Final Report for LNE97-080
[Note to online version: This online report consists of excerpts from a paper, “Impact of strawberry cultivar and incidence of pests on yield and economic profitability of strawberries cultvated using conventional and organic management practices,” which includes additional sections, tables, figures, and references that could not be included here. The regional SARE office will mail a hard copy of the paper at your request. Just contact Northeast SARE at (802)-656-0471 or email@example.com.]
Yield, incidence of pests, and profitability of strawberries under conventional and organic management systems were measured through four fruiting seasons, using three cultivars widely grown in the northeastern United States (dayneutrals [Tribute or Tristar], Earliglow and Honeoye). Fruits were more abundant in conventional than in organic plots, but the average weight of fruit was not affected by management system. Of all strawberry cultivars tested in this study, Honeoye was the most productive, both in terms of number and weight of harvested fruits. Incidence of tarnished plant bug damage by was much higher than for grey mould or slugs. Density of nymphs and proportion of fruits damaged by plant bugs were higher in organic than in conventional plots, and for Honeoye than dayneutrals or Earliglow. In contrast, management system and strawberry cultivar did not affect to a great extent the incidence of damage by grey mould or slugs. Productivity of strawberry plants was negatively correlated with incidence of damage by plant bugs, and positively with incidence of grey mould. Weed biomass was greater in organic than in conventional plots. Economic analysis indicated that a lack of reliable, effective measures for managing pests of strawberries without synthetic pesticides, especially weeds and plant bugs, may severely constrain yield and profitability of organic strawberries in the northeastern United States. Due to its high productivity and low susceptibility to plant bugs, Honeoye was by far the most profitable cultivar in this study, and appeared highly suitable for organic management.
The cultivated strawberry, Fragaria ananassa Duch., is grown as a perennial crop in the Northeast United States. Fruits are first harvested one year after strawberry plants have been established, and plants are typically kept in production for 4 years. Although the cost of establishing a strawberry farm is relatively high, growers typically realize a positive cash flow one year after strawberry plants have been established. The production cycle of strawberry plants is limited to 4 to 5 years, because yield steadily declines during successive years (Pritts & Handley, 1998).
Organic cultivation of strawberries can be a profitable operation. Although yield of organic strawberries tends to be lower than yield of strawberries cultivated using conventional management practices, differences are generally offset by the superior retail price of organic strawberries (Pritts & Handley, 1998; Gliessman et al., 1996a, 1996b). One factor that may constrain the profitability of cultivating organic strawberries is a lack of reliable, effective strategies for managing pests of strawberry without using synthetic pesticides. In the Northeast United States, the major pests of strawberry include weeds, tarnished plant bugs [Lygus lineolaris Palisot de Beauvais], gray mold [Botritis cinerea Pers.] and slugs.
Damage by tarnished plant bug (TPB) is caused by nymphs or adults feeding on achenes of developing strawberry fruits, which interferes with translocation of auxin and results in deformed fruits (Handley & Pollard, 1993). TPB causes direct economic loss to strawberry growers because damaged fruits are generally unmarketable (Schaeffers, 1980 1 Mailloux & Bostanian. 1988). In commercial strawberry farms, control of TPB is attained by applying insecticide after population density exceeds 0.25 nymphs per flower cluster (Mailloux & Bostanian, 1988). Alternative measures that have been investigated to reduce incidence of TPB on strawberries include: management of weeds that serve as host plant for TPB (Mailloux et al., 1979; Boivin et al., 1981a, 1981b), planting strawberry cultivars resistant to TPB (Handley et al.,1991, 1993), and placement of reflective mulch between rows of strawberry plants (Rhainds et al., 2001). To date, however,
alternative measures that provide control levels as consistent and effective as conventional insecticides have not been developed on a commercial basis.
Gray mold is an important fungal disease of strawberry. Dead leaves serve as an inoculum for the disease, and conidial infestation of flowers causes rotting of strawberry fruits (Maas, 1998). High relative humidity and abundant rainfall contribute to high incidence of the disease (Daugaard, 1998). Gray mold is effectively controlled by prophylactic applications of fungicides at the onset of bloom (Maas, 1998). Alternative measures to control gray mold include: planting strawberry in narrow rows to enhance movement of air through the canopy (Dijkstra & van Oosten, 1985; Sutton et al., 1988; Sønsteby et al., 1996), planting strawberry cultivars least susceptible to the disease (Sutton, 1991), and using arthropod pollinators to disseminate the antagonistic fungi Trichoderma harzianum (Kovach et al., 2000).
