Berry infection and the development of bunch rot in grapes caused by Aspergillus carbonarius

The effects of different levels of inoculum of Aspergillus carbonarius and time of inoculation on berry infection and the development of aspergillus bunch rot on grapevines (cv. Sultana) were studied under field conditions. Inflorescences at full bloom were inoculated with aqueous spore suspensions of A. carbonarius containing 0 or 1 × 10⁶ spores mL⁻¹ in 2004/05 and 0, 1 × 10² or 1 × 10⁵ spores mL⁻¹ in 2005/06. In both years, the incidence of infection in inoculated berries was significantly higher than in uninoculated berries. Incidence of infection in berries from veraison until harvest was higher than at earlier stages of bunch development (berry set to berries that were still hard and green). Inoculation of bunches at veraison did not significantly increase A. carbonarius infection prior to harvest, at harvest, 6 days after harvest or when berries were over-ripe. Bunches inoculated at harvest did not significantly increase infection 6 days after harvest or when berries were over-ripe. Aspergillus carbonarius was isolated more frequently from the pedicel end (53·1%) than from the middle section (37·5%) and distal end (35·0%) of berries that were inoculated with 10⁵ spores mL⁻¹.     

Visual and PCR assessment of light leaf spot (Pyrenopeziza brassicae) on winter oilseed rape (Brassica napus) cultivars

Methods to assess light leaf spot ( Pyrenopeziza brassicae ) on winter oilseed rape cultivars were compared in laboratory, controlled-environment and field experiments. In controlled-environment experiments with seedling leaves inoculated at GS 1,4, the greatest differences in percentage area affected by P. brassicae sporulation were observed with inoculum concentrations of 4 × 10³ or 4 × 10⁴ spores mL⁻¹, rather than 4 × 10² or 4 × 10⁵ spores mL⁻¹, but older leaves had begun to senesce before assessment, particularly where they were severely affected by P. brassicae . In winter oilseed rape field experiments, a severe light leaf spot epidemic developed in 2002/03 (inoculated, September/October rainfall 127·2 mm) but not in 2003/04 (uninoculated, September/October rainfall 40·7 mm). In-plot assessments discriminated between cultivars best in February/March in 2003 and June in 2004, but sometimes failed to detect plots with many infected plants (e.g. March/April 2004). Ranking of cultivar resistance differed between seedling experiments done under controlled-environment conditions and field experiments. The sensitivity of detection of P. brassicae DNA extracted from culture was greater using the PCR primer pair PbITSF/PbITSR than using primers Pb1/Pb2. P. brassicae was detected by PCR (PbITS primers) in leaves from controlled-environment experiments immediately and up to 14 days after inoculation, and in leaves sampled from field experiments 2 months before detection by visual assessment.     

Antifungal vapour-phase activity of allyl-isothiocyanate against Penicillium expansum on pears

The fungitoxicity of allyl-isothiocyanate (AITC) vapour against Penicillium expansum , the causal agent of blue mould on pears (cvs Conference and Kaiser), was evaluated. The best control of blue mould was obtained by exposing fruits for 24 h in a 5 mg L⁻¹ AITC-enriched atmosphere, the extent of control depending on the inoculum density. Lesion diameter was inversely related to AITC concentration. In treated fruits the percentage of infected wounds increased with conidial concentration, with fewer than 20% affected at 1 × 10³ conidia mL⁻¹ to almost 80% at 1 × 10⁶ conidia mL⁻¹. In comparison, >98% of wounds were infected in untreated fruits irrespective of conidial concentration. AITC treatments were effective up to 24 h after inoculation for Conference and 48 h for Kaiser. AITC treatments also controlled a thiabendazole-resistant strain of P. expansum , reducing the incidence of blue mould by 90% in both cultivars. The use of AITC produced from pure sinigrin or from Brassica juncea defatted meal may be an economically viable alternative to synthetic fungicides against P. expansum .     

