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 cauliflower rotation plots being intermediate. 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, hydroponic dutch 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 cauliflower , 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 cauliflower , 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 California . 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 California. 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 amplified 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 cauliflower, except when soil microsclerotia levels were high. Under high soil inoculum density, the colonization rate of cauliflower roots was about 1.5-fold higher compared with broccoli roots. Microsclerotia never developed within broccoli root tissues, even 60 days after decapitating plants at the crown. In addition, there was no apparent inhibition of growth of V. dahliae on a medium with broccoli root extracts. This led to the hypothesis that perhaps the reduction in V. dahliae soil populations was caused by the combined effects of broccoli acting as a trap crop to force the germination of microsclerotia and the activation of resident microflora with an ability to degrade lignin-rich broccoli residue in addition to the melanized microsclerotia of V. dahliae . Fungal ligninases have been found to have activity against melanin as well, but microorganisms with melanolytic activity also may be involved . Data from broccoli-rotated plots demonstrated a 1,000-fold increase in bacterial and 100-fold increase in actinomycete populations relative to the unamended control or cauliflower-rotated plots, bato bucket suggesting a biological basis for the suppression of V. dahliae . It also is possible that the reduction in V. dahliae soil populations is partly due to oxygen depletion, created by the increased microbial activity from the incorporated broccoli residue, or from increases in anaerobic activities induced within the oxygen-depleted environment. Blok et al. determined that broccoliamended or rye grass-amended soils covered with a plastic cover created anaerobic environment sufficient to reduce soil inoculum of V. dahliae, Fusarium oxysporum f. sp. asparagi, and R. solani. In contrast, Subbarao et al. found that the effects of incorporated broccoli residue were identical in both open and plasticcovered plots. Perhaps the differences in these two studies can be attributed to the quantity of broccoli residue incorporated and the different field soils. In addition to the effects of glucosinolates on plant pathogens, there may be impacts on the broader soil microbial community, perhaps favoring beneficial organisms. Other studies also have attributed a biological basis of pathogen suppression from Brassica residues or by other means in naturally suppressive soils. Suppression of take-all in wheat caused by Gaeumannomyces graminis in acidic soils was associated with fungal antagonism by Trichoderma spp. . Smith et al. failed to observe changes in microbial communities by Brassica tissues when the following crop was wheat. In in vitro studies , Trichoderma spp. were tolerant to isothiocyanates while Aphanomyces, Gaeumanomyces, and Phytophthora spp. were sensitive, suggesting both a direct suppression from the toxicity of isothiocyanates and favoring of antagonism by Trichoderma spp. The effects of Brassica residues on Pythium propagules in soil have been variable. Stephens et al. reported that mustard tissue incorporation decreased grapevine establishment in soils with high numbers of Pythium propagules. Similarly, Walker and Morey found that, in citrus orchards, the number of Pythium propagules in soil as well as in the root systems were increased by mustard and rapeseed tissue amendments.
Although P. sulcatum and P. violae were highly sensitive to isothiocyanate from Brassica residues, the highly pathogenic P. ultimum was tolerant . In a recent study, Brassicaceae seed meals stimulated Pythium populations in certain soils whereas B. juncea alone had no effect. In combination with B. napus, however, B. juncea eliminated the stimulation of resident Pythium spp. typically observed when B. napus seed meal was applied alone. Furthermore, elevated populations of Pythium spp. in S. alba or B. napus seed meal-treated soils contributed to significant weed suppression. This weed suppression was lost when Ridomil -methoxyacetylamino]-propionic acid methyl ester was applied to B. napus-treated soil and significantly diminished in S. alba-treated soils, confirming that the high Pythium numbers contributed to weed suppression . In the current study, incorporation of broccoli, Brussels sprouts, cauliflower, or lettuce residues did not alter the total Pythium populations in soil. Because the pathogenic Pythium spp. were not quantified separately, the possibility that incorporation of residue from various crops had some effect on this segment of Pythium population could not be ruled out. The impact of diseases or methods to ameliorate diseases in strawberry is ultimately measured by their effect on yield. As expected, the fumigated control provided the highest yield and correspondingly the highest profits. Even though none of the rotations equaled the level of pathogen and disease suppression observed in the fumigated control, strawberry yield in broccoli-rotated plots was a close second. The unique cost-benefit analysis employed in this study also supported this conclusion. Despite giving up yearly strawberry cultivation that is practiced in some commercial strawberry fields, rotations with broccoli and, to some extent, Brussels sprouts would be a profitable, environmentally friendly method of managing Verticillium wilt in strawberry that is effective in both conventional and organic strawberry production systems. The geometries and topologies of leaves, flowers, roots, shoots, and their arrangements have fascinated plant biologists and mathematicians alike. As such, plant morphology is inherently mathematical in that it describes plant form and architecture with geometrical and topological techniques. Gaining an understanding of how to modify plant morphology, through molecular biology and breeding, aided by a mathematical perspective, is critical to improving agriculture, and the monitoring of ecosystemsis vital to modeling a future with fewer natural resources. In this white paper, we begin with an overview in quantifying the form of plants and mathematical models of patterning in plants. We then explore the fundamental challenges that remain unanswered concerning plant morphology, from the barriers preventing the prediction of phenotype from genotype to modeling the movement of leaves in air streams. We end with a discussion concerning the education of plant morphology synthesizing biological and mathematical approaches and ways to facilitate research advances through outreach, cross-disciplinary training, and open science. Unleashing the potential of geometric and topological approaches in the plant sciences promises to transform our understanding of both plants and mathematics. The study of plant morphology interfaces with all biological disciplines . Plant morphology can be descriptive and categorical, as in systematics, which focuses on biological homologies to discern groups of organisms . In plant ecology, the morphology of communities defines vegetation types and biomes, including their relationship to the environment. In turn, plant morphologies are mutually informed by other fields of study, such as plant physiology, the study of the functions of plants, plant genetics, the description of inheritance, and molecular biology, the underlying gene regulation .