| Watermelon (Citrullus lanatus (Thunb) Matsum & Nakai)is an important horticulture and garden crop falling the scope of top 10 fruits particular in the predominant summer fruits with top cultivation area and total yield in the world, which originated from equator vicinity of Africa. Watermelon crop was moved and cultivated in China via minor Asia and northwestern Asia about 4-5 century. China possesses the biggest area field of watermelon production in the world, which covers over 45% of total cultivation area and yield. In 2006, China sees 1.78 million hectare of watermelon cultivation area and 6.26 million tons of yields.However, watermelon production has been restricted due to long-term monoculture system. Decreased seed germination rate, slow down of germination, no stretching and slow emergence of hypocotyledon, stunt, lodging and easily-death of seedling, weakening and dwarfing of cane, yellowish leaf, easy-disease, particular in fusarium wilt of watermelon which leads to watermelon plant death and decreased yield and quality, generally with loss of 15-30%, and 50-85% in heavily Fusarium-infested field even zero harvest. Fusarium wilt of watermelon, thus, has been the key limiting factor in watermelon production in China.From view point of host-pathogen-environment ecological interaction triangle, culturable microflora changes, microbial biodiversity changes, interactions between Fusarium oxysporum f. sp. niveum and other soil microorganisms, effects of fusarial toxin fusaric acid on nitrogen metabolism, cell transmembrane potential, antioxidase activity, pathogenesis-related proteins and photosynthesis of seedling, effect of watermelon root exudates and decaying watermelon tissues and their main allelopathic phenolic acids on the growth and virulence factors of Fusarium oxysporum f. sp. niveum, effects of ecological and environmental soil actors on growth of Fusarium oxysporum f. sp. niveum, microbial biodiversity and dominant population in Fusarium-infested soil under fallow condition, biological control of fusarium wilt of watermelon by a bio-organic fertilizer were investigated in this laboratory study. Related results or conclusions are listed as follows.A total of 52 strains was identified from 243 isolates via careful identification and confirming of culture and physico-biochemical characteristics and microscopical evaluation, which were isolated from 5-year-watermelon-cultivated and watermelon-uncultivated field soil in plastic greenhouse in Songjiang, Shanghai. Of 52 strains, 12 strains were bacteria, 14 strains were actinomycetes, 20 strains were fungi, 6 strains were Fusarium spp. 12 strains of bacteria were identified from watermelon-uncultivated soil, while 11 from watermelon-cultivated soil, which was 1 strain less than control (watermelon-uncultivated soil). 12 strains of actinomycetes were identified from watermelon-uncultivated soil and watermelon-cultivated soil. 18 strains of fungi were identified from watermelon-uncultivated soil, while 20 from watermelon-cultivated soil, which were 2 strains more than control. 5 strains of Fusarium spp. were identified from watermelon-uncultivated soil, while 6 from watermelon-cultivated soil, which was 1 strain more than control. A new pathogen Fusarium oxysporum f.sp. niveum was isolate and identified from wilted watermelon plant grown in fuarium-infested soil by investigating physiological and biochemical, morphological, host-specific infection and DNA sequencing and blast in Genbank. After long-term monoculture of watermelon, significant changes were observed in microbial richness, while almost unchanged in microbial abundance. Though total number of bacteria decreased, the number of actinomycetes, fungi and Fusarium spp. increased, which therefore led to an almost stability of total microbial populations in the soil. An 83.3% of bacteria, 85.7% of actinomycetes, 31.6% of fungi and 20.0% of Fusarium spp. were inhibited by Fusarium oxysporum f.sp. niveum on PDA plates. Meanwhile Fusarium oxysporum f.sp. niveum was also inhibited by soil autochthonous antagonistic bacteria, such as Bacillus subtilis, Psedomonas putida, antagonistic actinomycetes, e.g. Spirillospora Couch (1963) and Streptomyces longispororuber Krass (1941), antagonistic fungi, e.g. Penicillum Simplicissimum(Oud.)Thom, Penicillium nigricans