PCR-based specific and sensitive detection of Pectobacterium carotovorum ssp. carotovorum by primers generated from a URP-PCR fingerprinting-derived polymorphic band

A 24-mer primer pair was generated by sequencing a URP-PCR fingerprinting-derived polymorphic band that is uniquely shared in Pectobacterium carotovorum ssp . carotovorum strains (Pcc). The primer set (EXPCCF/EXPCCR) amplified a single band of expected size (0·55 kb) from genomic DNA obtained from 29 Pcc strains and three Pectobacterium carotovorum ssp. wasabiae (Pcw) strains, but not from other P. carotovorum subspecies atrosepticum , betavasculorum or odoriferum , or from other Erwinia spp. or bacterial genera. The Rsa I digestion profile of the amplified bands divided Pcc strains into five groups with a unique profile from Pcw strains. First-round PCR detected between 5 × 10² and 1 × 10³ colony forming units (CFU) mL⁻¹ and detection sensitivity was increased to as few as 2–4 CFU mL⁻¹ after second-round (nested) PCR. This PCR protocol was used directly to detect Pcc strains in infected plant tissues.     

Factors affecting the severity of bacterial canker of pear caused by Pseudomonas syringae pv. syringae

Several factors affecting the severity of bacterial canker of pear were studied. In the orchard, infection of shoots by Pseudomonas syringae pv. syringae occurred only when the inoculum dose exceeded 10⁶ colony-forming units/shoot. However, under favourable conditions in a growth chamber, cankers formed on detached shoots inoculated with 5 cfu/shoot. A second-order polynomial relationship was established between log₁₀ transformed canker length and log₁₀ transformed inoculum dose. In orchard and growth chamber experiments, shoots were susceptible from the time of bud swell until after fruit harvest. The severity of Pseudomonas canker of detached shoots increased if they were frozen at – 10°C for 24 h before inoculation. Shoots were most susceptible when inoculated immediately after wounding, and no cankers developed in the orchard when 3-day-old wounds were inoculated. Additionally, no cankers resulted from inoculation of leaf scars at leaf drop. Actively growing, current-season shoots were more susceptible than shoots that had set a terminal bud. The practical implications of these results are discussed as a basis for control of bacterial canker of pear.     

Aggressiveness and host specificity of Brazilian isolates of Phytophthora infestans

The population of Phytophthora infestans in Brazil consists of two clonal lineages, US-1 associated with tomatoes and BR-1 associated with potatoes. To assess whether host specificity in these lineages resulted from differences in aggressiveness to potato and tomato, six aggressiveness-related epidemiological components – infection frequency (IF), incubation period (IP), latent period (LP), lesion area (LA), lesion expansion rate (LER) and sporulation at several lesion ages (SSLA) – were measured on detached leaflets of late blight-susceptible potato and tomato plants. Infection frequency of US-1 was similar on potato and tomato leaflets, but IF of BR-1 was somewhat reduced on tomato. Incubation period was longer on both hosts with US-1, although this apparent lineage affect was not significant. Overall there was no host effect on IP. On potato, BR-1 had a shorter LP (110·3 h) and a larger LA (6·5 cm²) than US-1 (LP = 162·0 h; LA = 2·8 cm²). The highest LER resulted when isolates of BR-1 (0·121 cm² h⁻¹) and US-1 (0·053 cm² h⁻¹) were inoculated on potato and tomato leaflets, respectively. The highest values of the area under the sporulation capacity curve (AUSC) were obtained for isolates of US-1 inoculated on tomato leaflets (6146) and for isolates of BR-1 on potato leaflets (3775). In general, higher values of LA, LER, SSLA and AUSC, and shorter values of LP were measured when isolates of a clonal lineage were inoculated on their original host than with the opposite combinations. There is evidence that there are quantitative differences in aggressiveness components between isolates of US-1 and BR-1 clonal lineages that probably contribute to host specificity of P. infestans populations in Brazil.     

Diversity and differentiation in two populations of Gibberella circinata in South Africa

Gibberella circinata [anamorph Fusarium circinatum (=  F. subglutinans f.sp. pini )] causes pitch canker and is an important pathogen in South African pine nurseries. The initial outbreak of the pitch canker fungus was limited to a single nursery at Ngodwana in Mpumalanga Province. Subsequently, several other pine nurseries in South Africa became infected. Most of these outbreaks were relatively small except for the outbreak in the Klipkraal nursery (Mpumalanga Province). The genetic diversity, population differentiation and relative frequencies of the sexual and asexual cycles among two South African subpopulations were determined to establish whether immigration, mutation and/or recombination contributed towards population structure. The allelic diversity of the initial population (Ngodwana) was observed to be lower (0·16) than that of the more recent Klipkraal population (0·25). Approximately 4% ( G _ST = 0·04) of total gene diversity could be attributed to differences among the subpopulations. Furthermore, six new vegetative compatibility groups (VCGs) have been identified since the initial outbreak of G. circinata in South Africa 10 years ago. The relatively low allelic diversity and low level of genetic differentiation suggest restricted gene flow among subpopulations, and indicate that the pathogen has been introduced recently. However, the amount of allelic and VCG diversity suggests that multiple genotypes have been introduced into South Africa. The increases in effective population number, allelic diversity and new VCGs over the past 10 years suggest that sexual reproduction might be occurring.     

