Composting is an effective treatment option for sanitization of Phytophthora ramorum-infected plant material

Abstract

Aims: To determine the effects of heat and composting treatments on the viability of the plant pathogen Phytophthora ramorum grown on both artificial and various natural substrates. Methods and Results: Phytophthora ramorum was grown on V8 agar, inoculated on bay laurel leaves (Umbellularia californica) and on woody tissues of coast live oak (Quercus agrifolia). Effects on growth, viability and survival were measured as a result of treatment in ovens and compost piles. Direct plating onto PARP medium and pear‐baiting techniques were used to determine post‐treatment viability. No P. ramorum was recovered at the end of the composting process, regardless of the isolation technique used. By using a PCR assay designed to detect the DNA of P. ramorum, we were able to conclude the pathogen was absent from mature compost and not merely suppressed or dormant. Conclusions: Some heat and composting treatments eliminate P. ramorum to lower than detectable levels on all substrates tested. Significance and Impact of the Study: Composting is an effective treatment option for sanitization of P. ramorum‐infected plant material. Assaying for pathogen viability in compost requires a direct test capable of differentiating between pathogen suppression and pathogen elimination.

Scientific Opinion on the Pest Risk Analysis onPhytophthora ramorumprepared by the FP6 project RAPRA

“…There have been early reports of the ability of P. ramorum to grow over a wide range of temperatures (Werres et al, 2001;Harnik et al, 2004;Swain et al, 2006). This is confirmed by an experimental study of P. ramorum infection (using the NA1 isolate) of Cunningham's White Rhododendron, with P. ramorum able to infect this host over a wide range of temperatures and moisture levels Similar results are obtained from a study of the susceptibility to P. ramorum and sporulation potential of 25 understory plant species growing in Eastern North America .…”

“…RAPRA already identifies some potential sporulating hosts, however no clear discussion is provided on how to effectively deal with infected plant material. Two papers have addressed sanitation in California: one dealt with the ability of heat and heat vacuum treatments to reduce viability of foliar infection to zero (Harnik et al, 2004), while a second paper showed that composting following EPA guidelines successfully eliminates all traces of P. ramorum, even in cases where massive infection of foliage was tested (Swain et al, 2006). Risk when moving infected plant parts needs to be considered and it could be mitigated by adopting these proved techniques or investigating alternative approaches.…”

“…Composting has been considered as an effective treatment option for sanitation of P. ramoruminfected plant material (Garbelotto, 2003;Swain et al, 2002Swain et al, , 2006Aveskamp and Wingelaar, 2005).…”

“…• an important issue is that infection remains viable for at least 3 months on foliage (Swain et al, 2006) • the issues of recycling and composting need to be addressed for this pathway…”

Phytophthora ramorumcan survive introduction into finished compost

“…The composting process can eradicate even the toughest resting propagules commonly produced by P. ramorum if the process is conducted according to U.S. Environmental Protection Agency (EPA) PFRP guidelines (Swain et al 2006).…”