Root rot caused by Phytophthora species
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Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.
From Scion publication Forest Research Bulletin 220,
An Introduction to The Diseases of Forest and Amenity Trees in New Zealand,
G.S.Ridley and M.A. Dick 2001.
Species: Phytophthora cactorum (Fig. 3), P. cinnamomi, P. cryptogea, P. megasperma, and P. heveae (these are the most common species) (Oomycete)
Country of origin: Phytophthora spp. are a very common component of the soil microflora around the world.
Host(s): Most Phytophthora spp. have a very wide host range. Their commercially important host species include Eucalyptus spp., Larix decidua, and seedlings of Pinus radiata and Pseudotsuga menziesii. Phytophthora heveae has been reported as a pathogen of Agathis australis.
Disease development: All Phytophthora species require presence of free water in the soil for spread and for successful attack on roots. They are thus a problem only on heavy clay soils with impeded drainage or on better-drained soils subjected to prolonged waterlogging. In P. radiata plantations, Phytophthora spp. are of little consequence; mortality of P. radiata has been recorded only in narrow farm shelterbelts, particularly in those on the heavy soils of North Auckland and Hawke's Bay. Disease development in the shelterbelts is gradual as the fungi cause rot of the feeding rootlets and eventually the replacement rate of the rootlets falls below the rate of loss and the tree succumbs. Phytophthora root rot in a nursery develops more rapidly because of the limited root regeneration capacity of seedlings. Species of Eucalyptus, particularly E. fraxinoides, are more susceptible to attack by Phytophthora spp. than conifers. The gradual decline and death of E. fraxinoides in low-lying areas prone to periods of soil saturation has been recorded in Kaingaroa Forest. In E. botryoides and E. saligna, a crown dieback caused by Phytophthora spp. occurs in a few localities in the central North Island. Leaves and small twigs are attacked and crown dieback gradually develops. Infection occurs in the winter months.
NZ distribution: Found throughout New Zealand in both exotic and indigenous forests.
Economic impact: Phytophthora spp. are of little importance in conifer plantations. They can cause considerable losses in nurseries on heavy soils or on soils with impeded drainage, particularly in wet years.
Control: The most sensible way to control root rot in nurseries is to make sure that the site is well drained. Chemical control can be achieved by fumigation of the soil before sowing (methyl bromide or Basamid), or by pre-emergence application of Ridomil 25G (6 kg/ha). In recent years phosphite has proven to be a useful chemical control.
References: Dick &Vanner 1986; Gilmour 1966.
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