Abstract
BackgroundPhytophthora 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.MethodsStudies 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.ResultsPine-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.ConclusionThe 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.
Highlights
Natural occurrence of Phytophthora propagules on the bark surface Sites The bark on Pinus radiata trees affected by red needle cast was examined for evidence of viable propagules of Phytophthora pluvialis and Phytophthora kernoviae deposited naturally on the prospective log surface
All showed moderate to high levels of browning in the lower third of the canopy and characteristic lesions were present on a selection of needles collected from 429 of the trees. Confirmation that they were affected by red needle cast was furnished in the laboratory by the isolation of Phytophthora spp. from 78% of the needle samples after surface sterilising in 70% ethanol for 30 seconds, rinsing in two washes of sterile deionised water for 30 seconds, plating onto P10ARP agar selective medium (Jeffers and Martin 1986, but with pentachloronitrobenzene reduced to 25 mg L-1), and incubating at 17°C
No Phytophthora species were obtained from segments treated with Phytophthora kernoviae sporangia and zoospores (Treatment B, unheated), Phytophthora pluvialis oospores (Treatment C, heated), or the uninoculated control (Table 1)
Summary
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 stands in parts of New Zealand since at least 2008 (Dick et al 2014). This discoloration has appeared only in some years mainly in plantations in the northern, eastern, and central parts of the North Island (i.e. Northland, Auckland, Coromandel, Bay of Plenty, Taupo, Gisborne) and in the northern South Island (i.e. Nelson and Marlborough regions) (Crosby et al 1988). The disorder is known as red needle cast from the colour that develops over the crowns of affected trees. Crowns may remain brown until November (late spring) when diseased foliage is replaced by the new season’s growth
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