Potential for eradication of the exotic plant pathogens Phytophthora kernoviae and Phytophthora ramorum during composting

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Noble, Ralph ; Blackburn, J. ; Thorp, G. ; Dobrovin-Pennington, Andreja ; Pietravalle, S. ; Kerins, G. ; Allnutt, T. R. ; Henry, C. M. (2011)

Temperature and exposure time effects on Phytophthora kernoviae and Phytophthora ramorum viability were examined in flasks of compost and in a large-scale composting system containing plant waste. Cellophane, rhododendron leaf and peat-based inoculum of P. kernoviae and P. ramorum isolates were used in flasks; naturally infected leaves were inserted into a large-scale system. Exposures of 5 and 10 days respectively at a mean temperature of 35 degrees C in flask and large-scale composts reduced P. kernoviae and P. ramorum inocula to below detection limits using semi-selective culturing. Although P. ramorum was undetectable after a 1-day exposure of inoculum to compost at 40 degrees C in flasks, it survived on leaves exposed to a mean temperature of 40.9 degrees C for 5 days in a large-scale composting system. No survival of P. ramorum was detected after exposure of infected leaves for 5 days to a mean temperature of >= 41.9 degrees C (32.8 degrees C for P. kernoviae) or for 10 days at >= 31.8 degrees C (25.9 degrees C for Phytophthora pseudosyringae on infected bilberry stems) in large-scale systems. Fitted survival probabilities of P. ramorum on infected leaves exposed in a large-scale system for 5 days at 45 degrees C or for 10 days at 35 degrees C were <3%, for an average initial infection level of leaves of 59.2%. RNA quantification to measure viability was shown to be unreliable in environments that favour RNA preservation: high levels of ITS1 RNA were recovered from P. kernoviae- and P. ramorum-infected leaves exposed to composting plant wastes at >53 degrees C, when all culture results were negative.
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