vengurla. This is the research station who has developed some nice mango varieties, including the Sindhu (so called seedless), Ratna and the newly released Konkan Samrat. So i believe the disease was not coming with the planting material but got infected at home after planting or during rain from nearby mango trees.
http://www.dbskkv.org/Research/Varieties_Fruit-Crops.htmlBelow are the Indian varieties
1. Kesar
2. Ratna
3. Sindhu
4. Amrapali
5. Dasheri
6. Payari
7. Goa Mankur
8. Dudh Pedha
Below are the foreign varieties
9. Tommy Atkins (USA)
10. Keitt (USA)
11. Kingfon (dont know the country of origin)
12. Maya (i believe it is from Israel, but not 100% sure)
13. Lily (dont know the country of origin)
14. Konkan Samrat - this is the new hybrid released by this university in 2014, its a hybrid of Alphonso and Tommy Atkins. Very difficult to get the plants at this moment due to limited planting material availability which is made available mostly to nursery owners.
Unfortunately i am away from home (another 2 months to go) so don't know exactly which of the above plants are affected, but i have asked my wife to check the labels and let me know. According to that documentation, Keitt could be one of them.
So, as suggested in that document, a copper based fungicide should work, correct?
What about using the Copper Sulfate and if that is going to be effective then how many grams / liter of water and the frequency of spraying?
What about using any systemic fungicide like Bavistin (Carbendazim)? Will it be more efficient and long lasting than a copper based contact fungicide? Proportion/frequency?
Thanks for your help.
Kind regards, Sobars