2:00 p.m. Regulation of Crop Load and Effects on Fruit Growth and Quality of Apple. Jens Wunsche, Universitat Hohenheim, Germany

Transcription

1 Great Lakes Fruit, Vegetable & Farm Market EXPO December 4-6, 27 DeVo Place Convention Center, Grand Rapids, MI Apple II Wednesday afternoon 2: pm Where: Ballroom D Recertification credits: 1 (1C, PRIV CORE) CCA Credits: PM(.5) CM(1.5) Moderator: Steve Thome, MSHS Board, Comstock Park, MI 2: p.m. Regulation of Crop Load and Effects on Fruit Growth and Quality of Apple Jens Wunsche, Universitat Hohenheim, Germany 2:3 p.m. Fireblight Strategies for 28 George Sundin, Plant Pathology Dept., MSU 3: p.m. Extreme Long-term Storage of Apples with 1-MCP - Breaking New Ground Randy Beaudry, Horticulture Dept., MSU 3:3 p.m. Michigan Apples: Shaping the Future Denise Yockey, Michigan Apple Committee, Dewitt, MI

2 Regulation of crop load and effects on fruit growth and quality of apple Jens N. Wünsche University of Hohenheim, Department of Crop Physiology, Fruit Sciences, Emil-Wolff-Str. 25 D Stuttgart, Germany Tel.: 49-() , Fax: 49-() , Introduction Crop load is a key cultural component of final fruit quality and thus of managing the risks associated with achieving commercial requirements for fruit size, consumer-based quality attributes, and freedom from disorders. In this regard, information on crop manipulation and effects of harvest time and fruit maturity are of particular importance to growers in enhancing the proportion of the crop achieving desired qualities. The presence or absence of fruit on perennial fruit trees and vines has a major effect on the photosynthetic performance and growth response of these plants (see reviews by Flore and Lakso 1989; Forshey and Elfving 1989; Jackson 1989; Byers 23; Wünsche and Ferguson 25). Effects of time and severity of fruit(let) thinning or crop load adjustment, and concomitant alteration of fruit to leaf ratios, have been extensively studied in a desire to achieve high orchard productivity without compromising potential fruit size and quality or return bloom. Optimized crop loads for a given cultivar and production system in a particular environment can give enhanced financial returns to growers. Most research has focused on the impact of fruit load on tree and fruit physiology; surprisingly little has been directed specifically at the impact of cropping on final fruit quality in terms of postharvest storage, shelf life, and consumer preference. Compared to other preharvest factors such as pollination, mineral nutrition, and light and temperature environment, variability in crop load may have the greatest impact on both fruit quality and tree physiology. Tree Growth Response Vegetative growth with concomitant leaf and whole-canopy photosynthesis is very dependent upon time and severity of flower/fruitlet removal; the later the thinning occurs the greater the effect on depressing photosynthesis, since proportionally fewer actively growing sinks are available for alternative carbohydrate movement. Early in the growing season, leaf photosynthesis is not affected by crop load, presumably due to the compensatory response of trees with lower fruit numbers to maintain sink strength by significantly increasing vegetative growth (shoot length, leaf area, and trunk circumference). The significant decline of leaf photosynthesis in response to reduced crop load in mid-season is presumably due to the cessation of shoot growth and development and hence fewer alternate sinks and a lower carbohydrate requirement at that time. On the other hand, it has been found that leaves with down-regulation of photosynthesis can be rejuvenated to relatively high photosynthetic capacity in late season when source-sink ratios recover because more carbohydrates are required for flower bud development, root growth, and re-filling storage pools in root and stem tissue. The effect of fruiting on leaf carbon uptake and water loss is, however, not consistent for all fruit crops; e.g., similar leaf photosynthetic rates were reported for fruiting vs. nonfruiting mandarin, sweet cherry, grape and strawberries. Some results from apple also indicate surprisingly little effect of fruit on photosynthesis. In contrast, leaf photosynthesis was enhanced, at least during maximum seasonal carbohydrate demands of fruit sinks, in peach. Palmer (1992) found significant differences in leaf carbon assimilation rates among apple cropping levels only during maximum fruit dry weight increase. Under the prevailing climatic conditions of New Zealand, Palmer et al. (1997) showed that

