Answer: Cultural practices for the flax hybrids are similar to those for the species with a few exceptions. As a general rule the hybrid flax are not as durable as Phormium tenax and are usually less tolerant to extremely hot or cold temperatures, prolonged dry conditions and heavy soil. We have noted that during the hot and dry summers of Arizona, sunburn is likely on the cultivars that have the graceful weeping leaves; cold temperatures below 20 F are likely to scald the leaves of these plants as well. In hot inland valleys it would be wise to use these cultivars in bright shade where there is protection from the sun. The upright growing cultivars are affected less by these extremes in temperature. We have found the hybrids to be nearly as drought resistant as Phormium tenax. Plants look much better with occasional irrigation but can survive extensive periods without it. There is a tendency for the slower growing hybrids to languish in poor heavy soil. If possible, position plants on a slope or mound when given this soil condition. Phormium plants suffer from few other maladies. Occasional long tailed mealy bug attacks can be controlled through sprays, and snails that often use the underside of the leaves for an abode can be easily picked off or poisoned. The most serious pest we have seen on New Zealand Flax is the New Zealand Flax Mealybug (Balanococcus diminutus). An infestation of this pest threatens the long term health of the plant.
The most serious complaint we have on the hybrid flax is the tendency of the colorful leaf forms to revert back to a green or bronze color. When a shoot of an unwanted color is seen on a hybrid plant, this shoot should be cut off at the base. With some of the hybrids it is the new foliage that is the showiest, and it is often best to remove all of the older leaves as the new ones emerge. In the individual listings below we have indicated the color forms that are least likely to revert by noting them as "Stable". If diligence is used, even the plants most likely to revert can be kept the original color. It should be noted that not all of the shoots with a different color than the original plant are detrimental; Phormium 'Cream Delight', one of the showiest of all of the cultivars, was a vegetative sport from Phormium 'Tricolor'.
I'm confused about the diagnosis of a "systemic disease". Especially if the blotches disappeared when you moved the plant to a shadier spot. I suspect sun scald was the culprit, not a disease of any kind. In any event, try repotting your plant into potting soil and placing it in a bright but shady spot. I think you'll find a great improvement in its appearance and health. As new growth appears you can trim away the old damaged foliage. It may take several weeks, but I think your plant will improve and become healthy again.
Best wishes with your New Zealand Flax!
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