2005 Plant Management Network. This article is in the public domain. Accepted for publication 20 May 2005. Published 27 May 2005.
Strawberry Cultivars and Mycorrhizal Inoculants Evaluated in California Organic Production Fields Carolee T. Bull, USDA/ARS, 1636 E. Alisal St. Salinas, CA 93905; Joji Muramoto, Center for Agroecology and Sustainable Food Systems, University of California, Santa Cruz 95064; Steven T. Koike, University of California Cooperative Extension, Salinas 93901; Jim Leap and Carol Shennan, Center for Agroecology and Sustainable Food Systems, University of California, Santa Cruz 95064; and Polly Goldman, USDA/ARS, 1636 E. Alisal St. Salinas, CA 93905 Corresponding author: Carolee Bull.
[email protected] Bull, C. T., Muramoto, J., Koike, S. T., Leap, J., Shennan, C., and Goldman, P. 2005. Strawberry cultivars and mycorrhizal inoculants evaluated in California organic production fields. Online. Crop Management doi:10.1094/CM-2005-0527-02-RS.
Abstract Thirteen commercial strawberry cultivars were evaluated in side-by-side comparisons in five experiments in organic strawberry production fields in central California. Seven cultivars were common to all five experiments; six additional cultivars were included in one to four of the experiments. Of the seven cultivars that were evaluated in all five experiments, the largest market yield was consistently obtained from Aromas, Seascape, or Pacific. Preliminary analyses detected a strong positive correlation between total fruit yield and the nitrogen status of plants, suggesting characteristics in nitrogen uptake and metabolism may be a significant factor in determining yield of commercial strawberry cultivars tested in organic fields. None of the seven commercially prepared mycorrhizal inoculants tested resulted in an increased marketable fruit yield in organic or nonfumigated fields. However, the effects of the treatments on mycorrhizal colonization and total yield varied among experiments. For example, in one of six experiments, a commercial inoculant increased total yield over the nontreated control but did not influence marketable fruit yield.
Introduction Strawberry (Fragaria × ananassa Duch.) is an important crop to California’s agricultural economy, ranking 9th with an annual value of $862 million dollars (2). Production in central coastal California represents 39% of the state’s total strawberry production. The central coastal region is, likewise, the leader in organic strawberry production in the state and is responsible for 53% of the approximately $12 million California organic strawberry industry (1). Despite the economic benefit of organic strawberry production, little research has been conducted to optimize yields in this system. Currently, many high-yielding commercially available cultivars are adapted to the central coast climate (24). These cultivars were evaluated and selected for yield and fruit quality in chemically intensive conventional systems (15). These systems include annual soil fumigation with a mixture of methyl bromide and chloropicrin as well as regular applications of insecticides and fungicides (17). Because of the divergence in methods for pest management in conventional and organic production systems, a growing body of evidence suggests that cultivars bred specifically in organic systems are needed for optimization of these systems (14). In strawberries, for example, the use of soil fumigation can change the relative performance of strawberry cultivars (16,17). This is probably due to differences in the relative tolerance of the cultivars to soilborne plant pathogens (17). Some strawberry companies select for and evaluate new cultivars in organic
Crop Management
27 May 2005
and non-fumigated plots, in order to develop optimal cultivars for performance in these environments. However, these proprietary cultivars are unavailable to most California strawberry growers. Cultivars used in other strawberry growing regions have been evaluated in organic fields (5,21). However, evaluations of cultivar performance in organic production fields are rare and farmers are left to extrapolate from conventional systems. The first of two main objectives of this research was to compare the performance of strawberry cultivars in organic fields in central coastal California. As with other crops, strawberries have critical requirements in terms of nutrients. Nitrogen is essential in bud differentiation and flowering but excess nitrogen can lead to soft fruit, late ripening, increased foliage and disease pressure, and lower fruit yield (18). Phosphorus nutrition directly influences strawberry growth through energy transfer, and phytohormone balance indirectly. Adequate levels of nutrients in strawberry leaf tissues have been assessed in conventional systems in the U.S. (8,10,20,25) and in an organic system in Europe (6), but not in organic systems in California. There are data suggesting that total tissue nitrogen content appeared to better correlate with crop yield than nitrate content in organic systems (C. Shennan, unpublished data), which may relate to higher availability of ammonium than nitrate in these systems (7). Here we present preliminary data on the nutrient status of strawberries grown in organic systems in California. The impacts of management practices other than soil fumigation may also be dependant on the cultivar chosen for production. For example, the benefits of microbial inoculants may be expressed in a host-specific manner (23). In strawberry, inoculation with and root colonization by arbuscular mycorrhizae fungi (AMF) have been shown to increase plant growth, yield, and nutrition (9,12,22), as well as prevent soilborne diseases (19). However, the benefit is dependant on the strawberry cultivar as well as the AMF isolate used (11). Few studies have evaluated the benefit of individual AMF inoculants in organic production systems (26). The second objective of this work was to evaluate the impact of several AMF inoculants on strawberries grown in organic fields. Evaluation of Commercially Available Strawberry Cultivars in Organic Production Fields We evaluated the performance of thirteen strawberry cultivars over a series of five experiments. Strawberry cultivars were chosen by organic growers and researchers based on commercial or scientific interest, and were evaluated at three and two certified organic farms in 2000 and 2001, respectively. Farms were located in Monterey, San Benito, and Santa Cruz counties. Strawberry production practices varied among sites according to the growers’ standard practices. Seven cultivars (Aromas, Diamante, Douglas, Pacific, Pajaro, Seascape, and Selva) were evaluated in all five experiments. Six additional cultivars were tested in some but not all experiments depending on the growers’ interest and the results of the first season. Specifically, Carlsbad was included in four experiments, Hecker and Sequoia were included in three experiments in 2000, while Chandler and Oso Grande were each included in two experiments in 2001. The cultivar Irvine was evaluated in a single experiment in 2000. Transplants were obtained from commercial nurseries and were planted between 1 October and 20 November. Planting occurred in stages with each cultivar planted according to nursery recommendations for optimum chilling. Mycorrhizal colonization of transplants was evaluated on 10 plants per cultivar at the time of planting. At each site, each cultivar was planted in four replicate, 60-plant plots. Plots were established and evaluated as a split plot design, with half of the plots treated with Endomycorrhizal Inoculant (Bio/Organics, Inc., Camarillo, CA) at the time of planting. Each root system was moistened and dusted with approximately 0.91 g per plant of the inoculant as recommended by the manufacturer. Yield was evaluated from 20 plants in each subplot, once or twice weekly, using a commercial or trained harvesting crew and standard criteria for market quality fruit. Market quality fruit and culls not suitable for fresh market were weighed separately; combined weights were reported as total yield. Total
Crop Management
27 May 2005
yield represents the potential income gained through sales of fresh market quality fruit as well as income from processed fruit. An analysis of variance (ANOVA) procedure was conducted and means were separated by Tukey’s HSD using JMP 4.0.4 (SAS Institute, Cary, NC). Evaluation of Plant Nutrient Status Because of the significant differences seen in cultivar performance and the lack of influence of mycorrhizal inoculants in 2000, we evaluated the nutrient status of five cultivars and the soils in which they were grown in 2001. Leaves were sampled three times between March 26 and July 23, which represented the period between initial flowering and late harvest. Following Ulrich et al. (25), 10 to 15 young-mature leaves were sampled from each plot to make a composite sample. Petioles were detached from leaf blades. Both were dried at 60 to 70°C for 48 h. After grinding, samples were analyzed for total nitrogen (T-N) and total phosphorus (T-P) in leaf blades and nitrate-N (NO3-N) in petioles. Nutrient analyses were performed by the Department of Agriculture and Natural Resources Analytical Laboratory, University of California, Davis, CA. Results were expressed on a dry weight basis and statistically analyzed using an ANOVA. Leaf blade T-N and petiole NO3-N contents were compared to yield using correlation analysis. For correlation analysis, we used total yield rather than marketable yield because "marketable" criteria differed among growers. We chose the best fit and the simplest polynomial regression to describe the relationships. Evaluation of Mycorrhizal Colonization For all experiments, mycorrhizal colonization of transplants was evaluated on 10 plants per cultivar at the time of planting. Throughout the season, colonization was measured on two plants per plot at each sampling time. Roots were gently washed under running water to remove soil and then were cut into 1-cm sections. Roots were cleared and hyphae were stained following published methods (13). In brief, roots were cleared by treatment in 10% KOH for 2 h at 90°C followed by treatment with 3% H2O2 for 20 min at room temperature. Roots were then acidified in concentrated HCl for at least