3 2 leaf assimilation rate was positively and curvilinearly related to crop load (Fig. 1A). Moreover, Wünsche et al. (2) provided evidence that mid- to late-season whole canopy net NCER per unit area of leaf increased linearly with increasing fruit load per TCA (Fig. 1B), whereas absolute whole canopy photosynthesis only revealed differences between fruiting and nonfruiting trees. Total water consumption is substantially higher in fruiting trees than deblossomed trees despite a strong reduction of whole-canopy leaf area in the presence of fruit (Fig. 2A; Lenz, 1986). ) -1 Leaf assimilation (µmol m -2 s A Canopy NCER (µmol. m -2. s -1 ) dafb B Number of fruit/leaf area (fruit m -2 ) Crop load (kg fruit/ cm 2 TCA) Figure 1 - Effect of crop load on mean leaf carbon assimilation rate (A) (Palmer et al. 1997) and whole canopy net carbon exchange per unit area of leaf (B) (Wünsche et al. 2) of Braeburn / M.26 apple trees. Lenz (1986) convincingly showed that dry matter of vegetative organs was significantly reduced in fruiting Golden Delicious apple trees on M.9 rootstock when compared to deblossomed trees over a 2-year observation period. Dry matter was reduced by 57% in leaves, 52% in shoots, and 69% in roots (Fig. 2B). Despite the lower dry matter in vegetative organs, total dry matter production of the fruiting trees was approximately 65% greater, due to the 66% of total dry matter accumulated in fruit. Greater amounts of carbohydrates, in particular starch, have been shown to be retained in leaves on trees with reduced crop loads (Fig. 2C, Lenz 1986) and in some cases leaf starch concentration has been negatively correlated with crop load. A B C 4 LSD 5% 36 Total water consumption / tree (L) LSD 5% -Fr. + Fr. Dry matter / tree (g) Fr. Fruit Leaves Shoots Trunk Roots + Fr. Starch (% of dry matter) LSD 5% -Fr. + Fr. Figure 2 - Effects of fruiting on tree water consumption (A), dry matter partitioning into plant organs (B), and leaf starch accumulation (C). (Redrawn from Lenz 1986).

4 3 The stomatal regulation of the photosynthetic process is important, and is either controlled by xylemderived phytohormones and/or due to a build up/degradation of starch grains within leaf chloroplasts and hence varying exposure levels of the thylakoids depending on sink-source ratios. Fruit Growth and Quality Response The impact of crop load on fruit quality is not straightforward. With medium to high cropping loads, vegetative growth and leaf area are reduced; yet total dry matter production may be increased, suggesting higher leaf photosynthetic efficiency. Thus fruit may retain sufficient dry matter, soluble solids, firmness, and Ca contents to provide high quality after storage. Fruit from light cropping trees mature earlier, are larger, and tend to have higher background color, red blush intensity, soluble solids, titratable acidity, and firmness. However, these qualities do not always transfer into quality after storage, where low Ca and advanced maturity increase susceptibility to disorders. Fruit from light cropping trees not only have more bitter pit but may also show a higher degree of radial water core, moldy core, core rots, internal browning, vascular browning, and cracking. There is insufficient research into the specific effects of crop load on quality, particularly on the physiological consequences of varying photosynthetic dynamics, and on the impacts on fruit growth over the season. The challenge for both the research scientist and the grower still remains: to balance carbohydrate supply and demand, and hence to manipulate fruit properties, by adjusting the crop load of trees early in the season so that an optimum distribution of commercially acceptable fruit can be produced. Major Conclusions Research on crop load has revealed a powerful interplay between fruit development and shoot growth and photosynthesis (Fig. 3). Tree and fruit response to crop load in apple Vegetative response As crop load increases, there is a decrease in: Leaf area, with heavier & thicker leaves Shoot growth, seen in shoot number and/or mean shoot length Trunk and root growth although proportionally the least increment Dry matter Reproductive response A heavy crop results in: Fewer flowers, and lower flower, quality Reduced fruit set, growth rate, size/weight and dry matter Retarded maturity, seen in colour, SSC, TA and firmness Less storage disorders such as bitter pit, watercore, and internal breakdown Physiological explanation Lesser amounts of assimilates / dry matter partitioned into vegetative sinks due to strong fruit sinks Hormonal regulation Carbohydrate supply limitation due to low leaf : fruit ratio High Ca:K conc. ratio Figure 3 - Summary of plant responses to crop load in apple. Generalized effects of cropping can vary dependent upon time and severity of flower/fruitlet removal, environmental conditions and sourcesink interactions.

