Population diversity, sources and evaluation of metalaxyl-based management of pythium damping-off of cucumber in Oman

Al-Sa'di, Abdullah Mohammed (2007). Population diversity, sources and evaluation of metalaxyl-based management of pythium damping-off of cucumber in Oman PhD Thesis, School of Integrative Biology, The University of Queensland.

       
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Author Al-Sa'di, Abdullah Mohammed
Thesis Title Population diversity, sources and evaluation of metalaxyl-based management of pythium damping-off of cucumber in Oman
School, Centre or Institute School of Integrative Biology
Institution The University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Elizabeth Aitken
Collection year 2008
Subjects L
Abstract/Summary This study was undertaken to identify Pythium species causing damping-off of cucumber in Oman, to characterize diversity in Pythium species, to identify the potential sources of Pythium inoculum in greenhouses and to evaluate the efficacy of metalaxyl in management of damping-off disease. A survey in 2004 and 2005 characterized Pythium aphanidermatum (77% of isolations), P. spinosum (22%) and P. splendens (1%) to be the pathogens associated with damping-off of cucumber in Oman, with the latter two being first reports in Oman. P. aphanidermatum was isolated from all districts in Oman during different times of the year and was generally aggressive at 25 and 30JC. However, the P. spinosum distribution was limited to seven out of 13 districts surveyed, isolated more during cooler months of the year and was found to be more aggressive at relatively lower temperatures (25JC). The second part of the thesis dealt with characterizing genetic diversity using Amplified Fragment Length Polymorphisms (AFLP) fingerprinting in the predominant species P. aphanidermatum. Isolates of P. aphanidermatum from muskmelon in Oman and from different hosts in the USA and Norway were included in the study. The study revealed that P. aphanidermatum populations in greenhouses in Oman consist of a large number of different phenotypes with generally a high level of genetic similarity (mean = 96.6%), which suggested the introduction of P. aphanidermatum in greenhouses via multiple sources. A high number of identical phenotypes were recovered between muskmelon and cucumber fields in the same area compared to the small number of identical phenotypes recovered across different regions. Quantitative population analysis using AMOVA and FST values supported the hypothesis of frequent movement of P. aphanidermatum inoculum between fields in the same area compared to limited circulation between geographically separated areas. Variation was also found between different isolates in aggressiveness on cucumber and muskmelon, but no host specialization was found. Investigating genetic diversity in 24 isolates of P. spinosum from different regions in Oman revealed that the population consists of a small number of phenotypes sharing a very high level of genetic similarity (L 99%). This was suggested to be due to the recent introduction of P. spinosum in Oman via a common source. All P. aphanidermatum and P. spinosum isolates were found to be highly sensitive to metalaxyl (EC50 < 1 Ng ml-1), which indicates that no resistance had developed among populations of these two species to metalaxyl. Further experiments into the potential sources of Pythium species in greenhouses in Oman revealed that Pythium species can be introduced into greenhouses via contaminated soil previously cultivated with crops other than cucumber on which greenhouses are established. Other avenues for the introduction of inoculum include uncultivated soil introduced into greenhouses, soil left in the greenhouse after excluding the top 30 to 60 cm of old soil, potting mixtures and soil adhering to irrigation pipes, growers’ shoes and cultivation equipment. No Pythium inoculum was detected in irrigation water or organic fertilizers. Correlation analysis based on existing field observations indicated that the practice of soil replacement is efficient in reducing inoculum density and damping-off levels in greenhouses (P < 0.01). The final part of this thesis dealt with identifying factors associated with decreased efficacy of metalaxyl to manage damping-off of cucumber. The study evaluated the contribution of three main factors to the problem: development of resistance to metalaxyl in the pathogen population, timing of metalaxyl application by growers and the rate of metalaxyl biodegradation in greenhouse soils. A survey over six growing seasons showed that growers lost up to 14.6% of seedlings under the use of metalaxyl, with mortality being high in the warmer seasons compared to the cooler seasons. All Pythium isolates were found to be highly sensitive to metalaxyl (EC50 < 1 Ng ml-1). Over 57% of growers in Oman were found to delay metalaxyl application in greenhouses until up to four days following transplanting; this action significantly increased seedling mortality during the first week following transplanting compared with metalaxyl application before transplanting. Pre-transplanting treatment with metalaxyl was found in greenhouse experiments to be superior in reducing seedling mortality during this period compared to application following transplanting. However, the re-appearance of damping-off symptoms 9 to 14 days after initial suppression at the time of transplanting was found to be associated with a rapid rate of metalaxyl biodegradation in greenhouses experiencing re-appearance of damping-off symptoms. The half life of metalaxyl in soil was found to have a negative correlation with the number of seasons of metalaxyl use, where the half life of metalaxyl dropped from 93 days in greenhouse soil with no previous history of metalaxyl use to 14 days in greenhouse soil in which metalaxyl was used for eight consecutive seasons. Following up the first treatment of metalaxyl at the time of transplanting with a second treatment eight days after transplanting was found to reduce the appearance of the second phase of damping-off in greenhouses suffering from enhanced rate of metalaxyl biodegradation. Information generated from these studies helped to understand the nature of dampingoff of cucumber in greenhouses in Oman and provided the foundation for the development and implementation of management strategies which would enable growers to minimize the incidence of damping-off.

 
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Created: Fri, 21 Nov 2008, 15:27:12 EST