Cross-sections of live (top) and dead (lower) peach flower buds showing cold injury. Photo by E. Winzeler.
Occasionally one-year-old shoots (also called twigs or hangers) and older wood are injured, i.e., if temperatures are extremely cold. Growers often modify late-winter and early-spring tree management, such as pruning and fertilizer application, when there is extensive bud loss or wood injury. Therefore an accurate assessment of injury is needed to make intelligent decisions pertaining to orchard management, anticipated labor requirements, and fruit marketing.
I know from personal experience that it is difficult to accurately assess peach bud mortality during the winter after a cold event and even in the spring after a frost during bloom. I know that I tend to overestimate the damage, so if I find that 10% of the buds are alive, I figure there is potential for a full crop and if I find only 1 to 2% of the buds alive, I figure there is potential for a half crop. Sometime I have found no live buds and I still had a third of a crop. I usually just go out in the orchard and randomly select a few buds from about 10 or 12 trees until I have looked at about 100 buds, but I think if we took a more systematic approach to evaluating cold injury, we might get more accurate results. The purpose of this article is to review some of the research that has been published relative to assessing cold injury in peach and use this information to develop a provisional protocol for assessing injury. During the 1970s the Ontario peach breeding program concentrated on flower bud hardiness and the Michigan program worked on wood hardiness and much of the relevant information was generated by those two programs.
Tissue browning indicates injury
When looking at a cross-section of a twig, branch or trunk, there are different tissues with different functions. If we think of a shoot, branch or trunk as a cylinder, the different tissues appear as concentric circles or a series of cylinders. The outermost portion of the stem is bark and to the inside of the bark there is a cylinder of phloem tissue. Phloem cells are living and function primarily to transport materials down the tree. To the inside of the phloem there are xylem cells. Xylem cells are short-lived and after they die they act as a pipeline to transport water and materials primarily from the roots to the top of the tree. The interior portion of the twig is referred to as the pith and is composed mostly of dead xylem cells. Between the phloem and xylem is a layer several cells thick called the cambium. The cambium produces phloem cells to the outside and xylem cells to the inside. Through cell division within the cambium, the shoots, branches and trunks increase in diameter. When grafting, it is important that the cambiums of the scion and rootstock are in contact. If the cambium is killed by low temperatures, the shoot or trunk will no longer be able to produce new cells and death will occur.
I have seen instances where the center portion of the trunk or twig is dark brown in late winter, but they survive because the cambium is still green. When evaluating injury of twigs or branches it is important to look at the cambium and not the pith. When evaluating winter injury of flower buds it is important to look at the ovary at the base of the flower. When evaluating flowers for frost injury, it is important to look at the style. Browning of these tissues indicates injury.
To properly assess winter injury, we first need to know how to sample an orchard. The exact relationship between twig hardiness and hardiness of older wood is not known, but researchers have often used excised twigs of the previous season's growth to evaluate wood hardiness of the entire plant. Cain and Anderson (1976), at Michigan State University, systematically sampled trees to determine differences over the length of the twig, between twigs within a tree, and among trees of the same variety in an orchard. They collected twigs from different parts of 'Redhaven' trees from different locations within an orchard on four dates. The samples were taken to the lab where they were frozen to temperatures that would injure the wood. Upon thawing, thin internodal cross-sections were evaluated under a scope for browning. They then used statistical techniques to evaluate the sources and magnitudes of variation. From this they learned several important things:
- Temperatures lower than -15 degrees F were usually required to injure twigs.
- The basal section of the twig was least injured and the tip section had the greatest browning.
- There were greater differences among twigs on a tree than among different trees, but tree differences became greater in the late winter
- Twigs sampled from the upper southwest sector of the tree had the least injury and twigs taken from the lower northeast sector of the tree had the more injury.
- Thick twigs had less injury than thin twigs, especially in the lower northeast sector of the tree.
They did this study primarily to determine how to sample trees to reduce variation so they could determine which selections from their breeding program were most cold hardy. They concluded, for breeding programs, where each seedling is unique, that a sample of 6 to 21 twigs could detect important difference in twig hardiness in breeding programs if they observed tissue browning at the basal section of medium diameter twigs collected from the upper southwest sector of the tree.
Cain and Anderson were trying to develop sampling methods to reduce variation so they could detect important differences in hardiness of selections in their breeding program. Since our goal is to estimate the extent of winter injury in an orchard block, we would probably want to sample twigs of different diameters and from sectors of the canopy that vary in hardiness.
