Type: Foliicolous Fungi; Description: Conidiomata stromatic, acervular, very variable in size and shape, in small groups, subepidermal, becoming erumpent, black, 0.1–1.2 × 0.05–0.5 mm; on clearly-defined transverse reddish bands on needles. Conidia filiform, straight or slightly curved with rounded ends, 1–5-(usually 3-) septate, 14–39 (commonly 20–28) × 2–3 μm, hyaline.

Distribution: Recorded from all regions except Kaikoura and Stewart Island.; 1st Record: Gilmour (1966b).

Notes: Dothistroma septosporum has priority over D. pini because of the earlier publication of the basionym Cytosporina septospora Doroguine. However, the type of C. septospora has been lost and was unavailable when Morelet made the re-combination in Dothistroma. Doroguine’s (1911) description and illustrations are inadequate for reliable identification, casting doubt on the contention that Cytosporina septospora is conspecific with Dothistroma pini. New Zealand collections closely match the type of D. pini.

Significance: Dothistroma needle blight is one of the most serious diseases of Pinus radiata in New Zealand. The major effect of the disease is a reduction in volume increment. This is directly proportional to the degree of crown infection. Van der Pas (1981) found that for an average disease level of 10%, the loss in volume was 9.7%; for a 20% disease level, it was 19.4% and for a 50% disease level, 48.5%. Because of the potential magnitude of growth loss, copper fungicides, which effectively control the disease, have been applied aerially since 1967. Although the disease is kept at a low level by the chemical treatment, growth loss still occurs since tree stands are not sprayed until the infection level reaches 20–25%. New (1989) estimated that the total cost of the disease to the forest industry (including chemical control costs) was $6.1 million per year (1988 dollars). More recent (2003) estimates of the annual cost approach $30 million (L.S.Bulman, New Zealand Forest Research Institute, pers. comm.). Infection process: Conidia produced on the exposed stroma in a mucilaginous matrix are released when the needle surface becomes wet. They are dispersed by water splash, the dispersal distance normally being quite short. Infection in a stand is usually only from neighbour to neighbour and natural transport of conidia over long distances is infrequent. Infected needles attached to the tree are the principal source of inoculum; when needles drop to the forest floor, the stromata are soon overgrown by saprobes and conidial production stops within 2 months (Gadgil 1970). Under favourable conditions (temperature 18 –20–C; needle surface moist), most conidia landing on the surfaces of needles of susceptible hosts germinate within 3 days, the germ tubes continuing to grow on the needle surface and a very few (about 0.1%) forming appressoria over stomatal openings. An infection peg develops between the guard cells and a swollen vesicle forms just below the guard cells (Gadgil 1968). Further hyphal growth occurs in the mesophyll tissues if the needle surface is wet. Lateral spread of hyphae is limited to a few millimetres from the point of infection but a much greater length of the needle is killed through direct and indirect action of a toxic compound, dothistromin (Bassett et al. 1970) produced by the fungus (see below for more details of the effects of dothistromin). Factors governing infection: Three major interacting factors control the infection of a susceptible host by D. pini: temperature, duration of needle wetness, and the number of viable spores landing on the needle surface. Although infection is also dependent on light intensity (Gadgil & Holden 1976), light conditions in the field rarely limit the infection process. Under experimental conditions, production of visible stromata was observed under the four different day/night regimes tested (24 /16/, 20,/12/, 16,/8/ and 12 /4/C) when foliage was kept wet for 8 h or longer after inoculation with a conidial suspension. The optimum temperature regime for infection was 20e/12/C and the severity of infection was greater where the foliage was kept continually moist (Gadgil 1974b). Study of the effect of moisture at 20m/12/C in more detail showed that infection progressed only as far as the formation of a substomatal vesicle when the leaf surface was not kept wet. When the foliage was kept continuously moist, stromata were first observed 19 days after inoculation. The length of a period of dryness after inoculation influenced stromatal development; stromata appeared after 5 days of continuous wetting following a dryness period of 30 days, but took 10 days to develop if continuous wetting followed a dryness period of 60 days (Gadgil 1977). A study by Gilmour (1981) demonstrated the relationship between the effects of temperature, needle wetness and inoculum density on infection in a stand of young Pinus radiata. In 1969–70, when infection levels (and therefore inoculum levels) were low, threshold values for infection were 12eC and a needle wetness period of 20 h. In 1970–71, when the level of infection