It is fairly common for monopodial plants, such as phalaenopsis, to lose their bottom leaves, resulting in them carrying only a few at a time. A similar phenomenon can be seen in sympodial plants, like cattleyas, oncidiums, and the like, when they lose foliage on old pseudobulbs. In paphiopedilums, growths appear to be limited and be in a “replacement mode”, where an old fan fades away as a new one matures. Some will tell you that is fine, as that’s just how they grow, and while they can be maintained that way, it is anything but ideal. It all relates to the production, utilization, and storage of “resources” as was described in our article, Support the Reserves!
Plants routinely take up and absorb nutrients, water, air, and light, then convert them into chemical resources. Some are consumed right away to keep the biological processes functioning, and the vast majority of the absorbed water (>95%) is lost through transpiration. Any excesses are stored within the tissues of the plant.
If culture is very good, the plant will continue to add tissue and become more robust, giving it greater capacity to take up-, produce-, and store those resources. In monopodials, the results are the growth of more and more leaf pairs. In sympodial orchids, that means more and more growths, whether they be pseudobulbs or “fans”. Those plants tend to be larger and more robust, leading to the exquisite specimens that win awards and see in displays.
If, on the other hand, the culture is lacking in some way, the plant may be forced to “withdraw” stored resources from its existing tissues, and that usually starts with the older leaves.
While such leaf loss can be caused by diseases, in an otherwise healthy plant, more often than not, the “shortfall” is in the ability of the plant to take up sufficient water, whether that be due to underwatering or a weakened root system. Underwatering can be a problem among relatively new growers who do not want to “overwater” plants (a misnomer for poor Air Management), but can also be seen in plants that are overcrowded in a pot. Root systems can be weakened in many ways, such as being exposed to temperature extremes or pathogens, but it most common when incorrect potting media are chosen for the grower’s particular growing environment, or when they have waited too long, as the media have started to decompose and suffocate the roots.