black regions on cattleya roots/rhizome
I recently received a Rlc. Hawaiian Lighting Fiesta which I repotted yesterday to coincide with new root growth. Upon unpotting I noticed hard, black areas on the older parts of the rhizome, going up the base of the oldest non-withered back bulb, and above the soil line on some old roots (I had assumed they were just dirty). The plant had been in a a mix of bark, soil, and pebbles and had not been repotted by the previous owner. The bark was brittle and seemed degraded.
I clipped rhizome just below the oldest pseudobulb that still has a leaf, roughly where the dark area stopped on the rhizome (removing two withered bulbs and the green bulb with spreading black). I did not removed all of the old roots that have black near their base because they seemed firm and otherwise healthy. I sprayed the rhizome and old roots with H2O2, rinsed after a minute, dried, covered the cut with cinnamon, and repotted the plant into a ventilated terracotta pot with medium bark, keeping the dark areas on the remaining old roots partly exposed.
My very inexperienced guess at a culprit is Rhizoctonia, but I felt it prudent to check here for advice before advancing to a targeted treatment for the remaining old roots. Am I correct in my diagnosis? If so, what are the thoughts on treating the problem with thiophanate (cleary's/thiomyl) vs. chlorothalonil (daconil)? From the treatment chart linked in the sticky daconil seems easier, but might the thiomyl be more effective due to mobility?
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