The data were expressed on a per-hectare basis, and cost-benefit estimates obtained for the following three scenarios: growing strawberry with fumigation, growing strawberry without fumigation, and growing broccoli and strawberry sequentially. Effects on resident microsclerotia. At the Watsonville site, soil inoculum levels of V. dahliae were moderate at the start of the experiment in 1997. The inoculum levels either increased or decreased depending on the vegetable rotations tested. Rotations with lettuce increased the number of microsclerotia in soil, with upper ranges of 10 to 17 microsclerotia g–1 of soil being most frequent. In contrast, rotations with Brussels sprouts significantly reduced microsclerotia of V. dahliae both in 1997 and 1999 relative to the pretreatment numbers . Of the threerotations tested, the lowest number of V. dahliae microsclerotia was observed in plots rotated with broccoli. The reduction of V. dahliae microsclerotia at the end of two cycles of broccoli in 1997 was approximately 74%. Following this dramatic reduction, cultivation of strawberry in these plots increased V. dahliae microsclerotia by 29% over pre-strawberry levels in 1997–98. However, a second cycle of broccoli rotations reduced the microsclerotia by 83% over the initial 1997 levels, followed by a 33% increase of microsclerotia over pre-strawberry levels at the end of the second strawberry crop. Rotations with Brussels sprouts reduced V. dahliae propagules by 55 and 65% in 1997 and 1999, respectively. However, plastic flower buckets wholesale following strawberry crops in these plots, V. dahliae microsclerotia increased by 87% in 1997– 98 and by 93% in 1999–2000.
No detectable microsclerotia of V. dahliae were present in the Salinas field soils during either vegetable or strawberry seasons. Effects on Pythium propagules. At both locations, rotations had little influence on population levels of total Pythium spp. . At the Watsonville site, populations ranged from 14 to 80 CFU g–1 of dry soil whereas, at the Salinas site, they ranged from 54 to 118 CFU g–1 of dry soil in 1997 and 190 to 440 CFU g–1 of dry soil in 1998. Differences in populations among treatments for the same year were not significant or related to cropping history. Although exact population counts were not made, the most commonly recovered species was P. ultimum. Effects on strawberry growth. Strawberry plants grown in plots rotated with lettuce at both locations had a significantly smaller canopy diameter than other rotation treatments. In Watsonville, broccoli- and Brussels sprouts-rotated plots and fumigated control plots, plants had a greater canopy diameter than strawberry plants in lettuce rotation plots . These differences were maintained until the vegetative phase of strawberry growth . In 2000, the canopy diameter of strawberry plants was higher in fumigated control and broccoli rotation plots than in the lettuce and Brussels sprouts rotation plots. Subsequently, however, the highest canopy diameter was observed in the fumigated control, followed by broccoli, Brussels sprouts, and lettuce rotation plots . At the Salinas site in 1998, strawberry plants in broccoli-rotated plots had the highest canopy diameter, and the differences between fumigated control and other rotations were variable.
Conditions during fumigation in 1998 were not optimal; hence, strawberry plants did not show the typical robustness in these plots . In 2000, the response of strawberry canopy diameter to various rotation treatments was typical of what was observed at the Watsonville site, with the plants being more robust in the fumigated control followed by broccoli, cauliftlower, and lettuce plants. There were significant differences between each of the treatments on both assessment dates . Effect of rotations on disease severity. Repeated-measures ANOVA indicated that the rotation treatments significantly affected disease severity on strawberry for all observation dates at both locations. The highest wilt severity was observed in the lettuce rotation plots throughout the season in 1998 at the Watsonville site and the lowest was in the fumigated plots . Broccoli rotation plots had the lowest wilt severity among the three rotation treatments. Even though the differences in wilt severity between fumigated and broccoli rotation plots were significant through much of the season, final wilt severity was nearly identical between the two treatments . Wilt severity in the Brussels sprouts rotation plots was intermediate between broccoli and lettuce rotation plots throughout the season . In 2000, the onset of Verticillium wilt occurred 3 weeks later, and the severities were lower relative to 1998 . The response of different treatments, however, was nearly identical to 1998, with the least wilt severity recorded in fumigated plots followed by broccoli, Brussels sprouts, and lettuce . Final wilt severity in fumigated plots and plots that had broccoli residue incorporated was nearly identical and not statistically significant from each other . As in the Watsonville site, disease severity in the Salinas site was highest in the lettuce rotation treatment during both 1998 and 2000 . Because of the inefficient fumigation in 1997, disease severity on strawberry plants in fumigated plots was higher than in plots that had broccoli residue incorporated in 1998. However, there were no significant differences between these two treatments during the 2000 season .
