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The most characteristic symptom of this disease is 1-3 mm wide brick-red bands that appear on the needles and persist after the needles have withered and turned brown. The red coloration is due to the presence of a mycotoxin, dothistromin. Generally the red zone is distinctly marked off from the rest of the needle and roughly spherical black fruiting bodies (stromata) erupt in the red infected band. Adjacent to the red band are bands of yellow necrotic needle tissue. Flanking these can sometimes be seen a band of more intense green pigmentation, representing increased lignification of the needle tissue as a defence response. The end of the needle dies beyond the point of infection and the whole needle may develop extensive necrosis (browning) 2-3 weeks after the first appearance of symptoms. Diseased needles drop prematurely (Edwards and Walker, 1978; Kershaw et al., 1988).
In a few reported cases the characteristic red pigment is not seen (Pehl and Butin, 1992; Ivory, 1994). On Pinus radiata, diseased needles may show various degrees of damage, from clear red bands around the needle to complete discoloration and death of the needle (Edwards and Walker, 1978). On Austrian pine (P. nigra) early symptoms include deep green bands and yellow and tan spots on needles. Later the spots and bands turn brown to reddish-brown (Peterson and Graham, 1974).
The first symptoms are found on needles of lower branches and the pathogen gradually moves up the crown. In some cases the disease starts in inner parts of lower branches and moves up the inner crown then subsequently outwards along the branches (Marks et al., 1989). Successive years of severe disease and premature defoliation result in decreased growth and, in extreme cases, death of the tree.
Cultural Control and Sanitary Methods
Dothistroma needle blight is commonly found in nurseries (Evans and Oleas, 1983; Dick and Vanner, 1986; Ivory, 1990) and movement of the disease around the world is probably due largely to diseased planting material (Gibson, 1974), hence rigid quarantine procedures are required in those nurseries currently free of disease (Wardlaw and Phillips, 1990). Pine nurseries should be as far as possible from pine forests, as diseased planting stock may disseminate inoculum into new areas (Hunt, 1995).
The type of tree planted has a significant effect on the level of disease. Some pine species are much more susceptible than others and some show resistance to infection upon maturity.
Because conidia remain viable for 6 months on damp leaf litter, Gadgil (1970) recommends waiting for this period after clear felling before replanting. This would not apply to species such as Pinus radiata that develop resistance at 15-20 years as they would not be expected to leave viable inoculum.
Standard commercial pruning removes many of the diseased branches and thereby decreases the inoculum in the forest environment (Kershaw et al., 1988). The beneficial effect of pruning on individual trees is disputed (Pas et al., 1984) although the effects can be influenced by their position in the stand. In a study of 5- to 7-year-old P. radiata, pruning reduced the level of infection within rows close to the edge of the plantation, possibly due to improved stand ventilation, but not those deeper within the plantation (Marks and Smith, 1987).
Genetic variation in susceptibility to Dothistroma needle blight is documented in many species including P. ponderosa (Peterson, 1984), P. radiata (Wilcox, 1982), P. flexilis (Taylor and Schwandt, 1998) and P. muricata (Ades and Simpson, 1991), therefore the use of trees with increased resistance is recommended in high-risk areas. An extensive selection and breeding programme with P. radiata has yielded Dothistroma-resistant breeds that are expected to show a 15% decrease in mean stand infection and a 56% reduction in chemical spraying costs (Carson et al., 1991).
Some pine species, such as P. radiata and P. muricata, develop resistance with age and maturity (usually at 15 years). This is a feature of the whole tree in that resistance is seen even on new needles of mature trees (Ivory, 1972b). Other highly susceptible species remain susceptible throughout life (e.g., P. attenuata, P. nigra, P. ponderosa).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
The economic impact of M. pini has been most serious in countries such as New Zealand, Chile and South Africa that have used Pinus radiata and other susceptible species as a major commercial forest crop. In New Zealand diseased foliage exceeds 10% on over 450,000 ha in the North Island (New and Griffith, 1989).
In P. radiata, the loss in wood volume growth is directly proportional to the average level of disease (estimated as percentage of crown affected) over a period of 8 years (Pas, 1981), i.e. 10% disease led to 10% loss of volume, 30% disease to 30% loss, etc. Other authors have published variations on these figures (Gibson, 1974). However, most agree that the impact is not considered significant until greater than 25% of the foliage becomes infected in 50% of the total number of trees in a stand (Kershaw et al., 1988) and at this stage fungicide spraying is considered economic.
Serious outbreaks have resulted in tree death in many parts of the world. There was 67% mortality in 7- and 8-year-old P. radiata in California, USA (Cobb et al., 1969) and Dothistroma needle blight caused complete failure of most P. ponderosa plantings in eastern states of the USA and up to 40% mortality of P. flexilis in Montana (Taylor and Schwandt, 1998). In Kenya in 1963, over 1500 hectares of P. radiata aged 1-5 years were so badly diseased that they were cut out and replanted with alternative species.
The annual cost of Dothistroma needle blight to the forestry industry in New Zealand was estimated to be NZ$6.1 m (about £2 m), in terms of direct control costs and residual growth loss (New and Griffith, 1989). In addition to these direct costs are the indirect effects. One report suggested that increased wood density occurs following severe defoliation, which would impact on processing costs (Harris and McConchie, 1978). Another indirect effect on wood yield is increased infection with secondary pathogens and pests, for example Sirex wasp (Sirex noctilio) which is more prevalent in stands with severe Dothistroma disease levels (Neumann et al., 1993).