Decline in vitality of propagules of Phytophthora pluvialis and Phytophthora kernoviae and their inability to contaminate or colonise bark and sapwood in Pinus radiata export log simulation studies

Abstract

Background: Phytophthora pluvialis Reeser, W.L. Sutton & E.M. Hansen is the cause of a newly described disease, red needle cast, in certain stands of Pinus radiata D. Don in New Zealand that experience periodic foliage browning, while Phytophthora kernoviae Brasier, Beales & Kirk is also infrequently isolated from symptomatic needles. Methods: Studies were undertaken to test the possibility that these species may be transported on pine logs either as superficial contaminants or as colonists of bark or wood.Results: Pine-needle baiting found no evidence of Phytophthora species in bark samples or aqueous bark washes from stems of 603 symptomatic trees in 17 affected stands implying that survival after natural deposition of sporangia or zoospores is low or absent. The persistence of zoospores or oospores was evaluated at intervals after applying them at artificially high surface densities to the bark on log segments and incubating at five temperatures between 15°C and 35°C in the laboratory. The ability to re-isolate Phytophthora kernoviae decreased with time and increasing temperature, but this species was still obtained at low frequencies after 4 weeks at 15°C and 20°C following treatment with oospores of Phytophthora kernoviae. Phytophthora pluvialis could not be isolated under any conditions of time or temperature tested. Percentage vitality of oospores of both species as determined using tetrazolium bromide vital staining also decreased with time, although some oospores of both species remained alive after 4 weeks at all temperatures tested. In a further study to test potential log colonisation, Phytophthora spp. were not isolated from bark or xylem at or near points where zoospores, oospores or mycelium of either species were applied to the bark or sapwood of pine segments and incubated for 6 weeks under ambient or humid conditions at 17°C. Conclusion:The results of these studies suggest that occurrence of Phytophthora kernoviae or Phytophthora pluvialis on export logs from affected stands is negligible. In addition, although some remained alive, the substantial decline in vitality among artificially applied oospores implies that the survival of any few that may be naturally present on logs is likely to be slight. Based on the evidence from this work there appears to be little risk of transporting these Phytophthora species on New Zealand radiata pine logs.

Pathogenicity of Phytophthora pluvialis to Pinus radiata and its relation with red needle cast disease in New Zealand

“…Field-based observations have indicated that the pathogen infects only the needle blade with shed needles, rain splash and air movement being the most likely drivers of pathogen movement. The potential for subsequent spread of the pathogen through the trade of export logs has been demonstrated to be negligible (Hood et al 2014). These findings are consistent to those made for Phytophthora pinifolia on Pinus radiata by Ahumada et al (2012) who similarly concluded that there was no evidence of pathogens surviving on sawn green timber.…”

“…Re-isolation confirmed that Phytophthora pluvialis is the cause of red needle cast in Pinus radiata. Sampling by pathologists targeting fresh lesion material resulted in the isolation of Phytophthora from 76% of characteristic needle lesions (Hood et al 2014). Recovery of P. pluvialis from symptomatic needles was variable between collections but clearly decreased with time after infection had occurred.…”

First evidence of genetic-based tolerance to red needle cast caused by Phytophthora pluvialis in radiata pine

“…Red needle cast (RNC) is a new needle disease caused by Phytophthora pluvialis Reeser, Sutton & E Hansen that causes defoliation of Pinus radiata D. Don (radiata pine) under conditions favourable to the development of the disease Hansen et al 2012;Hood et al 2014;Reeser et al 2013). The disease has become well established in certain areas of New Zealand and has the potential to cause production loss through needle shed and reduced growth.…”

Management of red needle cast caused by <i>Phytophthora pluvialis</i> a new disease of radiata pine in New Zealand

“…Phytophthora pluvialis is a foliar pathogen isolated from needles of Pinus radiata, and sporangia that produce zoospores have been observed on needles under laboratory and field conditions. Extensive attempts to isolate or recover Phytophthora pluvialis from other plant tissues have been unsuccessful (Hood et al 2014). Likewise attempts to artificially colonise plant material other than needles have been unsuccessful.…”

“…Likewise attempts to artificially colonise plant material other than needles have been unsuccessful. These findings, in conjunction with studies of spore survival on logs at different temperatures over time, have been used for risk assessments (Hood et al 2014). From a market access perspective, the possibility of transmitting Phytophthora pluvialis from New Zealand on radiata pine logs is negligible.…”