Therefore, longer-term research is required to develop non-chemical alternatives, and their adaptation will require effective integration with other methods of disease, pest, and crop management . In the post-methyl bromide era, Verticillium wilt is likely to reemerge as a major disease for conventional strawberry production. The disease already is a major problem in some organic production fields. In strawberry, symptoms begin to appear during early to mid-season, with outer leaves on infected plants turning yellow, drooping, and later turning brown and dry. Yield from these affected plants can be dramatically reduced and infected plants usually die before the end of the season . The fungus survives in the soil as microsclerotia for many years, and survives better in sandy loam soils typical of strawberry production fields in coastal Califtornia than in other types of soil . Large numbers of microsclerotia are formed in colonized Thissue of susceptible crops, and a few are formed even on non-hosts . Whether microsclerotia are formed on infected strawberry plants is not known. Resistance to Verticillium wilt is unavailable in currently used commercial cultivars and tolerance in these cultivars is low. With the phase out of methyl bromide and possible future loss or restrictions on the use of alternative fumigants, resistance to Verticillium wilt has now become a selection criterion in some breeding programs.
As a result, flower harvest buckets resistance to Verticillium wilt in locally adapted strawberry cultivars may increase over time. The concept of rotating crops to manage plant diseases is perhaps one of the oldest cultural practices in agriculture . The utility of this practice in reducing Verticillium dahliae inoculum and subsequent disease intensity has been equivocal . Microsclerotia of V. dahliae survive in the soil up to 10 years, and the extensive host range and lack of host specificity reduce the usefulness of some crop rotations for Verticillium wilt management . However, recent work has shown that rotations with broccoli dramatically reduce microsclerotial numbers and Verticillium wilt incidence in susceptible crops. If rotations of broccoli are successful in strawberry, they will be equally applicable to both conventional and organic strawberry production systems. Although the benefits of rotations are numerous and quantification of these benefits in dollar terms is difficult, simple cost-benefit analysis of adapting rotations will inevitably lead to a better understanding of their composite benefits. Such information also may lead to increased adoption of crop rotations. The objectives of this study were to determine the effect of crop rotation on soil borne fungal inoculum density, disease severity, and strawberry growth and yield; to assess the effectiveness of crop rotation in soil with no detectable Verticillium spp. to improve strawberry growth and yield; and to obtain a cost-benefit analysis of this method of managing Verticillium wilt in strawberry.
Experiments on vegetable–strawberry rotations were conducted at two sites from winter 1997 to fall 2000. One site was located at the United States Department of Agriculture, Agricultural Research Station at Salinas, CA with no history of either strawberry cultivation or Verticillium wilt ; however, the site had other soilborne pathogens such as Pythium spp., Rhizoctonia solani, binucleate Rhizoctonia, and Cylindrocarpon spp. The other site was located at the MonTherey Bay Academy in Watsonville, CA and has a history of strawberry production and disease pressure from V. dahliae as well as the above-noted root rot pathogens. Both sites lie in the major strawberry production areas of the central coast of Califtornia and prior investigations confirmed that black root rot pathogens were present and causing disease . Soil at the Salinas site was a Chualar loam with 57% sand, 31% silt, 1.1% organic matter, and a pH of 5.6. Soil at the Watsonville site was an Elder sandy loam with 62% sand, 26% silt, 0.6% organic matter, and a pH of 6.1. Average yearly precipitation at Salinas and Watsonville was 429 and 757 mm, respectively, and mean daily temperature ranged from 8.3 to 19.9°C and 8.3 to 19.7°C , respectively, during the study. The experimental design was a randomized complete block design with four replications. At both locations, two crops per year of vegetables were grown followed by rotation with annual strawberry. There were three rotation treatments at each location in 1997 and 1999. The crop sequences were broccoli-broccoli-strawberry, lettuce lettuce-strawberry and Brussels sprouts-strawberry or cauliftlower-cauliftlower-strawberry . The first cycle of vegetables, including broccoli , cauliftlower , Brussels sprouts , and lettuce , was grown in January or February and harvested in June in each year. The second cycle of vegetables was planted in July and harvested in September or October.