Feeding activity by slugs create deep holes in strawberry fruits, rendering them unmarketable (Maas. 1998). Mild weather in the winter or wet conditions in the spring increase the incidence of damage by slugs (Pritts & Handley. 1998). Slugs can be controlled by applying molluscicides or using traps baited with food, but neither approach is very effective (Pristypa et al., 1987; Pritts & Handley, 1998: Torres & Yanez, 1998: Ogrodnick & Kovach, 1998).
Weeds negatively affect the productivity of strawberry plants and interfere with harvesting (Pritts & Handley, 19981). Weeds may also increase the incidence of damage by TPB and gray mold, because both organisms have a wide range of host plants (Young., 1986 Daugaard, 1998). Weeds can be managed using a combination of approaches, including application of herbicides, planting high density of strawberry plants, manual collection, and planting cover crops to displace harmful weeds (Pritts & Handley, 1998).
Comparing yield and incidence of pests for strawberries cultivated through several fruiting seasons using conventional and organic management systems is essential to assess the economic profitability of both management systems. These types of study have been conducted in California (Gliessman et al., 1996a, 1996b), but unfortunately not in the Northeast United States. Furthermore, studies conducted in California tested only one cultivar, and yield of strawberry plants as well as incidence of pests may vary for different cultivars (Sutton. 1991: Handley et al., 1991, 1993; Pritts & Handley, 1998). The present study was carried out in New York State to compare yield, incidence of pests, and economic profitability of strawberries cultivated using conventional and organic management systems through 4 fruiting seasons, testing three strawberry cultivars widely grown in the Northeast United States.
Results obtained through this 4-year study indicated that, for all strawberry cultivars. yield decreased between 1997 and 2000, with a sharp decline in 1999 (Figure 1). Low yield in 1999, both in terms of number of fruits per ha and mean weight per fruit (Figure 1), may have been due to unusually dry weather in 1999 negatively affecting productivity of strawberry plants (Pritts & Handley, 1998; El-Fahran & Pritts, 1997). As a result of extremely low yield in 1999, accumulated profit per ha. steadily increased between 1997 and 1998, but remained stagnant in subsequent years (Figure 6). The dry weather in 1999 may also have contributed to low incidence of damage by slugs and gray mold during that year (Daugaard, 1998; Pritts & Handley. 1998). Density of TPB fluctuated among years, with lowest number of nymphs per flower cluster and proportion of damaged fruits in 1998 (Figure 2).
Of all strawberry cultivars tested in this study, Honeoye was the most productive and least susceptible to TPB, whereas Dayneutrals exhibited the lowest productivity and highest incidence of damage by TPB (Figures 1, 2). Incidence of damage by slugs and gray mold was higher for Earliglow than Honeoye or Dayneutrals, although differences between cultivars were obvious only in organic plots in 1998 (Figure 3). Due to its high productivity and low susceptibility to various pests. Honeoye was by far the most economically profitable strawberry cultivar tested in this study, independently of which management system was used (Figure 6).
Density of strawberry fruits harvested per row section was negatively correlated with proportion of fruits damaged by TPB during 4 years in organic plots, possibly because the impact of TPB feeding is "diluted" when strawberry plants produce numerous fruits; the relationship was significant during only one year in conventional plots where malathion was applied (Table 4). These results suggest that maintaining cultural practices that promote high yield of strawberry plants may be highly desirable in organic strawberry farms, both in pure economic terms and as a strategy to reduce relative damage by TPB. However, positive relationships between density of fruits harvested per row section and proportion of fruits infested with gray mold (Table 4) suggest that high productivity of strawberry plants may have some adverse effect, by increasing transmission of fungal diseases (Dijkstra & van Oosten, 1985; Sutton et al., 1988; Sønsteby et al., 1996).
