| Guzmania ‘Denise' is very popular among the consumers due to its beautiful spike and varied plant type. In the warm and humid environment, stem rot, root rot, heart rot, and rhizome rot were always occurred to this plant with 20% to 50% economic losses. In this study, the tissue isolation method and pathogenicity test were carried out to collect and confirm the pathogens from the infected plant, Guzmania ‘Denise', which were cultivated in the ornament plantations in Heshan, Guangdong province. Morphological characteristics comparation and molecular biological methods were applied to identify the pathogen. At the same time, the effective fungicides against the pathogen and the host susceptibility to the fungicides were tested in greenhouse. The main results are summarized as following:1. Fifty-four fungal isolates and three bacterial strains were obtained through tissue isolation method from the diseased plants. Then all the fungal cultures were classified preliminarily as 20 strains according to the colony morphological characteristics under the microscopy. 35 isolates were obtained through the single-spore isolation method.2. The pathogenicity test of 35 isolates and three bacterial strains were carried out on detached leaves and in vivo health host plants. The test results showed that all 35 fungal isolates possessed strong pathogenicity to the host with a slight degree of difference among them, and the three bacterial strains had no pathogenicity to the host plants.3. A phylogenetic tree was constructed with nucleic acid sequences from all 35 isolates and the type strains as well as interpretation strains. The phylogenetic analysis indicated that all 35 strains belonged to a fungal genus with the same group and the same branch in this tree, and the group gathered together with Pythium myriotylum Drechsler and their bootstrap reached ninety-eight percents, a high similarity among them.4. The pathogen morphology was observed on PDA medium at 26? with the representative strain from all 35 strains. The results showed that the pathogen was characterized with hypha width 3.6- 9.2 ?m, sporangia swollen as finger or rosette with the average size 228.4 × 13.3?m, oogonia round or nearly spherical, smooth, and occasionally proliferate, the diameter 27.50- 31.25 ?m, the rod-like or hook antheridium 10.91×5.65 ?m, oospores spherical smooth 23- 30 ?m. Basing on the comprehensive comparison of morphological characteristics, pathogenicity test and molecular phylogenetic analysis, the right pathogen of the disease was identified as Pythium myriotylum Drechsler.5. To obtain effective fungicides against the pathogen Pythium myriotylum, 12 kinds of fungicides current on the market were chosen to do the indoor fungicides screening experiment. The results showed that 25% pyraclostrobin EC, 30% hymexazol AP, 30% benzoyl propiconazole EC, 72% mancozeb cymoxanil WP and 58% mancozeb metalaxyl WP had high inhibitory effects and reliability on Pythium myriotylum and its EC50 values were 50.73078, 10.95828, 28.27244, 10.7134, 14.54234, respectively and correlation coefficient were 0.788, 0.989, 0.997, 0.877, 0.977, separately. Besides, the slope of toxicity regression equation were 1.54711, 0.72033, 1.73296, 2.59574, 1.02112, separately, which meant Pythium myriotylum was sensitive to the above five fungicides. All the results indicated that the right five fungicides were effective to the pathogen Pythium myriotylum.6. In order to avoid the phytotoxicity caused by the fungicides on the hosts, plant sensitivity assay to fungicides were carried out in greenhouse. The results showed that the three fungicides, 25% pyraclostrobin EC, 30% hymexazol AP, 30% difenoconazole propiconazole EC, had no poison on the host plants, even at the concentrations of 500 times, 1000 times and 1500 times of each fungicide. |
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