A pesticide wont do much if its not a pest. Try some Copper Oxychloride, it is a broad spectrum fungicide and anti-bacterial, so it will sort out anything within those two categories. I use it on my sick plants and i swear by it.
My recommended Regime: Mix in 4-5gms of a strong copper oxychloride or 5-6gms of a regular copper oxychloride in 5ltrs of water (this is half strength).
Apply to foliage and media once a week for 6 weeks, then after this, apply a foliar fertilizer specifically formulated for orchids (i have found others dont work as well during this regime) at half strength.
Continue for another 6 weeks on this, then after just grow on as normal.
NEVER USE THE FERTILIZER DURING APPLICATION PERIOD OF COPPER. Again, i have found it messes with the plant and it can kill them.