Newhook (1959) showed that this fungus was abundant in many New Zealand soils in indigenous and exotic forest. In forests where soil conditions were favourable to tree growth it did not appear to affect the growth of trees, but if soil conditions became waterlogged then the fungus caused considerable rootlet death. Hepting and Newhook (1962) showed the similarity of this disease condition in five species of Pinus in Waipoua, North Auckland, with little leaf disease of the same Pinus spp. in North America. Bassett (1961) reported considerable damage in conifer nurseries. Phytophthora cinnamomi may be extremely damaging to a number of ornamental shrubs such as Banksia, Camellia, Isopogon, Leucospermum, and Protea. In nurseries it can be of considerable economic importance; soil drenching with captan sometimes reduces the effect of the disease.

Type: Radicicolous Fungi; Description: Colonies on agar with a rosette pattern, mycelium coralloid with abundant hyphal swellings which are typically globose, 30–60 μm in diameter, in clusters. Chlamydospores produced by some, but not all isolates, globose to pyriform, >35 μm in diameter. Sporangia terminal, proliferating internally, ovoid to ellipsoid, 40–85 × 25–40 μm, non-papillate. Oogonia produced in compatible paired cultures, globose, 21–58 μm in diameter, hyaline to yellow brown, smooth. Antheridia amphigynous, some with two cells. Oospores plerotic, globose, 19–54 μm in diameter, hyaline to yellow brown.

Distribution: Northland, Auckland, Waikato, Bay of Plenty, Taranaki, Taupo, Rangitikei, Wanganui, Wellington, Gisborne, Hawkes Bay, Wairarapa, Nelson, Buller, Westland, Marlborough, North Canterbury, Mid Canterbury, Dunedin, Southland.; 1st Record: Newhook (1958).

Significance: The significance of P. cinnamomi in the New Zealand forest industry has been the subject of vigorous and occasionally acrimonious debate in the past. It is agreed that the fungus is an important pathogen of tree seedlings in nurseries established on heavy clay soils prone to waterlogging. In such nurseries, severe root-rot of seedlings of Acacia baileyana, A. verticillata, Betula verrucosa, Eucalyptus ficifolia, Metrosideros excelsa, and Pinus radiata has been attributed to P. cinnamomi (Gilmour 1966a). Bassett (1961) reported that P. cinnamomi was associated with root-rot and collar canker of 2-year-old seedlings of Larix decidua and Pseudotsuga menziesii. The status of the fungus as a pathogen of older (>3-year-old) trees in New Zealand is not clear. Phytophthora cinnamomi has been recovered from soil beneath many exotic and indigenous plant communities throughout New Zealand. With one exception, none of these communities showed symptoms of ill health attributable to infection by the fungus (Newhook & Podger 1972). The exception involved certain farm shelterbelts in which episodic mortality was associated with P. cinnamomi. In 1953–54 and 1955–56, sudden death of 20-year-old and older trees of Chamaecyparis lawsoniana, Cupressus macrocarpa, and Pinus radiata occurred in farm shelterbelts growing on heavy clay soils in the Northland region (Newhook 1959). There had been droughts during the previous summers, followed by abnormally wet periods in autumn, winter, and spring (Sutherland et al. 1959). Mortality under similar climatic and edaphic conditions was reported from Hawkes Bay in the 1970s (New Zealand Forest Research Institute, unpublished data). Affected shelterbelts were characterised by soil compaction associated with the camping of farm animals under the trees. As a consequence the roots had poor mycorrhizal development. It has been suggested that under these conditions heavy rainfall and soil saturation provided ideal conditions for rootlet infection by P. cinnamomi while reducing rootlet regeneration capacity. The imbalance between rootlet death and rootlet regeneration led to mortality (Newhook & Podger 1972). The specific combination of conditions is not likely to occur in extensive exotic plantations or indigenous forests where P. cinnamomi, although present, causes no observable damage.; Host(s): Abelia schumannii, Abies sp., Acacia baileyana, A. verticillata, Acmena smithii, Actinidia deliciosa, Actinostrobus sp., Agathis australis, Astelia trinervia, Banksia sp., Bauera sessiliflora, Betula pendula, Boronia megastigma, Brachyglottis kirkii, Calluna vulgaris, Camellia japonica, Carpodetus serratus, Carya illinoinensis, Castanea sativa, Casuarina cunninghamiana, C. glauca, Chamaecyparis lawsoniana, Coprosma robusta, Coriaria sarmentosa, Corokia buddleioides, Cupressus macrocarpa, Cyathodes juniperina, Daphne odora, Dryandra sp., Elaeocarpus hookerianus, Erica bowieana, E. canaliculata, E. lusitanica, Eucalyptus ficifolia, Fatsia japonica, Forsythia sp., Fuchsia excorticata, Gaultheria antipoda, Geniostoma rupestre var. ligustrifolium, Gevuina avellana, Griselinia lucida, Hakea sericea, Hebe topiaria, Juglans regia, Juniperus communis, J. squamata, Knightia excelsa, Larix decidua, L. kaempferi, Leptospermum scoparium, Leucadendron adscendens, L. argenteum, L. decorum, Leucopogon fasciculatus, Leucospermum sp., Lhotskya alpestris, Lophomyrtus obcordata, Lupinus angustifolius, Lycopodium deuterodensum, Macadamia integrifolia, M. tetraphylla, Melicytus macrophyllus, Meryta sinclairii, Metrosideros excelsa, Myrsine australis, Nestegis lanceolata, Nothofagus fusca, N. menziesii, N. solandri var. cliffortioides, Olearia rani, Passiflora edulis, Persea americana, Phormium tenax, Phyllocladus alpinus, P. trichomanoides, Pieris japonica, Pinus echinata, P. palustris, P. radiata, P. taeda, Platanus sp., Protea sp., Prumnopitys ferruginea, Prunus persica, Pseudopanax arboreus, P. crassifolius, Pseudotsuga menziesii, Rhododendron indicum, Rosmarinus officinalis, Rubus idaeus, Sciadopitys verticillata, Serruria sp., Taxus baccata, Telopea speciosissima, Thryptomene calycina, Vaccinium corymbosum, Vitis rupestris, V. vinifera.