Andrew J. Storer

The Pitch Canker Epidemic in California

Native Monterey pine (Pinus radiata D. Don) forests are currently found at three disjunct locations in coastal California and on two islands off the coast of Mexico. The mainland forests comprise, in total, approximately 4,500 ha (11), whereas the two island populations are limited to 150 ha on Cedros Island and fewer than 400 trees on Guadalupe Island (28). Collectively, these populations are significant ecological and recreational resources, but they are also a valuable repository of useful genetic traits for improved varieties of Monterey pine, which are widely used by the timber industry. In California, Monterey pine is important as a landscape tree, with an estimated 50 million standing trees as of 1985. Monterey pines were especially popular for plantings on freeway rights-of-way, where they served as visual and sound barriers for the adjacent properties. It was among such trees that a dieback problem became apparent in Santa Cruz County (Fig. 1) during the mid-1980s. In 1986, A. H. McCain, extension plant pathologist at University of California Berkeley, established that the affected trees were suffering from pitch canker (30), caused by Fusarium circinatum Nirenberg & O’Donnell (=Fusarium subglutinans (Wollenweb. & Reinking) Nelson, Toussoun, & Marasas f. sp. pini). Subsequent surveys showed the disease to be widespread in coastal Santa Cruz County, with a clearly disjunct infestation being found farther inland, in Alameda County (Fig. 1). Curiously, the disease was also found in Christmas tree farms in Los Angeles and San Diego counties, more than 450 km to the southeast (8).

Population structure of the pitch canker pathogen, Fusarium subglutinans f.sp. pini, in California.

Isolates (170) of Fusarium subglutinans f. sp. pini, the cause of pitch canker, were collected from infected Pinus spp. throughout the known range of the disease in California, between 1993 and 1995. Of these, 152 isolates were vegetatively compatible with tester strains representing a previously characterized vegetative compatibility group (VCG) of F. s. pini: VCG C1, 83 isolates; C2, 12 isolates; C3, 51 isolates, and C4, 6 isolates. The remaining 18 isolates were associated with three new VCGs: C6, 9 isolates; C7 4 isolates, and C8, 5 isolates. The populations of F. s. pini in the original disease centres in Santa Cruz/Monterey Counties and east of San Francisco Bay (Alameda and Contra Costa Counties), were dominated by VCG C1, as they were when originally sampled (1987–9). In contrast, nearly all isolates from a large infestation in San Luis Obispo Co. were associated with VCG C3, which was previously identified as the sole VCG in an isolated infestation near Santa Barbara and at a tree nursery in Los Angeles Co. This distribution implies that movement of infected trees might have contributed to the spread of pitch canker in California. The limited diversity in the F. s. pini population statewide is consistent with a recent introduction of the pathogen into California and the absence of sexual reproduction in this population.

Efficacy of trap and lure types for detection of Agrilus planipennis (Col., Buprestidae) at low density

Development of effective trapping tools for forest pests and evaluating the key components of these tools is necessary to locate early‐stage infestations and develop management responses to them. Agrilus planipennis Fairmaire (emerald ash borer) is an introduced pest of ash (Fraxinus spp. L.) in North America. The effectiveness of different trap and lure combinations were tested in areas with low and high density populations of A. planipennis. At low density sites, purple prism traps outperformed green traps and girdled ash trap trees in capture rates (adults per day) and rates of detection of A. planipennis. Also, manuka oil lures, used as a standard lure in a national survey programme, captured higher rates of A. planipennis than did previous standards of girdled ash trap trees. There was no logistic relationship between the detection of A. planipennis on a trap and the diameter of the ash tree from which the trap was suspended, possibly because of the use of artificial lures with these traps. There was also no difference in the mean number of A. lanipennis captured per day between ash species and between vigour rating of ash associated with the traps. However, traps placed in open grown and dominant trees captured more beetles than traps placed in lower canopy class trees. At sites defined as low and high density, there was no difference in the larval density per cm3 of phloem. This suggests that exposure time to A. planipennis has been shorter at those low density sites. By exploiting the trap and tree characteristics that improve A. planipennis capture rates and detection efficacy, there can be future improvement in management of this pest. If detection can occur before infested ash trees exhibit signs and symptoms, there may be a potential for reducing the mortality of ash within stands.