Factors influencing infection of eucalypts by Cylindrocladium pteridis

The pattern of Cylindrocladium pteridis adhesion, germination and penetration in eucalypt leaves was assessed using scanning electron microscopy. The effects of inoculum concentration, leaf wetness period, plant age and branch position of cylindrocladium leaf blight and defoliation severity were assessed in greenhouse studies using two Eucalyptus grandis × E. urophylla hybrid clones. Penetration occurred through stomata, and there was no difference in the number of penetrations between young and old leaves. Percentage leaf area with lesions and defoliation increased with the increase in inoculum concentration (1 × 10² to 10⁵ conidia mL⁻¹), duration of leaf wetness period (6 to 48 h) and plant age (60 to 180 days). Branch position in plants also significantly affected the percentage leaf area with lesions and defoliation, the latter variable being significantly higher at the stem base. The highest values of lesion area were also observed on leaves at the stem base in both clones. The Pearson correlation between defoliation and leaf area with lesions was significant in all experiments ( r  > 0·9) indicating a high association between these two variables.     

Host–parasite relationships of Meloidogyne incognita on spinach

Host–parasite relationships in root-knot disease of spinach caused by Meloidogyne incognita race 1 were studied under glasshouse conditions. Nematode-induced mature galls were large and usually contained one or more females and egg masses with eggs. Feeding sites were characterized by the development of giant cells containing granular cytoplasm and many hypertrophied nuclei. The cytoplasm in these giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density ( P _i) in a series from 0–128 eggs and second-stage juveniles per cm³ soil and growth of spinach cv. Symphony F₁ seedlings was tested under glasshouse conditions. A Seinhorst model [ y = m  + (1 −  m ) z ^P–T ] was fitted to fresh top- and total plant-weight data for inoculated and control plants. Tolerance limits ( T ) of spinach cv. Symphony F₁ to M. incognita race 1 for fresh top and total plant weights were 0·25 and 0·5 eggs and second-stage juveniles per cm³ soil, respectively. The minimum relative values for fresh top and total plant weights were zero in both cases at P _i ≥ 32 eggs and second-stage juveniles per cm³ soil. Root galling was least at low initial population densities and greatest at 16 eggs and second-stage juveniles per cm³ soil. Maximum nematode reproduction rate was 33·1-fold at the lowest P _i.     

Effects of avirulent bacteriocin-producing strains of Pseudomonas solanacearum on the control of bacterial wilt of tobacco

Root systems of tobacco dipped in suspensions containing 2 × 10⁹ colony forming units (CFU)/ml of avirulent bacteriocin-producing strains (ABPS) of Pseudomonas solanacearum and assayed immediately after planting in steam-sterilized soil had 8 × 10⁶ CFU/root system of ABPS. The bacterial population declined to an average of 5·3 × 10⁵ CFU/root system after 30 days. Roots of seedlings dipped in bacterial suspensions of ABPS were more effectively protected against wilt caused by P. solanacearum than those dipped in suspensions of an avirulent nonbacteriocin-producing strain (ANBPS). Lipopolysaccharide (LPS) isolated from one ABPS (121) inhibited the attachment of bacteria on roots by 70% but had no effect on the reduction of wilt, whereas bacterial cells significantly reduced the disease severity as compared to LPS or water treatment. In steam-sterilized soil containing a 1:1 mixture (5 × 10⁵ CFU/g of oven-dried soil) of ABPS 121 or 237 and the virulent strain K-60, ABPS 121 reduced multiplication of the virulent strain in soil and in the rhizosphere of seedlings. When roots of seedlings were dipped in a suspension of 2 × 10⁹ CFU/ml of ABPS before planting, root colonization by the virulent strain added to steam-sterilized soil at 2 × 10⁶ CFU/g of oven-dried soil was significantly reduced. When roots were dipped in a suspension of ABPS and assayed 20 days after planting, 98% of the bacterial population was found in the original zone of inoculation and only 2% was detected in new growths of the root system. Plants which were grown in soil infested with ABPS 121 or K-60 had both strains present at variable populations along all sections of roots.     

