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J. Egea and L. Burgos

Laboratory and orchard tests have shown that the apricot (Prunus armeniaca L.) cultivars `Hargrand', `Goldrich', and `Lambertin-1' are cross-incompatible. All three cultivars are from North American breeding programs and have `Perfection' as a common ancestor. In orchard tests, compatible pollinations resulted in 19% to 74% fruit set, while incompatible pollinations resulted in <2% fruit set. Microscopic examination showed that, in incompatible pollinations, pollen tube growth was arrested in the style, most frequently in its third quarter, and that the ovary was never reached. It is proposed that self-incompatibility in apricot is of the gametophytic type, controlled by one S-locus with multiple alleles, and that these three cultivars are S1S2.

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Stephen M. Southwick and Kitren G. Weis

Selection and propagation of rootstocks for apricot (Prunus armeniaca L.) varies worldwide in response to local climate, soils, and cultivars. In this paper we review published research focused on these local selective practices. Additionally, we review the current development of apricot rootstocks and suggest new research avenues to satisfy the needs of commercial apricot growers. Rootstocks are identified by their responses to biotic and environmental stresses, with specific adaptive characteristics that enable establishment and production under unique zonal ecologies. Desirable characteristics include scion compatibility, adaptation for heavy or wet soils, pest and disease resistance, ease of propagation, control of vegetative vigor, effects on dormant season physiology of the scion, precocity, fruit quality, and productivity. Interstocks that can overcome incompatible rootstock-scion combinations are covered. As worldwide consumer demand for apricots increases with improved apricot cultivars, rootstock selections and propagation must be developed for niche fruit with specific characteristics, intensive production systems, mechanized harvest, and marginal site selection.

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Patrick M. McCool and Robert C. Musselman

Almond (Prunus amygdalus Batsch cv. Nonpareil), apricot (Prunus armeniaca L. cv. Royal Blenheim), and peach [Prunus persica (L.) Batsch cv. Halford] grafted nursery stock seedlings were exposed once per week for 4 hours to a maximum O3 concentration of 0.25 μl·liter-1 in field exposure chambers. Exposures were repeated for a total of 4 months in 1986 (year 1) and 1987 (year 2). Trunk caliper, number of shoots, and net growth (total seasonal weight increase) were measured at the end of each year. Almonds appeared to be the most sensitive to O3. Almond seedlings exhibited extensive foliar injury from O3, while apricot and peach seedlings were relatively insensitive. Total net growth of O3-exposed almond was reduced during both years relative to the controls and an impact on caliper was evident after year 2. Apricot seedlings exposed to O3 developed a thinner trunk but more shoots than the controls in both years. Peach tree seedlings exposed to O3 had fewer shoots than the controls at the conclusion of year 2 but thicker trunks after both years. No significant difference in variance or shape of distribution of net growth within the treatment populations between O3-exposed seedlings and controls was detected for any of the three fruit crops. The impact of O3 on young, nonbearing perennial fruit crops may be most evident in specific growth characteristics, such as net growth or trunk caliper.

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Massimo Tagliavini and Bruno Marangoni

Most deciduous fruit crops in Italy are grown in the north and especially in the eastern part of the Po River Valley (mainly in the Emilia Romagna and Veneto regions) and in the Adige River Valley (South Tyrol and Trento provinces). Soils in the wide Po River Valley, where pear (Pyrus communis), peach and nectarine (Prunus persica), kiwifruit (Actinidia deliciosa), plum (Prunus domestica and P. insititia), apricot (Prunus armeniaca), cherry (Prunus avium), and apple (Malus domestica) are grown, are alluvial, generally fertile, fine textured, alkaline, often calcareous and well enriched with Ca. Apple plantings are concentrated in the Adige Valley and located on a variety of soil types, including sandy loam, loamy sand soils or sandy clay, sometimes calcareous. Integrated fruit production is gaining importance and represents more than 80% of apple production in South Tyrol and about 60% of peach and nectarine production in Emilia Romagna. Under these conditions, the main objectives of mineral nutrition are to reconcile production and environmental concerns (minimize nutrient leaching, soil pollution, volatile emissions). In particular, fertilization aims to improve external and internal fruit quality and storage ability, reduce production costs, maintain soil fertility, avoid nutrient deficiency and excess and control tree vigor. Nitrogen applications have strongly decreased in recent years and there is a need to improve the efficiency of N fertilizers while avoiding deficiencies. Research is focussing on application technology, timing of N uptake, internal cycling of N and methods for assessing the need for N application (e.g., using estimates of native soil N availability). Early diagnosis of bitter pit is recommended for guiding applications of Ca sprays. Iron deficiency and chlorosis is a major problem in pear, peach and kiwifruit grown in alkaline and calcareous soils and Fe chelates are usually applied annually to the soil or to the canopy. Current research is focused on agronomic means for controlling the problem and on developing rootstocks tolerant to Fe deficiency.

