Although typically easy to grow, the monkey puzzle tree can develop a fungal disease called canker, and it is also susceptible to insects that cause the foliage to turn brown, according to sfgate.com. The first issue is due to poor growing conditions while the latter is easily treated.
Canker can appear on a monkey puzzle tree that has been planted in soil that is too moist. To avoid canker, the grower must ensure the tree is planted in an area with good drainage.
Several kinds of insects are known to infest monkey puzzle trees, including mealybugs, thrips, spider mites and scale, says sfgate.com. Such insect invasions are treated with insecticidal soap or horticultural oil.