Bainier, Rhizoctonia DC.EX Fr., Stysanus stemonite s(Pers.) Corda, Cephalosporium Corda, Westerdykella Stolk, Aspergillus effusus Tiraboschi on plates. However, antagonistic effect of bacteria was much smaller than actinomycetes and fungi that were responsible for a decrease of total bacterial population number and an increase of total actinomycetes and fungus population number in watermelon-cultivated soil. Effect of Fusarium oxysporum f.sp. niveum on bacteria and actinomycetes was primarily contributed to nutrient competition, while effect on fungi and Fusarium spp. was nutrient competition and secondary metabolites antagonism.Fusarium phytotoxin fusaric acid collected from Fusarium oxysporum f. sp. niveum was added into the media to evaluate its effect on the growth of watermelon seed and seedlings in laboratory study. The results demonstrated that Fusarium oxysporum f. sp. niveum caused watermelon seedlings wilt with the highest incidence rate of 86% during 30 days of inoculation. The mycotoxin suppressed the germination of watermelon seeds with the lowest germination rate of 24% at the duration of 10 days. Resulting root dysfunction, decrease of uptake of water and ammonium, repression of root growth, reduction of 23% of plant biomass-of watermelon was found. Simultaneously, 72.3% of the content of leaves' chlorophyll and over 90% of relative photosynthesis parameters were observed. The greatly decreased activity of antioxidases and pathogenesis-related proteins led to the reduced stress-resistance and the resulting cell membrane lysis. Furthermore, possible new mechanism of nitrogen metabolism disorder in watermelon leaf was found. Finally the seedlings were dead due to disorder functions of those cells of watermelon seedlings.The purpose of this research was to evaluate the influences of root exudates produced by different genotypic watermelons on their pathogen Fusarium oxysporum f. sp. niveum (FON) in a laboratory. Root exudates from both susceptible (zaojia 84-24) and resistant (jingxing 1 and yuxing 3) cultivars stimulated growth of FON at low concentrations (<50 mg. L-1), but exudates from resistant cultivars inhibited FON at high concentration. Mycotoxin production was increased to a greater extent in the presence of exudates from susceptible cultivars than those from resistant cultivars. After the flowering stage, root exudates promoted greater production of mycotoxin by FON than those collected before flowering. The results suggested that root exudates from susceptible watermelon cultivars more greatly stimulated production of mycotoxins and activities of enzymes (protease, pectinase, cellulase and amylase) by FON than those from resistant cultivars. Further HPLC analysis showed that higher ratios of antifungal phenolic acids (p-hydroxybenzoic, phthalic, gallic, coumaric, cinnamic, sinapic, ferulic, salicylic acids) against fungus-stimulating phenolic acids (vanillic, syringic, caffeic acids) were found in resistant cultivars and after flowering than those from susceptible ones and before flowering.Extracts of decaying watermelon tissues were added into media to investigate the allelopathic impact of decaying watermelon tissues on Fusarium oxysporum f. sp. niveum (FON) in a laboratory study. Twelve types of phenolic acids from different decaying watermelon cultivars (Zaojia 82-24 and Jingxing 1) with different resistance to FON were analyzed by using HPLC. The results demonstrated that types and quantities of chemical components released from decaying susceptible watermelon tissues (Zaojia 84-24) were less than from decaying resistant watermelon tissues (Jingxing 1). Much more content of antifungal phenolic acids from decaying resistant watermelon tissues, with a ratio of antifungal phenolic acids to fungus-stimulation of 29.2:1, was found than from decaying susceptible tissues, with a ratio of antifungal phenolic acids to fungus-stimulation phenolic acids of 8.4:1. Decaying resistant watermelon tissues extracts inhibited the growth and conidia formation of FON, while decaying susceptible extracts stimulated the growth and conidia formation. Furthermore, Extracts from decaying resistant watermelon tissues stimulated virulence factors much less than from susceptible tissues. We conclude that the decaying plants extracts from susceptible watermelon