Late frosts that kill peach flowers are common in Kansas. Many areas will not have a full peach crop except for about one in every seven (or more) years. This year has been exceptional, with full fruit crops and excellent peach-growing weather in most areas. However, we have been receiving reports of trees with small peaches. Though small fruit could be due to poor weather (rare this year) or heavy fruit crops (common), there is a third possibility that is often overlooked. That possibility occurs when the top portion of the peach dies and the rootstock puts up new growth.
Peaches, like other fruit plants, must be vegetatively propagated. In other words, you cannot grow fruit from seed and expect the progeny to share the same characteristics as the parent. Therefore, good fruit trees have a top portion called the scion (the good fruiting part) and a bottom portion known as the rootstock. This combination is made by grafting or budding the scion onto the rootstock. Virtually everything above ground will be the scion and everything below ground will be the rootstock. The rootstock may keep the tree smaller, be more disease resistant than the scion, delay bloom or give some other good characteristic to the tree. However, the rootstock normally does not produce good, high quality fruit. Therefore, if the scion dies and the rootstock throws up new growth, the fruit produced will most likely be of poor quality.
How do you tell if the small fruit is due to a rootstock taking over? If the fruit produced is always poor quality, then suspect the rootstock problem. If this is the case, there is no remedy. The tree will not produce good quality fruit regardless of the care given. It would be best to remove and replace the tree. (Ward Upham)