I would like to share a new spray that I have composed and it works for me.
It started with the Zepherine Drouhin I have that is highly proned to mildew and blackspot. Since last year, I have tested with many different new ingredient to grade how effective they are to mildew and blackspot.
Finally, I realised that both problem are very sensitive to acidic and a tinge of oil will prolong and protect the leaf from catching the diseases for a considerable period of time.
If you are interested, you can try the below:
(750ml of water)
-Biocarbonate 1 tsp
-Vinegar acid or any salade vinegar 2 tsp (7 to 8% concentrated)
-Cooking oil 2 tsp
-Mild soap a few drop (optional)
-Mild body lotion a few drop ( I use baby body lotion)
Any preventive spray is advisable to spray under leaf and perferably early morning.