an hour but commonly overnight. After removing the acid, roots were stained for 20 min at 60°C using trypan blue in lactoglycerin. The number of root sections colonized by AMF was determined microscopically using a line intercept method. The percentage of root sections colonized was calculated from the numbers of root sections examined and colonized. Because the percent data were not normally distributed, the data were arcsin transformed prior to conducting analysis of variance procedures and separation of means with Tukey’s HSD; however, the untransformed data are reported. Data were analyzed using JMP 4.0.4. Evaluation of Commercially Available Mycorrhizal Inoculants on Root Colonization and Yield Six experiments were conducted to evaluate the ability of commercially available mycorrhizal inoculants to increase yield and AMF colonization of roots. In all six experiments we tested five inoculants: Endomycorrhizal Inoculant (Bio/Organics, Inc., Camarillo, CA), Ascend PA (BioScientific, Inc. Avondale, AZ), EndoNet (Reforestation Technologies International, Salinas, CA), Endos (AgBio, Inc. Westminster, CO), and Strawberry Saver (Plant Health Care, Inc., Pittsburgh, PA). In five of the experiments we tested two additional inoculants: AMFinoc (Microbio, Ltd. Royston Herts, UK) and BioAMF (EcoLife, Corp., Moorpark, CA). Inoculants were applied as root applications or to the planting hole at the time of planting at recommended rates: Endomycorrhizal Inoculant, 0.91 g per plant; Ascend PA, 2.2 ml per plant; EndoNet, 0.5 tsp per plant (approximately 2.5 g per plant); Endos, 0.25 tsp per plant (approximately 1.25 g per plant); Strawberry Saver, 0.5 tsp per plant (approximately 2.5 g per plant); VAMinoc, 1 g per plant; and BioVAM 1 g per plant. Four experiments were conducted in organic fields and two were in a conventional field that had
Crop Management
27 May 2005
not been fumigated for two years prior to the experiments. For all experiments, mycorrhizal colonization was estimated from two plants per plot at three times during the growing season as described above. Each experiment was designed and analyzed as a randomized complete block design with blocking over beds using four replicate blocks. Each plot consisted of 30 plants (Aromas or Diamante), 20 of which were monitored for yield. An analysis of variance was conducted and means were separated by Tukey’s HSD using JMP 4.0.4 as previously described. Influence of Cultivar on Yield in Organic Production Fields In these experiments, inoculation with AMF did not provide any yield benefit (P = 0.20 to 0.76). Because no differences were detected between yields in inoculated and nontreated plots or in the variances of the data at a P = 0.05 significance level, the yield data within each experiment were pooled and reanalyzed. Of the seven cultivars that were tested in all five experiments, the cultivars Pacific, Aromas, and Seascape performed the best. In each of the experiments one of these cultivars had the numerically greatest market and total yields of those seven cultivars (Fig. 1). In four of five experiments, yields from at least one of these three cultivars were significantly greater than the yields of either Diamante or Selva, which were the most widely planted cultivars during the time of the study. For example, market yields of Seascape were significantly greater than Diamante and Selva in two experiments (Farm B and C in 2000). Likewise, market yields for Aromas and Pacific were significantly greater than those of Selva in two experiments (Farm B in 2000 and Farm A in 2001) and Diamante in one experiment (Farm B in 2000). Cultivar Irvine may be a promising candidate for organic production and needs to be evaluated further. In the one experiment in which it was tested, Irvine had the highest total yield.
Crop Management
27 May 2005
Fig. 1. Market and total yield of strawberry cultivars grown under organic management in 2000 and 2001. Bars representing total or market yield which have the same capital or lower case letters, respectively, are not significantly different (P = 0.05) according to Tukey's HSD. Each bar is the mean of four replicates. Market and total yield were analyzed separately. To convert yield into crates (12 lb) per acre, multiply values given by 0.18.
Crop Management
27 May 2005
Influence of Cultivar on Plant Nutrient Status Leaf blades collected in March contained 26 to 35 mg/g T-N that decreased to 17 to 28 mg/g in May and July in both farms (Fig. 2). At each growth stage, leaf T-N differed significantly by cultivar (P < 0.04), but neither the mycorrhizal treatment (P > 0.2), nor the interaction (P > 0.4) was significant. Regardless of the farm, the cultivars were ranked in the following order of decreasing mean leaf T-N across all sampling dates: Seascape, Aromas, Diamante, Chandler, Pajaro.
Fig. 2. Total N content in leaf blades of five strawberry cultivars grown under organic management. Red horizontal line indicates tentative critical level (28 mg/g) by Ulrich et al. (25). Bars with the same letter within each date are not significantly different at P = 0.05, Tukey's HSD. Each bar is the mean of eight replicates.