Management of phytophthora root rot in radiata pine seedlings

Chemical and biological agents were evaluated for their ability to suppress root rot, caused by Phytophthora cactorum , in field-grown radiata pine seedlings in New Zealand. Trials were conducted over two seasons in an area of a forest nursery with a natural infestation of P. cactorum , and a history of root rot. In each season, symptoms of root rot developed during April, one month after root pruning, when seedlings were approximately six months old. In trial one, root rot incidence by mid July 2007 was 9·1% in untreated plots and 8·4% in plots that had been treated with metalaxyl-M/mancozeb (14 kg ha⁻¹) at seedling emergence. Disease incidence was lowest (2·1%) in plots that received seven monthly applications of phosphorous acid (6·5 L ha⁻¹). Other treatments, including seed coating with thiram or Trichoderma spp., and foliar applications of methyl jasmonate, did not control disease. In trial two, effects of treatment timing relative to root pruning were investigated. By late June 2008, three months after root pruning, root rot incidence was 22·2% in the untreated plots. Phosphorous acid was the most effective treatment and almost completely suppressed disease (0·1% incidence) when applied fortnightly from February until May (seven applications). Metalaxyl-M/mancozeb (15 kg ha⁻¹) was not effective (21·4% incidence) when applied five months before root pruning. However, disease incidence was reduced when the chemical was applied one week after root pruning (14·9% incidence) and greater control was achieved (8·2% incidence) when the application rate was increased to 50 kg ha⁻¹.     

Effects of the mycoparasite Verticillium biguttatum on barley stunt disease caused by Rhizoctonia solani anastomosis group 8 in a model system

The height of barley stunted by Rhizoctonia solani anastomosis group 8 was significantly increased by up to 72·8% after incubation for 8 days at 20°C in seedling tray tests following application of the mycoparasite Verticillium biguttatum. The pathogen and mycoparasite were applied at the rate of 1% Perlite maizemeal inoculum (w/w potting mixture) resulting in propagule densities of approximately 24·0 and 6·6 × 10⁵ colony-forming units (cfu) per g potting mixture, respectively. Sieving (2 mm) R. solani inoculum prior to dilution in potting mixture increased the recovery of propagules from 1·2 × 2·1 × 10³ cfu per g inoculum compared with recovery when inoculum was sieved after dilution. Applications of a V. biguttatum isolate from the UK (vbl) and a Dutch isolate (M73) reduced stunting to a similar extent but did not stimulate the growth of healthy plants. The height of stunted plants was significantly increased after application of V. biguttatum inoculum after 6 days if inoculated trays were preincubated for 1 day prior to planting but a similar increase was only detected after 7 days if seeds were planted immediately. The number of stunted plants which emerged after 4 days was significantly increased by treatment with V. biguttatum but preincubation had no additional effect. These results suggested that control of R. solani was effected both before and after the initiation of disease.     

Pathogenicity of the root-knot nematode Meloidogyne javanica on potato

Host–parasite relationships and pathogenicity of Meloidogyne javanica on potatoes (newly recorded from Malta) were studied under glasshouse and natural conditions. Potato cvs Cara and Spunta showed a typical susceptible reaction to M. javanica under natural and artificial infections, respectively. In potato tubers, M. javanica induced feeding sites that consisted of three to four hypertrophied giant cells per adult female. Infection of feeder roots by the nematode resulted in mature large galls which usually contained at least one mature female and egg mass. In both tubers and roots, feeding sites were characterized by giant cells containing granular cytoplasm and many hypertrophied nuclei. Cytoplasm in giant cells was aggregated alongside the thickened cell walls. Stelar tissues within galls appeared disorganized. The relationship between initial nematode population density ( P ) [0–64 eggs + second-stage juveniles (J2s) per cm³ soil] and growth of cv. Spunta potato seedlings was tested under glasshouse conditions. A Seinhorst model [ y = m  + (1 −  m ) z ^{( P − T )}] was fitted to fresh shoot weight and shoot height data of nematode-inoculated and control plants. Tolerance limits ( T ) for fresh shoot weight and shoot height of cv. Spunta plants infected with M. javanica were 0·50 and 0·64 eggs + J2s per cm³ soil, respectively. The m parameter in that model (i.e. the minimum possible y -values) for fresh shoot weight and shoot height were 0·60 and 0·20, respectively, at P  = 64 eggs + J2s per cm³ soil. Root galling was proportional to the initial nematode population density. Maximum nematode reproduction rate was 51·2 at a moderate initial population density ( P  = 4 eggs + J2s per cm³ soil).     

