Cold Hardiness

With the exception of the cold hardy varieties, peach trees can handle temperatures down to zone 5a (-15F to -20F), but their flower buds are only hardy down to -5F to -12F, depending on the weather of the previous few days.  Bud hardiness begins around -5F (zone 6b), but they develop a few extra degrees of hardiness, down to -12F, for every day the temperature is below 29F. If the temperature rises above freezing once again, even for just a few hours, the process restarts.

Cold hardy varieties can survive down to zone 4 (somewhere between -20F to -30F) and frequently bear after experiencing zone 5a temperatures (-15F to -20F, sometimes lower), but fruit set can vary widely.  I do not consider varieties to be cold hardy unless they also appear to bloom late or have additional frost resistance to their flower buds. 

Disease and Pests

Brown rot is the most devastating disease that affects peaches.  It can ruin the entire crop in unfavorable weather when chemical control is absent.  Bacterial spot can also be a problem, but there are a number of varieties that are fairly resistant.  It should not be confused with peach scab, which is often quite minor on its own.  Peach leaf curl is a problem in areas that experience cool and wet conditions around the time the tree comes out of dormancy.  In some high pressure regions, such as the Willamette Valley, most varieties may only last a few years before they die, and those that are considered resistant will also struggle (without intervention).  Peaches on standard rootstock are quite vigorous and often callus over areas infected with canker.  This is more likely to occur in warmer regions. 

The oriental fruit moth and the greater peachtree borer are the pests of most concern.  The oriental fruit moth behaves differently with peaches than they do with other stonefruit in that the first few generations will feed on young shoots, which becomes apparent by the wilting leaves.  The greater peachtree borer generally targets the first 6 inches of the trunk.  While they also feed on other species, they are particularly damaging to peaches due to their soft wood.  Vigourous trees will often outgrow the damage, but borers are occasionally responsible for the death of the tree.

Plum curculio may also be a cause for concern, but based on my observations, they are significantly less of a problem for peaches than they are for apples and plums, even in high pressure areas.  I cannot say this is the norm, but I haven't seen anyone talk, in detail, about the curc's effect on their peaches.  Plum curculio are generally limited to the eastern half of the United States.