Cyclaneusma (Naemacyclus) needle-cast of Pinus radiata in New Zealand. 2: Reduction in growth of the host, and its economic implication.

Twenty pairs of 9-year-old final-crop trees were selected in a stand of Pinus radiata D. Don heavily infected with Cyclaneusma minus (Butin) DiCosmo et al. at Kaingaroa State Forest in 1977. Each pair consisted of one heavily diseased tree and one healthy tree. Disease severity, in terms of percentage of crown infected, was recorded annually. Relationships between disease severity and growth were studied by complete stem analysis at tree age 15 years. Annual volume increments of the diseased trees were significantly reduced from age 7 onwards. Reduced diameter growth in the lower parts of the stem of the diseased trees caused small changes in the relative diameter distribution. The relationship between disease severity and volume loss showed a reduction of the average volume increment of approximately 60% at an average disease severity of 80%. Stand growth was projected to age 30 for various proportions of diseased trees. For each 10% increase in the proportion of diseased trees a reduction in total volume of 10-14 m/ha and a subsequent reduction in revenue at clearfelling of $60O-$70O/ha may be expected under the given stand conditions. Nine pairs of trees were injected with a solution of carbendazim fungicide in 1980 and 1981 in an attempt to control the disease. Injection with the fungicide gave a significant growth response associated with a trend to reduced disease severity. Some of the growth response may have been attributable to a nutritional effect as it occurred in healthy and diseased trees. INTRODUCTION Needle-cast of Pinus radiata associated with Cyclaneusma minus has been observed in many forests for many years (Gadgil et al. 1977). The severity of the needle-cast has varied from year to year and periodic serious outbreaks are causing concern to forest management. It has been reported elsewhere that Cyclaneusma minus can cause reduced growth in young Pinus sylvestris L. and destroy the trees' utility as Christmas trees (Kistler & Merrill 1978). Little is known, however, about the effect of this fungus on the growth of P. radiata or its economic impact. A trial was therefore set up to establish the relationship between growth loss and disease severity in a stand of P. radiata infected by C. minus in Kaingaroa State Forest. New Zealand Journa l of Fores t ry Science 14(2): 197-209 (1984) 198 New Zealand Journal of Forestry Science 14(2) METHODS Site An area of approximately 0.5 ha was selected in the centre of a stand (65 ha) of 9-year-old P. radiata in Kaingaroa State Forest. The stand was on a level site that was aerially seeded in 1968. The initial stocking of 4200 stems/ha had been thinned in 1971, 1974, and 1977 to respectively 2604 stems/ha, 1144 stems/ha, and 400 final-crop stems/ha. Trees in this part of the stand had not shown symptoms of Dothistroma needle blight, and thus had not been sprayed with fungicide at any time. The study was carried out over 6 consecutive years starting in 1977 when the trees were 9 years old. Field observations indicated that approximately 50% of the trees in the stand were diseased, although the severity of infection varied. The affected trees were scattered among others that showed no visible symptoms of ill health and had normal green crowns. In 1977, after the last thinning, 40 of the final-crop trees were selected in pairs. Each pair consisted of a healthy and a heavily diseased tree of approximately equal d.b. h. , crown height, and crown width at time of selection. Paired trees, representing the full range of tree sizes, were randomly selected. The healthy trees were free from any visible infection whereas the diseased trees showed premature needle-cast of more than 6 5 % of the crown. Volume loss due to defoliation caused by the fungus was measured at age 15 by comparing annual growth of the healthy and diseased trees. Treatments In March 1980 nine randomly selected pairs of healthy and diseased trees were injected with a carbendazim solution. This fungicide has been shown to control Cyclaneusma needle-cast (Hood & Vanner 1984). The solution consisted of 3.5 g methyl benzimidazol-2-yl carbamate dissolved in 8.75 ml concentrated hydrochloric acid made up to 1 litre with water. The dosages applied were 4 litres for trees with a diameter at breast height of less than 18 cm, 5 litres for those from 18 to 25 cm d.b. h. , and 6 litres for those larger than 25 cm d.b. h. The solution was gravity fed through five equidistant holes (1.2 cm diameter, 3 cm deep) drilled through the bark 30 cm above ground level. The uptake of the solution took place within 5 days for all trees. A second dosage was applied to the same trees in November 1.981. Dosages were increased and ranged from 4 to 17 litres/tree depending on the size of the tree. The uptake this time was slow and some trees took much less than the applied dosage. Disease Scoring Disease severity was visually estimated for each tree using a method similar to that used for the assessment of Dothistroma infection (Kershaw et al. 1982). The percentage of the normal green crown that showed visible symptoms of infection by C. minus was estimated in 5 % steps by three trained observers whose individual scores were averaged. Scoring was carried out weekly over a 6-week period in September and October each year, the period when maximum symptom expression could be identified for each individual tree. Foliage samples were taken from each tree and isolations made of the van der Pas et al. — Cyclaneusma. 2: Growth reduction 199 fungi present inside the needle tissues in June and November 1977 and 1978, May and July 1979, and September 1980 using methods outlined by Gadgil (1984). Analyses In July 1983 all sample trees were felled for a complete stem analysis of past growth. After felling, sections were cut at mid-internodes and diameters were measured within each annual shoot (Whyte 1974). Cumulative volumes and annual volume increments were calculated, and growth increments were plotted using program package GRANRAD (unpubl. data, available on request). Growth increments were tested by analysis of covariance with height and volume of previous years as covariates. Volume growth was calculated for healthy and diseased stands. In the latter the proportion of diseased trees in the final-crop component increased from 10% to 60% in 10% steps. Stand basal areas were extrapolated from the basal area of healthy and diseased trees measured at age 15, and basal areas of diseased stands were reduced proportionally with increased percentage of diseased trees. Basal areas were implemented in the Kaingaroa Growth Model (Elliott & Goulding 1976) for growth projection to the end of the rotation at age 30, and predictions were carried out for stockings of 400 stems/ha, 300 stems/ha, and 200 stems/ha. Program PROD (Goulding & Shirley 1978) forecast tree size distribution and program SILMOD (McGregor & Williams 1982) predicted changes in the net value of the simulated stands. RESULTS AND DISCUSSION The average annual disease severity of the diseased trees was significantly higher than that of the healthy trees (p <0.01) from 1977 to 1982 (Table 1). Some of the initially healthy trees became infected in 1978 but the disease severity remained low. TABLE 1—Average annual disease severity of healthy and diseased trees (percentage of green crown infected) Category Year Average 1977-82 1977 1978 1979 198