Some properties of a virus-like agent found in Brachypodium sylvaticum in the United Kingdom

A spherical virus–like agent ($$32 nm in diameter) was isolated from a plant of Brachypodium sylvaticum (Huds.) Beauv. growing in a botanic garden in England and showing yellow streaks in the leaves. The agent was readily purified and sedimented as a single component in sucrose density rate gradients. The particles had a sedimentation coefficient at infinite dilution of $$122S and a buoyant density of 1.35 g/cm³ in CsCl and 1·33 in CS₂SO₄. The particles were stable at acid pH but above pH 7·0 in the presence of EDTA dissociated. A protein having a major polypeptide with a molecular weight of $$3·76 × 10⁴ and a species of single stranded RN A with a MW of 1·67 × 10⁶ were detected in the particles. The agent was not transmitted by manual inoculation, by the insects Myzus persicae Sulzer, Rhopalosiphum padi L. or Nephotettix virescens Distant, through soil by leakage from roots or by seed. The particles had physicochemical properties in common with tombus– and sobemoviruses but were not serologically related to 10 members of these groups or to 57 other small spherical RNA plant viruses.     

Sensitivity of Sclerotinia homoeocarpa to demethylation-inhibiting fungicides in Ontario, Canada, after a decade of use

In late 2003, nine populations of Sclerotinia homoeocarpa in Ontario Canada (seven of which had been previously sampled in early 1994, prior to the registration of sterol demethylation-inhibiting (DMI) fungicides for turf disease control in Canada) were sampled and tested for sensitivity to propiconazole. Four of the nine populations had not been treated with DMI fungicides during the intervening years, and isolates from these locations were sensitive to propiconazole (geometric mean EC₅₀ values of 0·005–0·012 µ g mL⁻¹, compared with 0·005–0·008 µ g mL⁻¹ for the original 1994 populations). Among the five populations from 2003 that had been exposed to DMI fungicides, mean EC₅₀ values were significantly greater, ranging from 0·020 to 0·048 µ g mL⁻¹. A significant correlation of determination was found between estimated number of fungicide applications and log EC₅₀ ( R ² = 0·832, P  = 0·0001), and the equation predicted that 42·3 applications of propiconazole would be needed to bring a sensitive population (EC₅₀ < 0·01  µ g mL⁻¹) to a resistant level (EC₅₀ > 0·10  µ g mL⁻¹). Fungicide sensitivity vs. duration of fungicide efficacy was also tested, and it was found that isolates with decreased sensitivity were able to more quickly overcome the inhibitory effects of fungicide application, reducing the duration of control from 3 weeks to 2 weeks.     

Biological control of southern blight disease of tomato caused by Sclerotium rolfsii with simplified mycelial formulations of Trichoderma koningii

Two simple formulations of an antagonistic strain of Trichoderma koningii were employed against southern blight disease caused by Sclerotium rolfsii in seedling, potted outdoor and field-grown tomatoes (cvs. Ife No. 1 and Ibadan Local). Corn cob germling inoculum and mycelium powder of T. koningii significantly controlled ( P ≤0·05) symptoms of damping off, blight and wilting in both tomato cultivars. The populations of the antagonist increased from an initial 1 × 10⁴ to about 1 × 10⁶ colony-forming units per g of soil in the protected plants. Moreover, sclerotial counts decreased significantly ( P ≤0·05) in these soils and those sclerotia found had been parasitized by T. koningii. Trichoderma -protected plants were more vigorous than those in the other treatment categories. The significance of these results is discussed in relation to the use of Trichoderma in appropriately simplified formulations.     

