Microsclerotia never developed within broccoli root Thissues, 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 cauliftlower-rotated plots, 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, french flower bucket or from increases in anaerobic activities induced within the oxygen-depleted environment. Blok et al. determined that broccoli amended 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. This suppressive effect was lost if the amended soils were not covered .

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 Thissues 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 Thissue 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 Thissue 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, cauliftlower, 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. 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. ACKNOWLEDGMENTS The funding provided by the United States Department of Agriculture–SARE and the Califtornia Department of Food and Agriculture–Department of Pesticide Regulation is gratefully acknowledged. The Califtornia Strawberry Commission, Coastal Berries, and Golden Field Greenhouses provided material support for this project. We thank F. Westerlund for suggestions throughout this study; S. Yamamoto and P. Kohatsu for managing the strawberry crops; and K. G. Shetty, C. Blackford, T. Price, M. E. Abarca, and M. Orozco for technical assistance. The detection of resistance to Erysiphe necator, the causal agent of grape powdery mildew, in two cultivars of ViThis vinifera from Central Asia was intriguing given that this fungus was thought to have co-evolved with North American grape species, and that all V. vinifera cultivars were considered to be susceptible to this fungus. This discovery suggests that powdery mildew resistance is more complex than once thought and that other grape species may have played a role in the resistance found in these Central Asian cultivars. Several grape species native to Central Asia and China are known to express powdery mildew resistance, leading one to question the historical presence of powdery mildew in Asia and the role Asiatic species might have played in the evolution of resistance in present day cultivated grapes.

Addressing these questions would provide insight into the evolution of powdery mildew resistance and the forces driving grape diversity.It is widely accepted that the cultivated form of V. vinifera subsp. sativa derived from its wild form V. vinifera subsp. sylvestris, which was once spread widely across Western Europe, the Mediterranean, the Caucasus, Himalaya and Hindu Kush mountain ranges, and Central Asia. The mountainous region between the Caucasus and China is considered the center of diversity for many temperate fruit crops. Transitional types of grapes that included wild forms of the subsp. sylvestris, feral and cultivated land races and ancient local varieties were once common in this region. One of the key features separating domesticated grapes from their wild relatives is their reproductive system. Wild relatives are dioecious with anemophilous pollination while the domesticated grapevine is hermaphroditic in nature. However, the origin and evolution of hermaphrodism in grape remains an open question. It is not known whether hermaphrodism evolved through sexual recombination, as a mutation of the wild form that was then introgressed into cultivated varieties, or as a mutation that originated in cultivated forms. Cultivated grapevines have a very wide range of variation in fruit, leaf and growth characteristics, and there are thousands of varieties found worldwide. The high amount of diversity is due to the long history of grapevine cultivation that dates back to 4000 – 6000 BC. Initially grape cultivation relied on both seed and vegetative propagation and was influenced by religion, regional traditions and human migration. Seeds may have been the more common means of propagation early in the cultivation of grapes as they were easier to transport over large distances and intentional and unintentional crosses generated great diversity within the cultivated types. Historical records of grape growing in the Orient and Central Asia are very limited, however there is no indication of powdery mildew in available records from this region of the world. Powdery mildew, caused by Erysiphe necator, was first described on grapes in North America in 1834. It was discovered in Europe in 1845 and by 1852, it was reported throughout Europe and the Mediterranean region. Considering the long history of viticulture, the great attention paid to wine grapes, and the lack of any mention of this disease in historical records, it is unlikely that E. necator existed in Europe prior to the early 1800s. Frequent trade activity, bucket flower including the exchange of plant material, facilitated the rapid spread of E. necator over long distances. Many North American ViThis species are resistant to mildew diseases and other pests. Their resistance to powdery mildew is attributed to coevolution with this fungal disease. On the other hand, the Central Asian forms of V. vinifera subsp. sativa were domesticated in the absence of powdery mildew pressure in the mountains of Caucasus and surrounding areas, and these grapes lack resistance to powdery mildew. In the early to mid- 1900s, extensive grape breeding programs were maintained in multiple states of the former Soviet Union, which used germplasm acquired from Central Asia, China, the trans-Caucasus region, Africa, and Europe. Powdery mildew resistance was an important goal for these breeding programs and resistance from the Chinese species, especially V. amurensis was introgressed into cultivated varieties. There are no historical records that indicate any other powdery mildew resistant Chinese species were part of grape breeding in early 1900s. China was linked to Central Asia by both northern and southern silk routes and grape culture was flourishing by the second century AD. Although there are many diverse grape species in China, their impact on grape domestication is unknown. This is in part due to the inaccessibility of germplasm, and historical and scientific records to the non-Chinese speaking world.

The presence of powdery mildew resistance in Chinese grape species is unexplained. We do not know whether these species acquired resistance to fungal diseases after introduction of the disease from the New World or whether powdery mildew and other fungal diseases were present in Asia for a longer time period, but was not recorded in accessible historical records. The records available regarding grape breeding in Central Asia are limited to the early 1900s when the renowned Russian geneticist Nikolai I. Vavilov initiated germplasm acquisition trips in Central Asia and neighboring regions. There were three major objectives to this investigation into the origins of powdery mildew resistance in cultivated V. vinifera subsp. sativa. The first was to evaluate a large collection of cultivated V. vinifera germplasm from Central Asia to identify additional powdery mildew resistant accessions using simple sequence repeats markers linked to the powdery mildew resistance locus Ren1 on chromosome 13. We speculated that since there was breeding for powdery mildew resistance in Central Asia before the mid-1900s, there might be undocumented resistant selections, resistant parental material or new sources of resistant germplasm from this region. We analyzed accessions maintained in two of the world’s largest grape germplasm repositories, and the Department of Viticulture and Enology, and Foundation Plant Services at the University of Califtornia, Davis. The second objective was to evaluate the powdery mildew resistance in a range of Chinese ViThis species and in accessions of V. vinifera subsp. sylvestris collected from the regions of grapevine domestication, to determine if they were resistant to the disease and to identify potential contributors of powdery mildew resistance in Central Asiangrape cultivars. An analysis of population structure and diversity was conducted to obtain a global perspective on the mechanisms of domestication, and gene flow from accessions of wild Chinese species and V. vinifera subsp. sylvestris in an effort to determine the source of powdery mildew resistance detected in Central Asian V. vinifera cultivars. North American species and complex hybrids of these species resistant to powdery mildew were also included to determine their possible role in the resistance detected in Central Asian V. vinifera accessions. The third objective was to unravel potential parent-progeny relationships using kinship analysis to broaden the family of powdery mildew resistant cultivars for use by grape breeders.This study utilized grape accessions maintained in two of the world’s largest germplasm repositories: the INRA Domaine de Vassal collection; and combined collections housed at Davis, Califtornia . Most of the University of Califtornia, Davis accessions were collected by Harold P. Olmo during germplasm acquisition travels in 1948 . Many of the 559 accessions tested shared identical marker profiles, thus suggesting possible cases of synonymies were observed within and among the samples from the two collections .