On Farm A, leaf T-N did not differ significantly between Seascape and Aromas on two of the three sampling dates. Additionally, on two of the three sampling dates these two cultivars had significantly higher leaf T-N content than did the other cultivars. Similarly, on Farm B, leaf T-N did not differ between Seascape and Aromas on two of the three sampling dates. However, leaf T-N in Seascape was significantly higher than in Chandler, Diamante, and Pajaro on two of three sampling dates while the leaf T-N for Aromas did not differ from Chandler and Diamante on any date. In contrast, leaf T-N in Pajaro was significantly lower than in the other four cultivars on two of the three sampling dates. Petiole NO3-N was high in March (0.67 to 3.96 mg/g), decreased in May, and remained low in July, regardless of farm or cultivar. Again, at each growth stage a significant difference in NO3-N was found among cultivars, but there were no
Crop Management
27 May 2005
differences between mycorrhizal treatments. Regardless of farm, cultivars were ranked in the following order of decreasing petiole NO3-N: Aromas, Seascape, Chandler, Diamante, Pajaro. Pajaro consistently showed the lowest NO3-N and was significantly lower than Diamante on both farms. Positive correlations between N status, as measured by either leaf blade T-N or petiole NO3-N, and total yields were found across cultivars in each site in different growth stages. The most significant correlation was observed between leaf blade T-N in the early flowering stage and total yield (P < 0.001). This correlation was consistent across farms, whereas correlations of NO3-N in petioles in the same stage and the total yields differed between farms (Fig. 3).
Fig. 3. Correlations between total yield and N status in early flowering stage (March) of organic strawberries. Total N in leaf blades (top) and NO3-N in petioles (bottom) are used as indicators of N status.
Cultivars: = Aromas, = Seascape, = Chandler, = Diamante, and = Pajaro. Symbols in red = Farm A; black = Farm B. Solid symbol = with mycorrhizal inoculant; and open = without mycorrhizal inoculant. Solid line = linear regression curve for both Farm A and B (with a regression formula and a regression coefficient R2); - - - - = linear regression curve for Farm A, and - - - - linear regression curve for Farm B. *** Significant at P ≤ 0.001. Each point is the mean of four replicates.
Leaf blade T-P ranged from 2.5 mg/g (Chandler in July on Farm B) to 4.8 mg/g (Aromas in March on Farm A) across all sampling stages and treatments. At both farms, cultivar treatment was significant, but mycorrhizae treatment was not. Among cultivars tested, Aromas showed the highest leaf T-P at both farms (data not shown).
Crop Management
27 May 2005
Influence of Cultivar on Mycorrhizal Colonization Depending on the year, the impact of inoculation on strawberry root colonization differed. The transplants used in the 2000 season had low levels of colonization that did not differ among cultivars (P = 0.1). However, colonization by AMF increased significantly (P < 0.001) during the growing season. For example, Farm A in 2000 had mean root colonization levels of 27.2, 35.6, and 51.8% in February, April, and June, respectively. In 2000, regardless of the farm or cultivar, inoculation had no effect on the level of subsequent root colonization (P > 0.2). In 2001, Aromas and Pajaro transplants had the highest and lowest levels of colonization, respectively (P = 0.001), but these differences were not detected after planting in organic fields (P > 0.6). Inoculation resulted in significantly greater colonization at both locations when data for all cultivars were pooled prior to analysis (P < 0.008). However, this increase in colonization did not correspond to an increase in yield (P > 0.2). Influence of Commercially Available Mycorrhizal Inoculants on Yield and Root Colonization in Organic Production Fields In these experiments, no differences were detected in AMF colonization between inoculated and noninoculated plants (cultivars Aromas or Diamante), regardless of the commercial inoculum used. Levels of colonization on plants treated did not differ regardless of inoculant (P > 0.21; data not shown). Similar to cultivar studies, colonization levels increased over time in all experiments. For example on one farm in 2000, colonization means for all treatments ranged from 11.8 to 17.4%, 37.5 to 41.0%, and 46.1 to 56.8% for samples analyzed in February, April and June, respectively. No differences in market yield were detected between inoculated and noninoculated plants across experiments (data not shown). In one experiment (Farm A in 2000), plants treated with Agbio had significantly (P < 0.05) higher total yield (32,727 kg/ha) than noninoculated plants (27,272 kg/ha). However, in the five other experiments in which this product was tested, no difference in yield was detected compared to the noninoculated control (data not shown). Conclusions We report the first side-by-side comparisons of performance of California strawberry cultivars under organic management. Cultivars Aromas, Pacific, and Seascape were often the top performing cultivars, although Irvine may warrant further testing. This is in contrast to Diamante and Selva, which held the largest segment of the strawberry market during the time of this research. Differences in yield may be due in part to resistance to soilborne pathogens (17). For example, Aromas is more tolerant of Phytophthora