Systemic effects of Heterobasidion annosum on ferulic acid glucoside and lignin of presymptomatic ponderosa pine phloem, and potential effects on bark-beetle-associated fungi.

Concentrations of soluble phenolics and lignin in the phloem of ponderosa pines inoculated with the pathogen Heterobasidion annosum were assessed over a period of 2 years in a 35-year-old plantation in northern California, USA. The major effect of the pathogen on phloem-soluble phenolics consisted of a significant accumulation of ferulicacid glucoside: 503 ± 27 μg/g fresh weight (FW), compared with 366 ± 26 μg/g FW for mock-treated and 386 ± 27 μg/g FW for control trees. Lignin content was negatively correlated with ferulic acid glucoside concentration, and there was an indication of lignin reduction in the cell walls of inoculated trees. Lignin had a negative effect on the in vitro growth of two common bark beetle fungal associates, Ceratocystiopsis brevicomi and Ophiostoma minus. For this reason it, is hypothesized that lower lignification may facilitate the growth of beetle-associated fungi, resulting in greater susceptibility of the presymptomatic host to bark beetle colonization.

JUVENILE HORMONE REGULATES DE NOVO ISOPRENOID AGGREGATION PHEROMONE BIOSYNTHESIS IN PINE BARK BEETLES, Ips SPP., THROUGH TRANSCRIPTIONAL CONTROL OF HMG-CoA REDUCTASE

Evidence is presented for transcriptional regulation of de novo pheromone biosynthesis in Ips spp. bark beetles, but the comparative biochemical and molecular approach reveals a dichotomy between species in the pini and grandicollis subgeneric groups. Radiotracer studies with 14C-acetate demonstrate that feeding on host phloem stimulates biosynthesis in males of three Ips spp. However, treatment with juvenile hormone III (JH III) stimulates biosynthesis only in Ips pini. Thus, two species in the grandicollis subgeneric group (I. grandicollis and I. paraconfusus) appear to have a different mode of regulation related to JH III than does I. pini. Between 16 and 20 hr after feeding has commenced, pheromone production, as measured by accumulation in abdominal tissue, is stimulated about 150- (I. pini) and 350-times (I. paraconfusus) above the control level of 1–10 ng/male measured at 0 hr. Treatment with JH III results in accumulation in I. pini that is 3–4 times more than in phloem-fed males, whereas the identical treatment results in only weak accumulation in I. paraconfusus (45-times less than phloem-fed males). Comparative studies of gene expression and enzyme activity related to biosynthesis also support different modes of JH III-related regulation in I. pini and I. paraconfusus. In males of both species, feeding on host phloem results in increased transcript abundance and increased activity for the key de novo isoprenoid pathway enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-R). However, while JH III treatment results in comparable maximal increases in HMG-R transcript levels in both species (similar to feeding), the activity of HMG-R in crude extracts from JH III-treated male I. paraconfusus is low in comparison with male I. pini. Hypothetical explanations for the interspecific dichotomy in the regulation of pheromone biosynthesis include a second hormone or factor in grandicollis group species that functions either alone or with JH III; in both cases acting after HMG-R has been transcribed.

Effectiveness of differing trap types for the detection of emerald ash borer (Coleoptera: Buprestidae).

