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 may bear after experiencing zone 5a temperatures (-15F to -20F), but fruit set can vary widely.  The trees of some varieties have also survived zone 4 temperatures (somewhere between -20F to -30F).  Futhermore, most cold hardy varieties bloom late or have additional frost resistance to their flowers.

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 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 feed on young shoots, which becomes apparent by the wilting leaves, otherwise they feed on the fruit.  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 their death.

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 (deforms the fruit) and plums (drops the fruit), even in high pressure areas, but this may only apply to the northern strain of this pest, which only reproduces once per year.  Regardless, they can create entry points for pathogens later in the season as they feed on the fruit, which wouldn't pair well with a wet climate.  This hasn't been an issue for me, but the peaches here ripen when the average daily high is between 72F and 62F, which is, for the most part, low enough to inhibit brown rot growth.

Plum curculio are generally limited to the eastern half of the United States, and based on a map that is supposedly outdated, the southern strain, which reproduces twice per year, can be found south of Illinois and West Virginia (excluding the colder parts of the Appalachian mountains).  The claim is that the real boundary is a few hours north of this point.

Rootstock

In addition to seedlings from the most cold hardy varieties — Bailey, Krymsk 86, and Siberian C are some of the hardiest rootstocks for peaches.  Guardian is also believed to be quite hardy.  The American plum (Prunus americana), which is the most tolerant of them all, can also be used for some peach varieties — such as Redhaven, Contender, and Saturn — but others may show signs of incompatibility and fail to thrive.  It has a semi-dwarfing effect and can likely be used to resist any significant damage from the greater peachtree borer, if the peach is grafted far enough above the ground.  However, the American plum has a strong habit of producing suckers.  Lovell is often implied to be fairly hardy as well, but it is likely not to the same extent as the others.