spp. that attack strawberry than are Diamante and Pajaro (3). Preliminary analyses detected a strong correlation between yield and the nitrogen status of plants. Interestingly, one of the highest yielding cultivars, Aromas, also had the highest nitrogen status. The role of nutrient status and yield needs to be investigated further. A hydroponic study using a single cultivar (Oso Grande) suggested the inability of strawberry to increase growth and fruit yield in response to increasing nitrate concentration is not due to limitations in nitrate uptake rates, but rather to limitations in nitrate reduction and/or assimilation in plants (4). However, the characteristics in nitrate uptake and metabolism may vary among cultivars (6). Further, the present study and other data (18) suggest that the nitrogen status of the plant may be a significant factor in determining yield. Understanding the environmental and genetic factors that influence the relationship between nitrogen status and yield has the potential to influence breeding for this production system. In these experiments, none of the tested AMF inoculants influenced market yield and only once, increased total yield. Additionally, the inoculants only sporadically increased AMF colonization of roots. Although these inoculants were not effective under the conditions tested here, this does not rule out their usefulness with other cultivars or in other locations. However, these data suggest that naturally occurring inoculum from the field soil and/or transplants were sufficient. Organic production fields might be the source for the inoculum
Crop Management
27 May 2005
colonizing noninoculated plants as these fields are not fumigated with methyl bromide and chloropicrin as are conventional production fields. Of the transplants evaluated in November 2000 (plants for the 2001 season), approximately 91% were colonized to some degree (0.5 to 39% root length) by AMF. Preliminary data indicate that AMF inoculation did not significantly influence the nutrient and phosphorous status of the plants. Additionally, according to the conventional critical standard, the leaf T-P was sufficient throughout the growth period for all plants (data not shown). These data might explain the ineffectiveness of AMF inoculum in these experiments. This also leads us to recommend that growers consider the P level available in the soil or in applied nutrients prior to considering applications of mycorrhizal inoculants. Acknowledgments and Disclaimer This research was conducted with funding from CDFA, UC-SAREP, USDA/CSREES, The Alfred Heller Agroecology Endowed Chair at UCSC, The Halliday Foundation, and OFRF. We gratefully acknowledge the help of the organic growers who donated their resources to help this project. Addtionally we acknowledge the help of Joel Stryker, Adria Bordas, Diana Henderson, Steve Boutry, Rose Vega, Sarah Wiener Boone, Daniel Evan Hermstad, David Mendoza, Bree Eagle, and Jonah Luce Landor-Yamagata as well as the UCSC CASFS Apprenticeship Program and apprentices. We give special thanks to Jack Pinkerton and Kendra Baumgartner for their critical review of this manuscript and to Stephen R. Gliessman for his help with the nutrient analysis portion of this project. The mention of a trade name, proprietary product, or vendor does not constitute an endorsement, guarantee or warranty by the United States Department of Agriculture and does not imply its approval or the exclusion of these or other products that may be suitable. Literature Cited 1. Anonymous. 2002. 2002 State and County, Organic Crop Value and Acreage Reports. California Department of Food and Agriculture, Sacramento, CA. 2. Anonymous. 2003. Summary of County Agricultural Commissioners’ Reports, 20012002. California Agricultural Statistics Service, Sacramento, CA. 3. Browne, G. T., Becherer, H. E., Vazquez, M. R., McGlaughlin, S. A. Wakeman, R. J., Winterbottom, C. Q., Duniway, J. M., and Fennimore, S. A. 2001. Outlook for managing Phytophthora diseases on California strawberries without methyl bromide. Abstract 29 in: Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. 4. Darnell, R. L., and Stutte, G. W. 2001. Nitrate concentration effects on NO3-N uptake and reduction, growth, and fruit yield in strawberry. J. Amer. Soc. Hort. Sci. 126:560-563. 5. Daugaard, H., and Lindhard, H. 2000. Strawberry cultivars for organic production. Gartenbauwissenschaft 65:213-217. 6. Daugaard, H. 2001. Nutritional status of strawberry cultivars in organic production. J. Plant Nutr. 24:1337-1346. 7. Drinkwater, L. E., Letourneau, D. K., Workneh, F., van Bruggen, A. H. C., and Shennan, C. 1995. Fundamental differences in organic and conventional agroecosystems in California. Ecolog. Appl. 5:1098-1112. 8. Hochmuth, G. J., and Albregts, E. E. 2003. Fertilization of strawberries in Florida. Pub. CIR1141, Flor. Cooperative Ext. Serv. Spec. Ser., Gainesville, Fl. 9. Hughes, M., Martin, L. W., and Breen, P. J. 1978. Mycorrhizal influence on the nutrition of strawberries. J. Amer. Soc. Hort. Sci. 103:179-181. 10. Jones, J. B., Wolf, B., and Mills, H. A. 1991. Plant analysis handbook: a practical sampling, preparation, analysis and interpretation guide. Micro-Macro, Athens, GA. 11. Khanizadeh, S., Hamel, C., Kianmehr, H., Buszard, D., and Smith, D. L. 1995. Effect of three vesicular-arbuscular mycorrhizae species and phosphorus on reproductive and vegetative growth of three strawberry cultivars. J. Plant Nutr. 18:1073-1079.