Because of varying maturity times, planting dates were staggered so that all crops could be harvested and incorporated at the same time. At the Watsonville site, the individual plots consisted of two beds of 7.6 m in length with a single plant line for Brussels sprouts and two seed lines on the 1-m-wide beds standard for lettuce and broccoli production in coastal Califtornia. Plant spacing was approximately 30 cm between plant lines and 20 cm between plants within a plant line for lettuce and Brussels sprouts and 10 cm between plants for broccoli. At the Salinas site, the individual plots were 9 m long and eight 1-m beds wide and consisted of two plant lines for lettuce and broccoli production in coastal Califtornia. The beds contained a single plant line for cauliftlower. Plant spacing was approximately 30 cm between plant lines and 30 cm between plants within a plant line for lettuce and cauliftlower. Plant spacing was 15 cm between plants for broccoli. In both years, 4- to 5-week-old transplants of broccoli, cauliftlower, Brussels sprouts, and lettuce were planted. At the Salinas site, however, lettuce was direct seeded in two rows per bed and thinned to a spacing of 30 cm 3 weeks after emergence. After transplanting, the experimental sites were irrigated with sprinklers for 3 to 4 days to ensure proper establishment of plants. The rotational vegetable planting cycles were timed to include two rotational crops of broccoli, cauliftlower, or lettuce per year and one Brussels sprouts crop. All experiments were maintained using standard cultural practices for coastal Califtornia vegetable production. Preplant fertilizer at 450 kg ha–1 was applied to all beds before transplanting or direct seeding, and all plots were drip-irrigated twice a week. Two side dressings at 67 kg ha–1 followed by fertigation at the rate of 11 to 22 kg ha–1 were provided during each season. At maturity, all marketable vegetables were harvested and remaining residues were flail shredded, air dried on the soil surface for 48 h, and incorporated into the soil to a depth of 15 to 20 cm using a rototiller. Three to four weeks after incorporation, the beds in all plots were reworked for the next vegetable or strawberry production cycle. The second crops of vegetables were grown and incorporated similarly.Standard cultural practices for strawberry production were followed for bed preparation . Beds in the control plots were fumigated with methyl bromide plus chloropicrin at the rate of 450 kg ha–1. Fumigants were injected at a depth of 15 to 25 cm through two hollow shanks, and black or brown high-density polyethylene tarps were used to cover the soil immediately after application. Vegetable rotation plots also were covered with black or brown high-density polyethylene tarps before planting strawberry. Two weeks after fumigation, slow release fertilizer was applied at the rate of 672 kg ha–1 at both sites to all treatments. Fertilizer was band placed about 10 to 15 cm deep in each planting row and covered with approximately 4 cm soil to prevent direct contact with plant roots. Bare-root transplants of strawberry cv. Selva were planted 15 cm apart in all plots at both locations in two rows per bed spaced 40 cm apart in mid November of 1997–98 and 1999–2000 . The drip irrigation system consisted of two drip tapes with emitters spaced 30 cm apart, placed 8 cm from the bed center at a depth of 2 to 4 cm, with an emitter flow rate of 0.87 liter min–1 at 70 kPa. Approximately 30 mm of water per week was applied to each of the experimental plots. After plants were established, round flower buckets drip irrigation was applied once or twice per week for 1.5 to 2 h depending on soil moisture conditions. Hand weeding was done periodically when necessary. All management and harvest of the strawberry crop was done by cooperating commercial growers. To determine the densities of propagules of V. dahliae and Pythium spp., soil samples were collected at the beginning and end of the rotation crop and every month after the start of strawberry production.
Soil samples were taken to a depth of 6 to 10 cm from 10 random sites in each treatment, in the center four beds at the Salinas site and in both beds at the Watsonville site. Samples from each treatment were bulked and placed in paper bags and air dried for 4 weeks on greenhouse benches. The dried soil then was assayed for V. dahliae propagules by direct plating onto improved NP-10 medium using the modified Anderson sampler technique . After 3 weeks of incubation in the dark, the plates were washed in a gentle stream of water and examined under a sThereoscope . The number of microsclerotial colonies on each plate was counted and expressed as microsclerotia per gram of dry soil. For the determination of the number of CFU of Pythium spp., 1 g of air-dried soil was added to 10-ml sterilized water blanks and vortexed to mix the soil, and 500 µl of this soil solution was distributed evenly over a petri dish containing a semiselective Pythium medium . The medium consisted of corn meal agar amended immediately after autoclaving with 0.1% Tween 20 followed by pimaricin , ampicillin , rifampicin , rose bengal , and Benomyl 50WP after the medium had cooled to 50°C . After 24 h of incubation at 25°C, the surfaces of the plates were washed free of soil under a gentle stream of water and the number of colonies counted. Plates were placed back into the incubator and subsequent counts made again after 24 h. The total colony counts were expressed as CFU g–1 of dry soil and the presented data reflects the mean of the three replicates. Prior investigations at these sites revealed that Pythium ultimum was the most commonly encounThered species, but other species such as P. irregulare were recovered as well .To determine the relative effects of different rotation treatments, strawberry plant growth was monitored by recording the plant canopy diameter of 20 arbitrarily chosen plants per replication at least twice per season in each replication. Measurements were made in both the east-west and north-south directions because there were significant directional effects early in the season. Development of Verticillium wilt at Watsonville was assessed based on foliar symptoms . The symptoms of root diseases such as black root rot complex were similar but rarely progressed past stunting with leaf chlorosis later in the season . Assessments were made at 2-week intervals starting from June until the end of the season . No Verticillium wilt was observed at the Salinas site. Twenty plants per plot were visually rated to monitor disease progress. The disease severity estimate was made based on a scale of 1 to 8, where 1 = healthy plant, 2 = moderately stunted, 3 = moderately stunted and slight outer rosette of dead leaves, 4 = moderately stunted and moderate outer rosette of dead leaves, 5 = significantly stunted and slight outer rosette of dead leaves, 6 = significantly stunted and moderate outer rosette of dead leaves, 7 = significantly stunted and significant rosette of dead leaves, 8 = dead plant. Data on the cost of production of strawberry fruit, including fumigating, preparing of land, mulching, planting, irrigating, fertigating, weeding, and fruit harvesting, were obtained from four growers that produce both strawberry and vegetables. Similarly, total yields per unit area of production, corresponding revenue earned, and so on also were obtained. Similar data were obtained for strawberry production without fumigation. The cost of producing two crops of broccoli, revenue from broccoli, and the following strawberry crop also was obtained.