A bottlebrush plant turning brown is indicative of a number of potential issues that stem from roots to crown. The bottlebrush plant (Callistemon) is a hardy evergreen indigenous to Australia. It has a distinctive flower that resembles a brush used to clean the interior of a bottle, hence the name. Bottlebrush plants range from dwarf shrubs no more than 3 to 4 feet high to trees up to 16 feet tall and can live 15 to 45 years depending on care, soil conditions and temperatures. They can even be bonsai plants as well. Their colors range from bright red and pink to purple and yellow and many variations in between. Several conditions, fungi and diseases affect these plants and have the ability to turn them brown.
Avoid temperature zones that drop below freezing. Bottlebrush plants are originally desert-based, and while they are hardy plants that can thrive in soil that varies from loose and sandy to hard clay, they are sensitive to temperature variations that will cause them to brown and not sprout their telltale brightly colored flowers. Areas where there is very little frost damage are best.
Plant the bottlebrush tree or shrub in soil that is well draining. This is important, as the bottlebrush plant is sensitive to excess moisture and will turn brown if overwatered or if water pools around the trunk. Root rot is a real problem with these plants, and once the disease affects the roots, you won't have any recourse but to watch the plant brown and die. Make sure that water drains away from the roots adequately.
Poke holes in the bottom of the container tub if you plant a dwarf bottlebrush for an outdoor container garden to ensure that the plant won't brown. The same concept goes for a bonsai version of the plant. Let the water drain through the holes in the planter to wick away the moisture from the roots. Make certain that the soil in which you plant the bottlebrush is warm to the touch to emulate the desert origins of the plant.
Fertilize the plant two to four times a year to ensure that the trunk and stems are hardy and healthy to ward off fungi and insects that can harm the plant systemically. Apply fungicides as needed to maintain the healthy color and use a liquid copper spray fungicide for the leaf spots. Once leaves turn brown and other parts of the plant show discoloration, you can be sure that there is some sort of fungal issue affecting the plant's ability to thrive in the soil and temperature conditions.