Strawberry plants in the broccoli-rotated plots showed a consistently lower disease severity than in the remaining vegetable rotation plots during all observed dates. Effects on strawberry fruit yield. Overall strawberry yield was higher in Salinas than in Watsonville . The fumigated control treatment produced the highest marketable and total yields at both locations until comparable dates of harvest . The experiment at Watsonville was terminated earlier but was continued at the Salinas site; hence, the final yields appear comparable between the two sites . Among the vegetable rotation treatments during both seasons at both sites, plots that had broccoli residue incorporated produced the highest strawberry yield. In 1998, total strawberry yield was about 22% less in plots with broccoli rotation relative to the fumigated plots in Watsonville. However, in 2000, fruit yield in fumigated plots was only 12% higher than in broccoli-rotated plots. Plots with lettuce rotation consistently had the lowest strawberry yield at both sites during both seasons. In Watsonville, Brussels sprouts plots had intermediate strawberry yield in 1998 but lowest yield in 2000 when it was similar to that in lettuce rotation plots. In 2000, the difference in yield between the lettuce and broccoli rotation plots was greater than in 1998 at the Salinas site . In 1998, there were no yield differences among the vegetable rotation plots; however, in 2000,broccoli rotation plots had the highest and lettuce plots had the lowest strawberry yield, with the yield in cauliftlower rotation plots being intermediate. Cost-benefit analysis. Strawberry production under fumigation incurred the highest production costs but also provided the highest returns. Average total cost of production in 1998 and 2000 was estimated to be $81,000 per hectare with a net profit of $10,500 per hectare . In contrast, the cost of strawberry production without fumigation decreased to an estimated $77,000 to 79,000 but also led to losses between $17,000 and 19,000 depending on the production site . Production of strawberry under crop rotation involved giving up the annual strawberry production during the time rotation crops were grown but resulted in net profits because of income from rotation crops and higher strawberry yield. However, the overall profits were reduced by 20 to 30% a year relative to the production under fumigation. The total cost of producing strawberry following two crops of broccoli was estimated to be nearly $82,000 that resulted in a net profit of $6,800 to 7,800 per hectare per year, depending on location . This study demonstrated that rotations with broccoli and Brussels sprouts followed by the post harvest incorporation of the respective residues reduced the number of V. dahliae microsclerotia in soil that resulted in concomitant reductions in the incidence of Verticillium wilt and increases in fruit yield of strawberry. None of the rotations, however, black flower buckets reduced Verticillium wilt or increased yield as much as fumigation with methyl bromide + chloropicrin. The benefits of rotations were more evident with broccoli than with Brussels sprouts. Although the results with broccoli rotations are consistent with those obtained on cauliftlower , this is the first demonstration of successful rotations with broccoli and Brussels sprouts on a highly Verticillium wilt-sensitive, deep-rooted , and long-duration crop such as strawberry. Rotations with lettuce increased the numbers of microsclerotia in soil significantly over pre-rotation levels consistent with it being identified as a new host of V. dahliae and the strawberry strain being pathogenic on lettuce and vice versa . None of the rotations influenced the overall populations of Pythium spp. in soil, but it was unclear whether specific rotations influenced the species composition of this population. This often was not apparent on strawberry plants because disease caused by Pythium spp. does not have distinct symptoms on this host that enable diagnosis based on visual symptoms alone .
Adaptation of successful rotations with broccoli entails giving up the annual strawberry production following fumigation during rotation and nearly 30% of the annual profits on a per hectare basis. While these short-term losses accrue, growers reap the benefits of reducing soil inoculum over the long-term. As with cauliftlower , the greatest reduction in the number of microsclerotia at the Watsonville site was observed soon after the incorporation of broccoli and Brussels sprouts residues. This was followed by additional reductions in microsclerotia of V. dahliae during the second cycle of broccoli rotation. The numbers of microsclerotia increased marginally in broccoli plots during the subsequent strawberry season but remained lower than in the Brussels sprouts plots. In contrast, at the Salinas site, even with no detectable V. dahliae propagules, broccoli rotations increased strawberry yields as evidenced by higher plant health ratings, suggesting that broccoli may suppress pathogens other than V. dahliae or result in enhanced growth of strawberry plants. Even though this study focused on Verticillium and Pythium spp., other soilborne pathogens such as R. solani, binucleate Rhizoctonia spp., and Cylindrocarpon spp. also were present at this test site and common in strawberry production systems in Califtornia . One can infer from the results obtained at the V. dahliae–free Salinas site that rotations with broccoli have benefits beyond the pathogens tested in the current study. In contrast to the reductions in V. dahliae microsclerotia and wilt on strawberry observed in rotations with broccoli and Brussels sprouts, rotations with lettuce resulted in significant increases in V. dahliae microsclerotia and wilt on strawberry. Prior to 1995 , lettuce was not even considered to be a host of V. dahliae, but wilt caused by this pathogen currently is a major problem on lettuce in the central coast of Califtornia. Recent studies have clearly established that the strawberry and lettuce strains of V. dahliae belong to the same phylogenetic group based on the sequence similarities of the intergenic spacer region and the combined sequences of the IGS region and the β- tubulin gene. Furthermore, the two strains were also cross-pathogenic to both hosts. Previous molecular profiling based on random ampliftied polymorphic DNA analysis also concluded that lettuce and strawberry strains displayed the closest phylogenetic relationship relative to the other host-adapted isolates tested . Unlike in most other hosts of V. dahliae, microsclerotia develop along the veins of lower, senescing lettuce leaves prior to plant death and result in abundant augmentation of soil inoculum after an infected crop. Therefore, it is not surprising that microsclerotia of V. dahliae increased in the soil of lettuce-rotated plots and resulted in higher severity of Verticillium wilt on strawberry and reduced fruit yield compared with other rotations. Residues of other Brassica spp. have proven effective in reducing several other soilborne pathogens . Keinath reported significant reductions of gummy stem blight of watermelon in soil amended with cabbage residue. Chan and Close demonstrated the control of Aphanomyces root rot from Brassica residue amendments. Brassica spp. are well known for their characteristic sulfurcontaining compounds, known as glucosinolates, and for the disease-suppressive effects of the toxic byproducts derived from the breakdown of these compounds . Although this may explain, in part, the successful use of broccoli residues to reduce the number of microsclerotia in soil, other factors also may play an important role in the suppressive effects of Brassica spp. in general. Shetty et al. found that, despite the apparent lack of foliar symptoms and few root symptoms, broccoli roots still were colonized by V. dahliae to the same degree as cauliftlower, except when soil microsclerotia levels were high. Under high soil inoculum density, the colonization rate of cauliftlower roots was about 1.5-fold higher compared with broccoli roots.