cultivars stimulated FON due to its higher content of fungus-stimulation phenolic acids, while the decaying resistant watermelon tissues inhibited FON because of its higher content of antifungal phenolic acids. This implies that phenolic acids produced in decaying plant residues in soil would induce or inhibit pathogens in plant-microbe interactions, responsible for fusarium wilt of watermelon.Twelve types of phenolic acids (benzoic, p-hydroxybenzoic, coumaric, cinnamic, ferulic, gallic, caffeic, sinapic, salicylic, vanillic and tannic acids) were added into media to incubate Fusarium oxysporum f. sp. niveum to investigate their allelopathic effect on Fusarium oxysporum f. sp. niveum in the present laboratory study. Results suggested most of the phenolic acids (benzoic, p-hydroxybenzoic, coumaric, cinnamic, ferulic, gallic, caffeic, sinapic, salicylic acids) inhibited the growth and conidium germination of Fusarium oxysporum f. sp. niveum, while little phenolic acids (vanillic and tannic acids) stimulated the growth and conidium germination of the fungus. The phenolic acids that inhibited the growth of the fungus would not certainly inhibit its virulence factors, while those acids stimulated the growth of the fungus would not unavoidably stimulate the virulence factors, which would be contributed to different formation mechanism and process of mycelia growth and virulence and different action.To understand the natural remediation process of Fusarium-infested watermelon soil, microflora and microbial biodiversity changes and dominant populations and main causal factor Fusarium oxysporum f. sp. niveum variations in Fusarium-infested watermelon soil were investigated by using microcosoms of sickness soil during continuous 3-year open fallow. Results showed that microbial biodiversity in Fusarium-infested watermelon soil were significantly varied while soil microorganism ecological balance was broken. After continuous 3-year natural fallow remediation, soil microbial biodiversity was gradually restored. Dominant bacterial populations abundance was increased by 118—177%. The total number of actinomycetes, fungi and pathogenic fungus Fusarium oxysporum f. sp. niveum was decreased with the abundance only 68—77%, 63—67%, 60—85%, and 50% as little as the first year. The ratio of bacteria: actinomycetes : fungi: Fusarium spp. in soil was changed from 24000: 100: 4: 1 before fallow to 57000: 100: 3.5: 1 after fallow but yet far from 560000: 400: 8: 1 in normal soil. The Fusarium-infested watermelon soil was gradually changed from fungus type to bacterial type, which indicates the restoring microbial ecological balance of the sickness soil will lead to a normal soil production and ecological function.Long-term and excessive use of chemical fungicides have led to an increase of pathogen resistance against chemicals and the decreased effect on Fusarium oxysporum f. sp. niveum, a causal agent responsible for Fusarium wilt of watermelon, and threatening sustainable agriculture development and ecological equilibrium due to residue fungicides pollution in environment and harmful to human health. It is necessary to develop new control methods with high performance, low toxic and zero-pollution. In the current study, 12 isolates with antagonistic action on PDA were tested in soil-sterilized to compare their further antagonistic behavior in growth chamber and greenhouse. Two isolates, Panebacillus polymixa and Trichoderma harzianum, confirmed antifungal activity against F. oxysproum f. sp. niveum by growth chamber evaluation, were mixed with organic fertilizer (fermented oilseed cake) to further investigate the Fusarium suppression of the mixture (bio-organic fertilizer) in greenhouse. After 63 days of the mixture use in pot fusarium-infested soil, Fusarium wilt of watermelon was inhibited by organic and bio-organic fertilizer to an extent, particularly for treatments of 0.5%OF and 0.5%BOF, with suppressiveness rate of 35.7% and 64.5% respectively. We believe that the bio-organic fertilizer increased the activity of plant anti-stress enzymes and induced the systemic acquired resistance to inhibit the pathogen by detecting MDA content and antioxiases activity and pathogenesis-related proteins in root, stem and leaf in watermelon plant.
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