5 4 References Byers, R. E. 23. Flowering and fruit thinning and vegetative:fruiting balance. p In: D. C. Ferree and I. J. Warrington (eds.), Apples: Botany, Production and Uses. CABI Publ., Cambridge, MA. Flore, J. A., and A. N. Lakso Enviromental and physiological regulation of photosynthesis in fruit crops. Hort. Rev. 11: Forshey, C. G., and D. C. Elfving The relationship between vegetative growth and fruiting in apple trees. Hort. Rev. 11: Jackson, J. E The manipulation of fruiting. p In: C. J. Wright (ed.), Manipulation of fruiting. Butterworths, London. Lenz, F Fruit effects on transpiration and dry matter production in apples. p In: A. N. Lakso and F. Lenz (eds.), The regulation of photosynthesis in fruit trees. Symp. Proc. Publ., N.Y. State Agr. Expt. Sta., Geneva, N.Y. Palmer, J. W Effects of varying crop load on photosynthesis, dry matter production and partitioning of 'Crispin'/M.27 apple trees. Tree Physiol. 11: Palmer, J. W., R. Giuliani, and H. M. Adams Effect of crop load on fruiting and leaf photosynthesis of 'Braeburn'/M.26 apple trees. Tree Physiol. 17: Wünsche, J. N., J. W. Palmer, and D. H. Greer. 2. Effects of crop load on fruiting and gasexchange characteristics of 'Braeburn'/M.26 apple trees at full canopy. J. Am. Soc. Hort. Sci. 125: Wünsche, J.N. and I.B. Ferguson. 25. Crop load interactions in apple. Horticultural Reviews. Vol. 31,

6 Fire Blight Strategies for 28 George W. Sundin Michigan State University Fire blight, caused by the bacterium Erwinia amylovora, seriously limits apple production in Michigan. This disease is particularly difficult to manage, and the situation is exacerbated by three major problems: (i) most of the popular apple cultivars selected by growers are either rated as susceptible or highly susceptible to fire blight; (ii) many of the popular dwarfing rootstocks utilized in Michigan are also highly susceptible to fire blight; and (iii) the few chemical control options available are further limited by the development of streptomycin resistance in some areas of Michigan. The antibiotic streptomycin is the most effective control compound available for limiting blossom populations of the fire blight bacterium. Reducing blossom populations and thereby reducing fire blight inoculum is critical to the control of the blossom blight phase of the disease and also in reducing disease pressure for the shoot blight phase of the disease. The shoot blight phase is extremely difficult to manage because this is a systemic disease problem and the antibiotic control options are non-systemic chemicals. Unfortunately, due to the reliance of the apple industry on streptomycin for fire blight control, streptomycin resistance has developed in populations of the fire blight bacterium in Southwest Michigan in the mid-199's, and we have detected streptomycin-resistant fire blight bacteria in Southwest Michigan, the Fruit Ridge area, and in Oceana county in the last few years. In orchards with streptomycin resistance problems, fire blight management will depend on alternative antibiotics including oxytetracycline (Mycoshield). In addition, we hope to obtain a Section 18 specific exemption for a second alternative such as gentamicin (Agry-Gent) or kasugamycin (Kasumin). Elimination of inoculum by pruning will become more and more important as resistance problems continue. The use of other measures such as prohexadione-calcium (Apogee) for shoot blight control is also critical. Finally, biological control options such as Serenade MAX provide a level of blossom blight control that is good, but not comparable with that provided by antibiotics. In orchards without streptomycin resistance, fire blight management strategies for 28 involve using streptomycin for blossom blight control as this is the best material available and our work suggests that the current resistance problem is due to the movement of resistant strains between orchards and not of the generation of new strains in orchards.