Dr. Dick Layne, retired peach breeder at Harrow, Ontario, told me that peach flower buds rarely survive temperatures below -12 degrees F and twigs are injured at -20 degrees F; however I have seen flower buds survive -21 degrees F, so the lethal temperature for different tissues depends on a number of factors. One of these factors is the crop load the previous season.
When I was in New Jersey, we had a normal crop in 1982 and we experienced -23 degrees F in January 1983. We had a large peach germplasm collection on that farm, including hardy varieties like 'Redhaven', 'Harken' and 'Reliance'. The only tree that had any fruit in 1983 was the original 'Encore' tree, but it was also on a good site. I had a 2-year-old planting of several varieties and most of those trees had some shoot dieback and canker infection was a problem. In January 1984 we experienced -21 degrees F and all but the tender varieties, such as 'Blake', 'Jersey Queen', 'Sunhigh' and 'Loring', had a full crop. This indicated to me that trees with no crop are much more tolerant of low temperatures that trees that were cropped. When I was in Virginia, Dr. Ross Byers and I made similar observations following a spring frost, where flower survival was best on trees carrying light crops and trees that were bloom-thinned the previous season.
Dr. Layne suggested the following procedure for assessing natural flower bud and twig damage:
- Collect at least 5 to 10 shoots/tree randomly
- Collect entire 1-year shoots and place in plastic bags
- Hold at 70°F overnight to allow browning or hold at 40°F for up to several weeks in a moist plastic bag
- Assess bud and xylem injury on same shoots
He did not suggest how many trees per block should be sampled. He recommended that all buds on a twig should be evaluated and a cross-section from the middle section of the same twig should be used to evaluate xylem browning, since this represents the region of average injury (the tip will have more injury and the base will have less).
Dr. Layne also evaluated cold injury of flower buds, vegetative buds and twig xylem for many varieties of peach following cold winters and also following controlled freezing in the lab. He found a very poor correlation between percent bud mortality and twig browning with an R2 of 0.18. This means that when browning is used to predict bud death, only 18% of the variation in bud death is explained by variation in browning. This poor relationship is due to the fact that some varieties have hardy buds but not hardy xylem tissue. So bud injury is not a good indicator of wood injury or vise-versa.
Suggested protocol for assessing bud or twig injury
- Collect 4 one-year-old shoots (hangers) from the upper southwest sector and the lower northeast sector of 6 trees per block. Two of the shoots from each tree sector should be thin short shoots (about 6" long) and two shoots should be moderately thick shoots about 24" long. Choose typical looking trees from regions of the orchard that you suspect may differ in the amount of winter injury they sustained. For example, trees in low areas may have more injury than trees on higher ground. Trees with the heaviest crops the previous year will be less cold hardy than trees with a lighter crop. Trees near a windbreak may have less injury than trees in the open. Tie together the 4 shoots from a tree sector and label the bundle with the tree location and the sector of the tree, so you know where each shoot came from.
- Store the labelled shoots in a plastic bag at room temperature for 24 hours.
- To assess twig injury, move 1/3 of the way down from the shoot tip and cut a thin cross-section through an internode section and look for browning. Record injury as 1 = no browning; 2 = browning of the xylem, but the cambium and phloem are still green; 3 = patches of brown cambium and phloem around the circumference; and 4 = entire cambium and phloem is brown. Twigs rated as 3 or 4 will probably exhibit dieback in the spring. Razor blades and some type of magnification makes it much easier to see the various tissues in shoots and buds.
- Now cut every flower bud on each shoot and record each bud as dead or alive. Be sure to include the small buds at the base of the shoot because these are usually the most likely to survive low winter temperatures.
- To calculate the percentage of live buds divide the number of live buds by the total number of buds and multiply by 100. For example, if you evaluated 1,220 buds and 335 were alive, then the percent live buds would be 335/1,220 = 27.5% live buds. If there are no frosts during bloom and conditions are good for pollination and fruit set, then you have the potential for a full crop with 27% live buds.
I have not tested this protocol, but I think it takes into account the major sources of variation and should provide a reasonable estimate of flower bud mortality and shoot injury.
By knowing which sections of the orchard and which shoots on the tree have the most damage, growers can modify their pruning and bloom thinning strategies to increase the likelihood of a reasonable crop.