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David M. Hunter and Martin F. Gadsby

Mature seedling trees of pear (Pyrus communis and interspecific hybrids), and fruiting trees of peach and nectarine (Prunus persica), apricot (Prunus armeniaca), and pear were relocated during the dormant season using tree spades. During the growing season immediately following, some signs of drought stress were noticed but all trees grew well enough that they could be used as a source of budwood for limited propagation purposes. When drip irrigation was supplied, supplemented by overhead irrigation as required, normal growth and production resumed within two growing seasons of the move. Some tree losses (less than 10% of trees moved) were reported from one site where the soil type was Fox sand with very poor water holding capacity. These tree losses were attributed to an inadequate water supply to the root ball, even though the site was irrigated. Our experience has demonstrated the feasibility of relocating relatively large trees, which can be beneficial for germplasm conservation in a tree fruit breeding program. However, it is probably not economically viable to relocate such trees for commercial production.

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Peter L. Sholberg, Paul Randall, and Cheryl R. Hampson

Acetic acid (AA) fumigation of rootstocks and dormant shoots was explored as a method of eliminating plant pathogens from propagation material. Dormant shoots were tested in early winter to determine the rate of AA vapor that they could tolerate before being damaged. Apricot (Prunus armeniaca), apple (Malus ×domestica), and peach (Prunus persica) shoots collected from a single site in Dec. 1999 tolerated 30, 12, or 6 mg·L–1 AA, respectively. Vineland 3 (V3) and Malling-Merton 106 (MM.106) rootstock liners fumigated with 1 mg·L–1 AA were adequately surface-sterilized although the effect on growth was not recorded. A similar experiment with Malling 9 (M9) rootstocks showed that 12 mg·L–1 AA would eliminate most surface microorganisims from roots although it delayed shoot growth when the trees were planted. The higher 15 mg·L–1 rate delayed tree growth and appeared to kill some trees. The 12 mg·L–1 rate prevented growth of Erwinia amylovora and Pseudomonas syringae pv. syringae bacteria on shoots even when an enrichment technique was used to detect them. Finally, when 96 `Jonagold' apple shoots known to be infected by Podosphaera leucotricha were fumigated with AA in 2001, none developed powdery mildew, although 99% of the control shoots did. These promising results suggest that further research should be done toward adapting AA fumigation for use by commercial nurseries.

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Kitren G. Weis, Stephen M. Southwick, and George C. Martin

Gibberellic acid reduces return bloom in many fruit tree species. Reducing bloom may cut costs of hand thinning apricot, peach and plum fruit. Sprays of 250 ppm GA, during floral bud evocation (June 1993) resulted in bud death and abscission as determined by light microscopy sections in `Patterson' apricot (Prunus armeniaca L). GA treatment in May did not cause observable effects. August treatments, immediately prior to floral initiation, did not impede differentiation. Treatment of `Elegant Lady' peach (Prunus persica [L.] Batsch.) buds with 75-250 ppm GA, in late June, 1993 (evocation phase) did not have any discernable effects in that season with respect to abscission or differentiation. Treated peach buds differentiated simultaneously with untreated buds in early August. The patterns of response to GA treatment imply `windows of opportunity' with respect to effectiveness of GA treatments. The specific response suggests that apricot buds possess differing levels of sensitivity to GA treatment and probably reflect distinct phases in transition to flowering. In August buds were already `determined' and were in a potentially floral state that was irreversible.

