Winter Strawberry Production in Greenhouses Using Soilless Substrates: an Alternative to Methyl Bromide Soil Fumigation

In an effort to develop an alternative to methyl bromide-dependant strawberry (Fragaria x ananassa Duch.) field production in north-central Florida, we evaluated various soilless substrates, growing containers, plug transplants, plant densities, cultivars, and biological control strategies in a passively ventilated high-roof greenhouse near Gainesville, Florida. The type of soilless media used influenced total yield of field-grown and greenhouse-grown plugs when plants were grown in Polygal@ troughs or poly-bags placed at ground level, but did not influence total yield when plants were grown in poly-bags placed on elevated gutter sections. Type of growing container did not affect the yield of greenhouse-grown plugs, however, early yield of field-grown plugs was higher when grown in poly-bags placed on elevated gutter sections than when grown in the other types of containers. Twelve plant densities ranging from 0.86 to 2.55 per ft2 (37,462 to 111,078 per acre) were evaluated. Plant densities were derived by combinations of six between-row spacings (16, 18, 20, 22, 24, and 26 inches center-center) and two within-row spacings (7 and 12 inches plant-to-plant). In Fall 2001, the yield per ft2 increased linearly with plant density. However, in Fall 2002, the yield per ft2did not increase linearly due to a reduction in yield per plant at the between-row spacing of 16 inches. Strawberry yields of 1.961b/ft2 (7,115 12-lb flats per acre) were obtained from greenhouse production compared to a 10-year average yield of 2,392 12-lb flats per acre (FDACS, 2002) for field production. New cultivars such as FL97-39 and Carmine produced high early yields, but FL97-39 was highly susceptible to powdery mildew (Sphaerotheca macularis) , and 'Carmine' and 'Sweet Charlie' were more affected by aphids than other cultivars. Aphidius colemani and Lysiphlebus testaceipes parasitic wasps, and Neoseiulus californicus predatory mites were effective in controlling aphids (Aphis gossypil) and two-spotted spider mites (Tetranichus urticae) respectively. Thus, protected strawberry culture can enhance early and total yields, improve harvest efficiency, improve fruit quality by reducing pesticide usage, and eliminate dependency on methyl bromide. Since the early 1900s, the Florida strawberry industry has undergone many changes from growing strawberries on straw80 OF), fruit quality characteristics such as firmness and soluble solids content may be adversely affected. During severe cold weather, higher temperatures can be maintained in passively ventilated high-roof greenhouses by closing the side curtains and roof vents to trap the heatgeneratedbysolarradiation.lfnecessary, supplemental heat can be provided when night temProc. Fla. StateHort. Soc. 116: 2003. 99 Fig. 1. Passively ventilated, high-roof greenhouse with double layer polyethylene roof and insect screens (Protected Agriculture Project, Gainesville, Fla.). Powdery mildew (Sphaerotheca maculari\') infects the leaves, petioles, flowers, peduncles, and fruits of strawberry plants and is probably the most important and widespread fungal pathogen of strawberries grown under protected culture worldwide. Powdery mildew proliferates rapidly when humidity is moderate to high and air temperatures are between 59 of and 81°F (Hancock, 1999). AQ-I0 biofungicide (Ampelomyces quisqualis-, Ecogen, Inc., Langhorne, PA) can provide limited control of powdery mildew, but other fungicides like sulfur and potassium or sodium bicarbonate, which are mildly toxic to beneficial insects, may be more effective. Botrytis fruit rot or gray mold caused by Botrytis cineria is not as severe a problem in protected strawberries as it is in field-grown strawberries. However, it can be a serious problem in protected strawberries (Robert Hochmuth 2003, personal communication) that are grown under conditions of high humidity and moderate daytime temperatures (60 of to 75 OF) (Legard,2003). Soilless Media, Growing Containers (Systems), and Plug Transplants During Fall 2000 and 2001, a study was conducted to evaluate the effect of three growing systems, three soilless substrates, and two kinds of plug transplants on 'Sweet Charlie' strawberry yield and quality in protected culture. The growing systems were: (a) polyethylene bags (40 x 7 x 5 inches) suspended 4 ft above the ground on PVC gutter sections, (b) polyethylene bags (40 x 7 x 5 inches) placed at ground level, and (c) Polygal@ 'Hanging Bed-pack troughs (4-inch bottom width x 5-inch wall height, with 2-inch diameter planting holes) (PolYgal Industries, Ramat Hashofet, Israel) (Fig. 2) suspended 4 ft above the ground. The soilless media were: (a) peat-mix [2:1 peat:perlite (v:v)J, (b) pine bark, and (c) perlite. The two kinds of plugs were: (a) GH plugs: 4 months old greenhouse-grown, conditioned (77 OF day/59 of night, 9-h photoperiod for 2 weeks) (Bish et al., 1997) plug transplants grown in an evaporative-pad cooled greenhouse, and (b) FG plugs: 2.5 months old, field-grown, non-conditioned plug transplants grown in Cashiers, North Carolina (Norton Creek Farms, Cashiers, N.C.). Although these plugs were not conditioned