Chronic bud blast on cattleya?

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I suspect your problem is not enough water. They need a lot of water when in bud. I try not to let Cattleyas in bud ever dry completely. In LECA you could probably water a lot more than you are without causing root rot. There is a lot of air inside the container. Vandas in bud also need very large amounts of water, even more than usual.

Another thought - a small pest called thrips (singular and plural) loves to eat buds. They are famous for destroying Vanda buds. Consider using a spray insecticide in an aerosol can only on the developing flower stalks.