ABSTRACT The early detection of populations of a forest pest is important to begin initial control efforts, minimizing the risk of further spread and impact. Emerald ash borer (Agrilus planipennis Fairmaire) is an introduced pestiferous insect of ash (Fraxinus spp. L.) in North America. The effectiveness of trapping techniques, including girdled trap trees with sticky bands and purple prism traps, was tested in areas with low- and high-density populations of emerald ash borer. At both densities, large girdled trap trees (>30 cm diameter at breast height [dbh], 1.37 m in height) captured a higher rate of adult beetles per day than smaller trees. However, the odds of detecting emerald ash borer increased as the dbh of the tree increased by 1 cm for trap trees 15–25 cm dbh. Ash species used for the traps differed in the number of larvae per cubic centimeter of phloem. Emerald ash borer larvae were more likely to be detected below, compared with above, the crown base of the trap tree. While larval densities within a trap tree were related to the species of ash, adult capture rates were not. These results provide support for focusing state and regional detection programs on the detection of emerald ash borer adults. If bark peeling for larvae is incorporated into these programs, peeling efforts focused below the crown base may increase likelihood of identifying new infestations while reducing labor costs. Associating traps with larger trees (≈25 cm dbh) may increase the odds of detecting low-density populations of emerald ash borer, possibly reducing the time between infestation establishment and implementing management strategies.

The dynamics of an introduced pathogen in a native Monterey pine (Pinus radiata) forest

The plant pathogenic fungus, Fusarium circinatum, is the cause of a major epidemic of pitch canker in urban forests of Monterey pine (Pinus radiata) in California. This pathogen is now also well established in all three mainland, native populations of Monterey pine where it causes conspicuous branch die-back and, frequently in association with native bark beetles, increased tree mortality. In the present study, permanent plots were established on the Monterey peninsula to characterize the severity and progress of pitch canker in the largest of the native P. radiata populations. The results indicate that the disease is significantly more severe, and is progressing more rapidly, in managed stands than in the wildland areas. Furthermore, the disease is progressing significantly faster in the coastal zone than in more inland locations.

The role of olfactory stimuli in the location of weakened hosts by twig-infesting Pityophthorus spp.

1. Senescing, shade‐suppressed, or broken branches of Monterey pine Pinus radiata are infested by twig beetles in the genus Pityophthorus (Coleoptera: Scolytidae). The studies reported here tested whether twig beetles can discriminate between healthy and pitch canker‐diseased branches, whether diseased branch tips produce more ethylene than undamaged controls, and whether ethylene and other volatiles, produced by the plant in response to tissue damage, are utilised by twig beetles in host location.

The effect of larval aggregation behaviour on larval growth of the spruce bark beetle Dendroctonus micans

1. The great spruce bark beetle Dendroctonus micans is a primary pest of spruce in Europe. It is unusual among Eurasian scolytids in that apparently healthy trees are attacked by solitary adults, but larvae feed en masse, in response to a larval aggregation pheromone.

Feeding Response of Ips paraconfusus to Phloem and Phloem Metabolites of Heterobasidion annosum–Inoculated Ponderosa Pine, Pinus ponderosa

In studies of feeding by the bark beetle, Ips paraconfusus, two pine stilbenes (pinosylvin and pinosylvin methyl ether), ferulic acid glucoside, and enantiomers of the four most common sugars present in ponderosa pine phloem (sucrose, glucose, fructose, and raffinose) did not stimulate or reduce male feeding when assayed on wet α-cellulose with or without stimulatory phloem extractives present. When allowed to feed on wet α-cellulose containing sequential extracts (hexane, methanol, and water) of ponderosa pine phloem, methanol and water extractives stimulated feeding, but hexane extractives did not. Males confined in wet α-cellulose containing aqueous or organic extracts of culture broths derived from phloem tissue and containing the root pathogen, Heterobasidion annosum, ingested less substrate than beetles confined to control preparations. In an assay using logs from uninoculated ponderosa pines, the mean lengths of phloem in the digestive tracts increased as time spent feeding increased. Males confined to the phloem of basal logs cut from ponderosa pines artificially inoculated with H. annosum ingested significantly less phloem than beetles in logs cut from trees that were (combined) mock-inoculated or uninoculated and did not contain the pathogen. However, individual pathogen-containing treatments were not significantly different from uninoculated controls. It was concluded that altered feeding rates are not a major factor which may explain why diseased ponderosa pines are colonized by I. paraconfusus.