artificially, they may have received some natural chilling since they were produced at a high elevation nursery. Treatments were randomized in a split block design with three replications. Fruit with 80 percent color development were harvested at 4-5 d intervals. Fruit that weighed more than 109 and were not deformed or diseased were considered marketable. Fruit that weighed less than 10 g or were deformed or diseased were considered non-marketable. For each plot, the number of fruit and fruit weight was recorded for marketable and non-marketable fruit yield and quality. Data were analyzedwith SAS software (SAS Institute, Inc., 1999-2001). Early yield. The results from our greenhouse studies indicate that the type of soilless media influenced the early yield of plants during both seasons. The marketable yield was sigProc. Fla. State Horl. Soc. 116: 2003. 100 Fig. 2. Polygal@ Hanging Bed-pack trough system for strawbeny production. the total yield ofFG plugs was significantly higher than that of GH plugs (data not shown). Soilless media. Most soilless media are inert and free from weed seeds and pathogens. Hence, there is no need for methyl bromide. In central Europe, strawberries are grown commercially in a wide variety of soilless media such as peat moss, coconut coir, perlite, rockwool, or pine bark (Lieten, 2001). Besides the ability of a media to promote good plant growth and yield, the cost (Cantliffeet al., 2001) and availability of the soilless media is an important factor in soilless culture. In our study, plants grown in pine bark produced yields that were comparable to plants grown in peat-mix and perlite. Also, pine bark is easily available in Florida, and is relatively inexpensive ($6.50 per yd3) compared to perlite ($31 per yd3) and peat ($53 per yd3). On a per acre basis, using pine bark ($850 per acre) could reduce the cost incurred on soilless media by 85% compared with using a 2:1 mixture of peat: perlite ($5,500 per acre). Growing containe13" (systems). Strawberries can be grown in polyethylene bags, plastic pots, PVC troughs, or styrofoam containers of various shapes and sizes. Growing containers should be arranged in such a way that sunlight is distributed evenly throughout the plant canopy and plant population density and yield are maximized. Vertically arranged growing systems usually accommodate more plants per unit area than horizontal systems, but have problems with sub-optimallight levels in the lower sections, resulting in reduced yield and plant growth (Takeda, 1999). Growing systems arranged in a single horizontal tier (usually in the north-south direction) can also accommodate high plant population densities up to 2.8 plants per ft2 (Paranjpe et al., 2003) and, since all plants are at the same height, light distribution is uniform. In addi'tion to enabling high yields and uniform light distribution, a growing system should lend itself to spatial adjustments in order to facilitate cultural practices, and should have the ability to be reused over multiple seasons. Polyethylene bags degrade within a short time and cannot be used for more than one season. Also, they are bulky and difficult to handle, especiallywhen they are suspended above the ground. On the other hand, trough systems such as the Polygal@ Hanging Bedpack system are easy to handle and can be raised or lowered easily to a desirable height to facilitate cultural operations such as transplanting, scouting, and harvesting. Although the initial cost of trough systems is high ($1 per ft), their cost on nifica,ntly higher when plants were grown in perlite than with peat-mix or pinebark (data not shown). During both seasons, there was no difference between the early yield of GH plugs and FG plugs (data not shown). During Fall 2000, the growing system influenced the early yield of FG plugs. Greater yields were obtained when plants were grown in PolYgal troughs than with 'bag on gutter' or 'bag on ground' (Table 1). On the other hand, the early yield of GH plugs was not influenced by the type of growing system. During Fall 2001, the kind of growing system did not influence the early yield from either plug type. Total yield. The type of soilless media and growing system influenced the total yield during both seasons, wherein, in Fall 2000, plants grown in PolYgal troughs produced higher yields in peat-mix and pine bark than in perlite (Table 2). Plants grown in 'bag on ground' produced higher yields in peat-mix and perlite than in pine bark. The soilless media did not affect the yields of plants grown in 'bag on gutter'. During Fall 2001, plants grown in 'bag on gutter' produced higher yields in peat-mix and perlite than in pine bark, and the yields of plants grown in 'bag on ground' were higher in peat-mix than in pine bark, but not significantly different from those obtained in perlite (Table 2). On the other hand, the total yield of plants grown in Polygal troughs was not influenced by the type of soilless media. During both seasons, Table 1. Effect of plug type (PT) and growing container (GC) on the early and total marketable yield per plant of 'Sweet Charlie' strawberry grown during Fall 2000 and Fall 2001 in a passively ventilated greenhouse located in Gainesville, Fla. Marketable yield (lb.plant:1)z