In general, leaf loss in Cattleya Alliance orchids is not considered to be a major growing problem. Unless you have very good growing conditions and have solved most of your cultural problems, you are going to lose the leaves on the older pseudobulbs of your Cattleyas. It is an exceptional grower who can keep these plants happy enough to produce a specimen plant. That is not to say that the hobby grower cannot do better to reduce leaf loss than the average grower. There are three main reasons for leaf loss: diseases/insects, culture and genetics.
In the diseases and insects category, scale is the main culprits. Scale is easy to spot once you have ever seen it – a grouping of white fuzzy spots. Left untreated scale will overtake a plant and ultimately will kill it. As soon as you see evidence of these pests, remove the plant to a location away from your other plants. Manually clean up any obvious insects or spray the whole plant with Bayer’s Rose and Flower Insect Spray (dual action) which will kill any pests that it touches as well as be absorbed in the plant and will kill adults as well as any sucking insects. Continue to spray for about 3 months.
Cultural practices which will cause leaf loss include overwatering, under-watering and excess cold and or heat. Overwatering will cause the roots on your plants to die off so they cannot support the plant. In an attempt to stay alive, the plant will shed extra leaves and try to keep the new growth alive. If possible, look at the roots of your plants – some growers actually take their plants out of the pots periodically to assess the growth. An old rule of thumb says that if the second oldest pseudobulb (generally from the previous year) is wrinkled and desiccated, you are either under or overwatering. Take your plant out of the pot to see which it is. If the roots are good and plump, then you’re under-watering. If they are all dead and rotten, you have overwatered. Repot and change your culture.
Another cultural practice which will cause leaf drop is very cold temperatures in the winter and very hot temperatures in the summer. Most Cattleya Alliance plants will withstand a wide range of temperatures, but very high and low temperatures, even within their range, will cause them to be stressed and lose leaves.
Genetics will also determine whether the plant will grow large for you or lose leaves once they are a couple of years old. There is little that can be done for this aspect but to determine through trial-and-error which plants are best for your growing conditions and stick to them.
One point to keep in mind is that if you can keep from doing it, it is much better to allow the leaf to drop off naturally than to cut it off. If the leaf will not snap off with light pressure, leave it on until it is ready to come off. The plant will seal the wound through a natural drying process which will keep any pests or disease from using the area to invade the plant.