From September through October, this yellowing or browning phenomenon shows as a change of color that starts from the inside out. It can be alarming to a gardener, especially if drought or pests have affected nearby shrubs and trees.
Needles generally begin dropping from the interior of a branch (not from the branch tips, which is a different issue altogether) and an otherwise healthy conifer can look diseased if it still has brown needles clinging to its branches. It’s nothing to worry about—those brown needles will fall on their own.
All conifers lose needles, but they vary in severity and timing, and even by color. Some inner needles may turn brown all at once, while others may show a progression from gold to rust to brown before the needles drop.
Sometimes the change is so dramatic, and the needle loss so abrupt, that you think infection is to blame. White pines are notorious for this, as they shed all of their needles except for the current year and sometimes the previous year, and can look pretty sparse all winter.
Needles on an evergreen tree can live from one year to several years, so most trees lose their old needles every couple of years to every five years or more.