Of all non weed pests that damaged strawberry fruits in our experimental plots, TPB was by far the most prevalent (Figure 2). Damage by TPB was much higher in organic than conventional plots, both in terms of number of nymphs per flower cluster and proportion of damaged fruits (Figure 2); in contrast, incidence of damage by slugs or gray mold differed only to a minor extent in organic and conventional plots (Figure 3). Damage by TPB was higher in organic than conventional plots even in 2000 when malathion was not applied (Figure 2), suggesting that intrinsic parameters may have contributed to increase damage by TPB in organic plots, such as high abundance of weeds favoring reproduction of TPB (Figure 4; Boivin et al., 1981a), or low productivity of strawberry plants "concentrating," the impact of TPB feeding (Table 4; Rhainds et al., 2001). In conventional plots where malathion was applied, incidence of damage by TPB did not greatly vary for different strawberry cultivars; in contrast, variations in damage by TPB between cultivars were highly pronounced in organic plots. with Dayneutrals exhibiting greater proportion of damaged fruits than Honeoye (Figure 2). These results suggest that planting strawberry cultivars resistant to TPB may represent a useful strategy to alleviate damage by TPB especially for organic management systems. Field experiments testing up to 20 strawberry cultivars revealed that incidence of damage by TPB greatly vary for different cultivars, with Honeoye being the least susceptible cultivar, and that proportion of damaged fruits on untreated Honeoye strawberry plants is similar to that of susceptible cultivars treated 3 times with malathion (Handley et al., 1991 , 1993).
The major objective of the present study was to evaluate yield and economic profitability of strawberries cultivated using conventional and organic management systems. In a similar study conducted in California with one strawberry cultivar (Chandler), yield was significantly lower in organic than conventional plots (Gliessman et al. 1996a, 1996b). Our study also indicated higher yield for conventional rather than organic management systems for each of 3 strawberry cultivars widely grown in the Northeast United States (Figure 1). Differences in yield were likely not due to variations in soil fertility, because levels of foliar nutrients were similar in conventional and organic plots, and exceeded levels of deficiency (Rhainds, Kovach and English-Loeb, unpublished results). Variations in yield may have been due, in part, to plants in organic plants being smaller and producing less fruits than those in conventional plots (Gliessman et al., 1996a, 1996b), or to high incidence of weeds in organic plots (Figure 4) negatively affecting productivity of strawberry plants. Differences in yield between conventional and organic plots were more pronounced in 1997 than in subsequent years (Figure 1), suggesting that low productivity in organic plots may have resulted from a poor establishment of strawberry plants in ribbon rows one year after transplanting. Because incidence of pests was highest in organic plots (Figure 3), differences in yield between conventional and organic plots were more pronounced when fruits damaged by pests were considered unmarketable rather than marketable (Figure 5); these results strongly suggest that a lack of reliable, effective measures for managing pests of strawberry without synthetic pesticides, especially weeds and TPB (Figures 3, 4) may severely constrain yield and economic profitability of organic strawberries in the Northeast United States.
Economic analyses conducted with one strawberry cultivar (Chandler) in California indicated that differences in yield between conventional and organic systems were offset by the high retailing price of organic strawberries, resulting in superior economic profitability for organic rather than conventional management (Gliessman et al, 1996a, 1996b). In the Northeast United States, economic profitability of conventional and organic management systems greatly varied between strawberry cultivars, and was impacted by the incidence of pests damaging strawberry fruits (Figure 6). Break even price was high for Dayneutrals, especially in organic plots, indicating low economic profitability; when damaged fruits were considered unmarketable, organic growers cultivating Dayneutrals broke even only assuming a very high retail price (break even price of $1.86 and $4.06 per kg of conventional and organic strawberries, respectively) (Figure 6). Economic profitability of conventional and organic management systems was similar for Earliglow when damaged fruits were considered marketable, as indicated by small difference between break even price per kg of conventional and organic strawberries ($1.31 and $1.58, respectively), because incidence of pests (especially TPB) was high for Earliglow in organic plots (Figure 3), however, conventional management was economically superior to organic management when damaged fruits were considered unmarketable, as indicated by great differences in break even prices ($ 1.47 and $ 2.57 per kg of conventional and organic strawberries respectively) (Figure 6). Of all cultivars tested in this study, Honeoye was by far the most suitable for organic management, as indicated by small differences in break even price for
organic and conventional strawberries (Figure 6).
A recent study suggests that placement of reflective mulch between rows of strawberry plants may enhance the economic profitability of cultivating strawberries, by simultaneously increasing productivity of strawberry plants and reducing damage by TPB (Rhainds et al., 2001). Upon development of this cultural method on a commercial basis, organic cultivation of Honeoye plantings may represent a highly profitable management system available to strawberry growers in the Northeast United States.