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Charles G. Summers, Albert S. Newton Jr., and Kyle R. Hansen

Six table grape (Vitis vinifera L.) cultivars and 10 species of tree fruit were evaluated in cage tests to determine their susceptibility to colonization by the silverleaf whitefly (Bemisia argentifolii Bellows and Perring). The table grape cultivars Thompson Seedless, Perlette, Flame Seedless, Ruby Seedless, Christmas Rose, and Redglobe were all colonized. In a field nursery, with naturally occurring silverleaf whitefly populations, `Zinfandel', `Sirah', and `Chardonnay' were more heavily colonized than were `Merlot', `Thompson Seedless', or `Redglobe'. The tree crops `Kerman' pistachio (Pistacia vera L.), `Calimyrna' fig (Ficus carica L.), `Nonpareil' almond [Prunus dulcis (Mill.) D.A. Webb], and `Fuyu' persimmon (Diospyros kaki L.) were colonized in cage tests. Silverleaf whitefly failed to establish colonies on caged `O'Henry' peach [Prunus persica (L.) Batsch.], `Fantasia' nectarine [P. persica (L.) Batsch. var. nectarina (Ait.f.) Maxim.], `Casselman' plum (P. salicina Lindl.), `Tilton' apricot (P. armeniaca L.), `Granny Smith' apple (Malus domestica Borkh.), and `Hayward' kiwifruit [Actinidia delicoisa (A. Chevalier) C.F. Liang et A.R. Ferguson].

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Dario J. Chavez, Thomas G. Beckman, and José X. Chaparro

American plums), and Armeniaca . Recent phylogenetic studies supported the concept of Prunus as a monophyletic group (single genus). However, the genus Prunus contained several poorly supported subclades/terminals (subgenera/species) ( Bortiri et al

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Stephen M. Southwick and Kitren Glozer

Many commercially grown stone fruit including apricots (Prunus armeniaca L.), peaches and nectarines [P. persica (L.) Batsch], plums (P. salicina Lindl., P. domestica L.), prunes (P. domestica L.), and pluots (P. salicina × P. armeniaca) have a tendency to produce high numbers of flowers. These flowers often set and produce more fruit than trees can adequately size to meet market standards. When excessive fruit set occurs, removal of fruit by hand thinning is necessary in most Prunus L. species to ensure that remaining fruit attain marketable size and reduce biennial bearing. Over the years there have been numerous attempts to find chemical or physical techniques that would help to reduce the costs associated with and improve efficiencies of hand thinning, however, alternate strategies to hand thinning have not been widely adopted for stone fruit production. In the past 10 years, several chemical treatments have shown promise for reducing hand thinning needs in stone fruit. Management of flowering by chemically reducing the number of flowers has been particularly promising on stone fruit in the Sacramento and San Joaquin Valleys of California. Gibberellins (GAs) applied during May through July, have reduced flowering in the following season in many stone fruit cultivars without affecting percentage of flowers producing fruit. As a result, fruit numbers are reduced, the need for hand thinning is reduced and in some cases eliminated, and better quality fruit are produced. There are risks associated with reducing flower number before climatic conditions during bloom or final fruit set are known. However, given the changes in labor costs and market demands, the benefits may outweigh the risks. This paper reviews relevant literature on thinning of stone fruit by gibberellins, and summarizes research reports of fruit thinning with GAs conducted between 1987 and the present in California. The term thin or chemically thin with regard to the action of GA on floral buds is used in this paper, consistent with the literature, although the authors recognize that the action of GA is primarily to inhibit the initiation of floral apices, rather than reduce the number of preformed flowers. At relatively high concentrations, GA may also kill floral buds. Chemical names used: gibberellic acid, potassium gibberellate.