Biological control of damping-off of sugar beet by Pseudomonas putida applied to seed pellets

Pseudomonas putida 40RNF applied to seed pellets reduced the occurrence of Pythium damping-off of sugar beet. A density of 6 × 10⁷ 40RNF per pellet reduced Pythium damping-off from 70 to 26% when seeds were sown in artificially infested soil (250 propagules Pythium ultimum per g dry soil). The efficacy of 40RNF was dependent on its density in the seed pellet (in the range 2 × 10⁴–6 × 10⁸ per pellet) and on the number of propagules of Pythium in soil. 40RNF declined to or stabilized at approximately 1 × 10⁶ per pellet 3 days after planting, and this was independent of the inoculum density. This indicated that the crucial steps resulting in damping-off of sugar beet caused by Pythium ultimum must occur within 3–4 days of sowing. 40RNF reduced pericarp colonization by P. ultimum by 43% 48 h after planting and caused a 68% decrease in the number of sporangia of P. ultimum in the surrounding soil (0.0–5.0 mm). P. putida 40RNF also reduced pre and post-emergence damping-off (from 69.5 to 37.5%) caused by indigenous populations of Pythium species in an infested soil and this was as effective as the fungicide hymexazol (69.5 to 40%).     

Relationship between severity of blackleg (Leptosphaeria maculans/L. biglobosa species complex) and subsequent primary inoculum production on oilseed rape stubble

The relationship between severity of blackleg, or phoma stem canker ( Leptosphaeria maculans/L. biglobosa ), and subsequent primary inoculum production on oilseed rape ( Brassica napus ) stubble was investigated at two sites in France over 3 years. The quantity of primary inoculum produced in the following year increased with canker severity, from 1·9 to 10·8 pseudothecia cm⁻² on stubble with the least and most severe cankers, respectively. Stubble incubated at Le Rheu (cooler, more rain) had 1·7 times more pseudothecia than stubble incubated at Grignon. Stubble collected at Grignon had 2·7 times more pseudothecia than that collected at Le Rheu. The use of Darmor, a cultivar with a good level of quantitative resistance, reduced the severity of canker in the field, but not the subsequent inoculum production for stubble of the same canker severity class. At both sites, maturation of pseudothecia occurred after 63–75 days of incubation and increased with canker severity with a mean of 0·5 and 3% mature pseudothecia appearing per favourable day, on stubble with the least and most severe cankers, respectively. A simplified procedure for pseudothecial quantification proved satisfactory: for all three observers, most (91–96%) of the fructifications counted as pseudothecia were real pseudothecia. Only a few (4–14%) of the fructifications considered as non-pseudothecia were in fact pseudothecia of L. maculans . The total area occupied by pseudothecia, which was simpler and faster to evaluate, was correlated (coefficient of determination, R ² = 71%) with the number of counted pseudothecia. The results presented here make it possible to forecast the quantity of available primary inoculum for a given disease severity.     

Production of conidia by Peronosclerospora sorghi on sorghum crops in Zimbabwe

Factors affecting the production of conidia of Peronosclerospora sorghi , causing sorghum downy mildew (SDM), were investigated during 1993 and 1994 in Zimbabwe. In the field conidia were detected on nights when the minimum temperature was in the range 10–19°C. On 73% of nights when conidia were detected rain had fallen within the previous 72 h and on 64% of nights wind speed was < 2.0 m s⁻¹. The time period over which conidia were detected was 2–9 h. Using incubated leaf material, conidia were produced in the temperature range 10–26°C. Local lesions and systemically infected leaf material produced 2.4–5.7 × 10³ conidia per cm². Under controlled conditions conidia were released from conidiophores for 2.5 h after maturation and were shown to be well adapted to wind dispersal, having a settling velocity of 1.5 × 10⁻⁴ m s⁻¹. Conditions that are suitable for conidia production occur in Zimbabwe and other semi-arid regions of southern Africa during the cropping season.     

Pitch canker disease of pines

ABSTRACT Pitch canker, caused by Fusarium circinatum, is a disease affecting pines in many locations throughout the world. The pathosystem was originally described in the southeastern (SE) United States and was identified in California in 1986. Limited vegetative compatibility group (VCG) diversity in the California population of F. circinatum, relative to the SE United States, suggests the former is a recently established and clonally propagating population. Although the much greater VCG diversity found in the SE United States is suggestive of out-crossing, molecular markers indicate that many vegetatively incompatible isolates are clonally related. This implies that VCG diversity may derive, at least in part, from somatic mutations rather than sexual reproduction. Pitch canker is damaging to many pine species and one at particular risk is Monterey pine (Pinus radiata), which is widely grown in plantations and is highly susceptible to pitch canker. However, some Monterey pines are resistant to pitch canker and some severely diseased trees have been observed to recover. The absence of new infections on these trees reflects the operation of systemic induced resistance, apparently in response to repeated infection by the pitch canker pathogen.