Cattleyas in Hawaii
Why do the tips of new growths on cattleyas become dry and black? Sometimes this blackening spreads downward and the entire shoot will die. Plants are watered every three to four days with the fertilizer Nutricote 16-10-11, supplemented by Miracle-Gro 16-10-11. Malathion and Ultra-Fine mineral oil are used to control insects. —
What you portray can be caused by a number of factors. Newly divided plants that exhibit the symptoms described are usually suffering from virus, which expresses itself more aggressively with the stress of repotting. If you are also seeing the problem in established plants, it may be due to a calcium deficiency. Check the labels of both fertilizers to see if they contain any calcium and, if not, try a solution of calcium nitrate on its own at the rate of 10 ounces per 100 gallons of water for an alternate fertilizing, say once a month.
You do not mention whether you follow a preventive program for fungus, but you may need to use something like Phyton-27® every six weeks or so to assist in protecting the young growths from infection. Possibly some of the Hawaiian-area commercial growers will recommend another local product that is effective for this if you can contact them. Finally, if you're using Malathion® as an emulsion and with the mineral oils, spray early in the day so that the plants will be dry well before the sun is high enough to damage plants with a liquid film still on the leaves.