was noticeably greater, threshold values were 7CC and 10 h. The period between initial infection and the appearance of stromata varied from 6 weeks in December to 15 weeks in May. Progress of the disease: The usual progress of the disease in tree stands of central North Island is as follows: Very little or no infection occurs between May and August because of low temperatures. Many infected needles are shed and soon cease to be a source of inoculum. In September, few infected needles are left in the tree crowns and although new infections occur as temperatures rise, their number is not large. At daily mean temperatures around 12aC, a development period of about 12 weeks elapses before stromata begin to appear on newly infected needles in late November or early December. Conidia produced on these stromata provide the inoculum for the first major infection. Temperatures in early summer are near-optimal for infection and if the weather is wet (as it usually is), there will be a large increase in disease level. Stromata produced from November/December infections appear in February and a second cycle of infection may occur. The main infection period thus extends from late November to the end of February in central North Island; in warmer parts of New Zealand (e.g., Northland) it begins earlier (late October to early November) and in cooler parts (e.g., Southland) it is a little later (December). Chemical control: The main aim of the chemical control programme is to reduce the amount of inoculum available at the beginning of the main infection period by killing as many conidia as possible. Timing of the first (and often the only) treatment is crucial to the success of the spray programme (Gilmour & Noorderhaven 1971). Copper oxychloride, the fungicide usually employed, is very effective; germination of conidia obtained from stromata on needles was 74% before, and 1% after a routine aerial spray application (P.D.Gadgil & L.S.Bulman, unpublished data). Routine aerial spraying to control the disease in Pinus radiata stands has been common practice in New Zealand since 1966. Stands in susceptible age classes (<16 yr) are surveyed from the air in mid-winter, the average percentage of visible crown symptoms being assessed by experienced observers (see van der Pas et al. (1984) for an evaluation of assessment methods used). Tree stands are generally divided into three categories: (i) less than 20% of the crown infected: no spray needed; (ii) 20–40% crown infection: one application in early summer (usually late November–early December); (iii) more than 40% crown infection: two applications, one in early summer and one in late summer (usually mid-February). Details of assessment methods and spray application have been described by Bulman et al. (2004). Genetic diversity: Molecular studies of New Zealand isolates of D. pini showed no evidence of genetic diversity, suggesting that they are derived from a single introduction of one strain of the fungus into the country (Hirst et al. 2000). Dothistromin: Although there is no direct evidence for the involvement of dothistromin in pathogenesis, typical symptoms of the disease were produced when purified dothistromin was injected into needles (Shain & Franich 1981). Plant defence responses to dothistromin-induced damage have been characterised (Franich et al. 1986). The amount of dothistromin produced by different isolates varies, even within New Zealand where only one strain of the fungus exists. Strains from Germany and from central USA have been shown to produce far greater amounts of the toxin than those from New Zealand (Bradshaw et al. 2000). Dothistromin is a difuroanthraquinone having the same tetrahydro-2-hydroxy-bisfuran moiety as that of aflatoxin B1 produced by Aspergillus flavus (Gallagher & Hodges 1972). This particular structural feature of aflatoxin B1 is considered to be responsible for its hepatotoxicity and potential human carcinogenicity. Elliott et al. (1989) explored the possibility that dothistromin may be carcinogenic and thus pose a risk to forest workers. The amounts of dothistromin found in the forest environment (a maximum of 7 ng/ml in ponds and streams and lower concentrations in air and run-off from trees) were considerably lower than the minimum dose of aflatoxin (50 ng/kg body weight/day) needed to produce cancer in animals. They concluded that dothi-stromin was not a human health risk.; Host(s): Very slightly susceptible: Pinus ayacahuite, P. coulteri, P. devoniana, P. montezumae, P. patula, P. pseudostrobus, P. sabineana, P. serotina, P. strobus, P. sylvestris, P. taeda, P. torreyana. Slightly susceptible: Larix decidua, Picea sitchensis, Pinus contorta, P. elliottii, P. hartwegii, P. monticola, P. nigra subsp. nigra, Pseudotsuga menziesii. Moderately susceptible: Pinus canariensis, P. lambertiana, P. pinaster. Highly susceptible, but exhibiting a high degree of resistance with age: Pinus muricata, P. radiata. Highly susceptible at all ages: Pinus jeffreyi, P. nigra subsp. laricio, P. ponderosa. Very highly susceptible: Pinus attenuata, P. ×attenuradiata.