Crop Management
27 May 2005
12. Kiernan, J. M., Hendrix, J. W., Stoltz, L. P., and Maronek, D. M. 1984. Characterization of strawberry plants produced by tissue culture and infected with specific mycorrhizal fungi. HortScience 19:883-885. 13. Kormanik, P. P., and McGraw, A. C. 1982. Quantification of vesicular-arbuscular mycorrhizae in plant roots. Pages 37-45 in: Methods and Principles of Mycorrhizal Research. N. C. Schenck, ed. American Phytopathological Society, St. Paul, MN. 14. Lammerts van Bueren, E. T., Hulscher, M., Jongerden, J., Haring, M., Hoogendoorn, J., van Mansvelt, J. D., Ruivenkamp, G. T. P. 1998. Sustainable organic plant breeding. Pub. no. G18, Louis Bolk Inst., Driebergen, The Netherlands. 15. Larson, K. D., and Shaw, D. V. 1995. Relative performance of strawberry genotypes on fumigated and nonfumigated soils. J. Amer. Soc. Hort. Sci. 120:274-277. 16. Martin, F. N. 2001. Management of pathogens associated with black root rot of strawberry. Abstract 46 in: Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions. 17. Martin, F. N. and Bull, C. T. 2002. Biological approaches for control of root pathogens of strawberry. Phytopathology 92:1356-1362. 18. May, G., and Pritts, M. 1990. Strawberry nutrition. Adv. Strawberry Prod. 9:10-24. 19. Norman, J. R., Atkinson, D., and Hooker, J. E. 1996. Arbuscular mycorrhizal fungal-induced alteration to root architecture in strawberry and induced resistance to the root pathogen Phytophthora fragariae. Plant Soil 185:191-198. 20. Pritts, M., and Handley, D., eds. 1998. Strawberry production guide for the Northeast, Midwest, and Eastern Canada. Nat. Resour., Agric., and Engin. Serv., Ithaca, NY. 21. Rhainds, M. P., Kovach, J., and English-Loeb, G. 2002. Impact of strawberry cultivar and incidence of pests on yield and profitability of strawberries under conventional and organic management systems. Biol. Agric. Hort. 19:333-353. 22. Robertson, W. J., Boyle, C. D., and Brown, H. L. 1988. Endomycorrhizal status of certified strawberry nursery stock. J. Amer. Soc. Hort. Sci. 113:525-529. 23. Smith, K. P., and Goodman, R. M. 1999. Host variation for interactions with beneficial plant-associated microbes. Ann. Rev. Phytopathol. 37:473-491. 24. Strand, L. L. 1994. Integrated pest management for strawberries. University of California Statewide IPM Project, DANR Publication 3351. 25. Ulrich, A., Mostafa, M. A. E., and Allen, W. W. 1980. Strawberry deficiency symptoms: A visual and plant analysis guide to fertilization. Agric. Exp. Sta., Univ. Calif., DANR Bull. 1917. 26. Werner, M. R., Kluson, R. A., and Gliessman, S. R. 1990. Colonization of strawberry roots by VA mycorrhizal fungi in agroecosystems under conventional and transitional organic management. Biol. Agric. Hort. 7:139-151.
Crop Management
27 May 2005
