I have a pink baby doll Dracaena terminalis "ti plant" that has no visible pests but the older leaves develops scattered tiny spots with a dark center and yellow outline. They also have browning tips. I have the plant against my south facing bay window, keep it moist and mist it once a day. I fertilize it with miracle grow every two weeks. The lower, older leaves are all affected and eventually dry out and fall off. Now the plant has a long trunk with foliage just at the top. Before we look at what might be causing the tiny spots on the leaves, the brown leaf tips and dropping of the leaves, we should take a look at the plants true name. Many growers or retailers often label plants incorrectly. The species name 'terminalis' would indicate that your plant is really a Cordyline and not a Dracaena. Cordyline terminalis 'Baby Doll' is often incorrectly labeled as Dracaena terminalis 'Baby Doll'.
Although Cordyline and Dracaena plants do look quite similar, an easy way to separate the two genera is by examining the root system. The Dracaena's roots are smooth-surfaced, non-creeping, orange or deep yellow in color. Cordyline spp. and cultivars have a creeping rootstock with knobby, white roots. New plants are often propagated from sections of these fleshy rhizomes or tubers.
Cordyline and Dracaena species are often called False Palms, and for good reason. Older specimens resemble palm trees with their leaves crowned at the tops of leafless stems. The bottom leaves, through the natural aging process which is characteristic of these genera, will yellow and drop usually within two years of age. It is a bit deceiving when we purchase a young new plant because the plants often bear leaves from the base of the stem to its top. This is natural of most plants that have been newly propagated. Eventually, as the plant grows taller and ages, the bottom leaves will drop giving these plants their palm-like appearance.
It would also appear that the plant is actually beginning to look like a real 'Ti plant' or 'Ti tree'. The term 'Ti plant' is generally given to plants like Cordyline, Dracaena and Yucca after they are propagated from a piece of woody cane, or after the old plant has been air-layered and the top part of the plant removed. New leaves then grow at the top of the bear cane, giving the plant a palm-like appearance. These plants are named 'Ti plants' or 'Ti trees'.
It is best to know the natural characteristics of a plant as they grow and develop. In many cases there is really no problem with the plant, but only the natural changes it must go through as it matures and ages. The dark spots with yellow halo's on the leaves of your Cordyline could be one of many fungal or bacterial diseases known as 'Leaf Spot'. The only way of knowing which disease it is and how to control it (if it is a disease) is to have a laboratory examine the spots to verify what they are. If the spots are a disease, they would probably suggest destroying the plant as control methods are difficult. It is unlikely that this is a disease if the spots are only showing up on the leaves that are falling and no where else on the plant. Fungal and bacterial diseases are spread very easily by hand, splashing water, pruning tools or anything that might come in contact with the infected plant tissue. Be sure to dispose of the fallen leaves and badly spotted ones in any case.
Brown spots with yellow edges (halo's) and brown leaf tips can be caused by sun-scorch, low humidity, fluoride in the water, excessive amounts of fertilizer, overwatering or underwatering and high temperatures associated with dryness. When lower leaves dry up and fall off a plant, the main two causes are underwatering and too much heat.
My first suggestion would be to move the plant away from the south bay window. Plants with colored foliage most certainly do require bright light, but not in a location where the sun can beat down on them creating extremely high temperatures and excessively dry air. Cordylines do best near a west window or an east window where they will receive only approximately 4 hours of direct sunlight per day. They can be grown near a south window, but they must be shaded from the hot sun with a light curtain. Inside the bay window, the plant could be suffering from extreme fluctuations in temperature. When a plant is located this close to glass, sunlight is intensified and the area becomes very hot and dry. (Scorching or spotting of leaves can be a direct result of locating a plant to close to a window). During cold nights, temperatures near glass can be considerably cooler. A large rise or drop in temperature can put more sensitive plants into shock, resulting in leaf drop.
Misting the plant once a day in this location could hardly benefit it. Cordylines need high humidity levels, at least 60% for optimum health. The plant should be set on top of pebbles or on top of an upside down catch-basin that sits in the regular catch-basin (so the pot is not sitting in water). The water in the catch-basin will help to increase the humidity level around the plant. If you have ever entered a car with all the windows rolled up on a very hot day with the sun beating down on it, then there is no need to imagine how hot it really can get inside a bay window that faces the south. A plant must transpire at a terrific rate in order to keep itself cool. If humidity levels are very low and the soil becomes dry for even a short period of time, cell damage can occur, resulting in brown or yellow spots on the leafs, brown leaf tips and dead leaves.
Cordylines must always have a moist soil. In the winter months if the plant is dormant, it will not require watering as often as it does while it is actively growing in the summer, but the soil must not be allowed to dry out. Nor must it be overwatered. A constant soggy soil can lead to many problems and eventual plant death. When watering, keep pouring water on the top of the soil until it runs out of the pots drainage holes. Do not water the plant again until the top of the soil just approaches dryness.
Check to see if the plant is pot or root-bound by examining the root ball. If it is pot-bound, the plant may not be receiving the moisture it really requires even though it is being faithfully watered. This is also a good time to check the soil for compaction. Plant roots also need oxygen in order to live, so be sure the soil has good porosity. If it needs repotting, use a pot that is only one size larger.
Fertilizing the plant every two weeks can certainly cause spots on the leaves, brown tips and leaf drop, especially in the winter months if the plant should happen to be dormant. Even if the plant is not dormant, photosynthesis is occurring at a much reduced rate during the winter months because of the much shorter days. How can a plant use up the same amount of nutrient each day in winter as it can in summer when the days are much longer? Most Cordyline and Dracaena plants grown in the home are happy with an application of diluted water-soluble fertilizer every 3-4 months. After all, we do not want the plants growing to quickly. Excessive amounts of fertilizer in the potting media can soon harm and eventually kill a plant. If the potting media has a white crust on the surface or the drainage holes are covered with a whitish brown coating, this indicates a high concentration of salts or too much fertilizer.
A final suggestion (if you are very unhappy with the plants appearance) is to propagate the top portion of the plant using a method known as air-layering. You will have a beautiful, new, compact plant within a few months using this procedure. Be absolutely sure the plant is disease free before endeavoring to do so.