Presence of Phytophthora fragariae var. fragariae in Norway

The strawberry red core disease caused by Phytophthora fragariae var. fragariae has until recently not been reported from Norway. During 1995 and 1996, samples from 323 Norwegian strawberry fields were tested in a root-tip bait test for possible detection of red core. Symptoms of the disease developed in test plants cultivated in growth medium containing root tips from 21 fields at eight locations in four different counties. Disease symptoms included wilting of plants, root rot, red steles in the roots, and oogonia and oospores identical in size and appearance to those of P.f. fragariae developing along the root steles. In V8 juice agar, the mycelium was slow-growing, thick, fluffy and aerial. Sporangia were large, mainly inversely pyriform, and without papillae. Optimum temperature for mycelial growth was 21°C, and the fungus ceased to grow at 25°C. In infested fields, the most severe attacks were found in low, often poorly drained areas. The means by which red core may have been introduced and spread in the country are discussed, together with preventive action to avoid further dissemination.     

Survey for Phytophthora fragariae var. fragariae in Finland

A survey for the possible occurrence of red-core disease ( Phytophthora fragariae var. fragariae ) was carried out on 250 strawberry production sites in Finland. The fields were inspected visually. A total of 1080 samples of strawberry roots were taken in spring and autumn 1995 and examined visually, microscopically and by isolation in the laboratory. Root-tip bait-plant tests were performed in the glasshouse to look for the latent presence of the fungus. No red-core disease was detected in any of the inspected fields or in the examined samples.     

大豆疫霉根腐病菌单游动孢子的毒性遗传与变异

 采用离体叶柄伤口接种法测定大豆疫霉根腐病菌44号生理小种单游动孢子连续3代或4代分离后代的毒性,结果表明:从S1中选择与亲本相比毒性不发生变异的1号单游动孢子菌株(44号生理小种)和变异最大的30号单游动孢子菌株(1号生理小种)继续分离2代或3代,单游动孢子毒性变异趋势主要是从44号生理小种变异为3号生理小种,也有变异成毒性公式为7或1a,7的单游动孢子。大豆疫霉根腐病菌无性世代毒性变异几率很高,多数单游动孢子毒性在分离后代中都发生变异,产生不同的小种或毒性公式,并且毒性变异基本不能稳定遗传。     

大豆疫霉菌单孢分离物生物学性状的遗传变异研究

 本文研究了大豆疫霉菌单游动孢子无性分离物和自交单卵孢子分离物的菌丝生长速率、菌落形态、同宗配合性状、产孢量以及对甲霜灵敏感性的遗传变异。结果表明:菌落形态、生长速率和同宗配合性状在单游动孢子后代和自交后代可稳定遗传,控制上述性状的遗传因子是纯合的;大豆疫霉菌的游动孢子产生能力和对甲霜灵的敏感性在单游动孢子后代和自交后代中均发生连续性变异,表明这两种性状可能是数量遗传性状,也可能控制这两种性状的基因为杂合基因或细胞质遗传因子。     

Foliar aggressiveness of Northern Ireland isolates of Phytophthora infestans on detached leaflets of three potato cultivars

The aggressiveness of 20 Northern Ireland single-lesion isolates of Phytophthora infestans was compared following their inoculation onto detached leaflets of three potato cultivars chosen on the basis of their differing levels of race-nonspecific resistance to late blight: Bintje (highly susceptible); Cara (moderately resistant); and Stirling (more resistant). Five isolates from outside Northern Ireland were included for comparative purposes: two from the Republic of Ireland; two from the USA (representing the US-1 and US-8 clonal lineages); and one from Mexico. To control the variation between tests, a balanced incomplete block design was used, as opposed to either a complete block design or the method of inclusion of standard isolates, where such variation would have increased the error. Highly significant variation for disease parameters, including latent period, infection frequency, area under the lesion expansion curve (AULEC) and sporulation capacity, was found between isolates. These differences were much more marked on the cultivars exhibiting higher levels of race-nonspecific resistance. There was a significant interaction between isolate and cultivar for all parameters assessed and, overall, no one isolate was the most aggressive across all three potato cultivars. However, a group comprising seven of the 20 Northern Ireland isolates was consistently found to exhibit the highest levels of aggression towards all three cultivars for each of the disease parameters. These results demonstrate that significant variation for foliar aggressiveness exists within the Northern Ireland population of P. infestans , and indicate the importance of selecting appropriately aggressive isolates for evaluation of host resistance to late blight within breeding programmes.     

Frequency of virulence phenotypes of Phytophthora fragariae in the field

The virulences of 102 single-zoospore cultures of Phytophthora fragariae from one field site were determined on a range of strawberry differentials, and used to assign the cultures to four clusters (I, II A, IIB, IIC) using cluster analysis. On cv. Favourite, which is susceptible to all known races of the pathogen, isolates in cluster I were recovered most frequently and had the narrowest spectrum of virulence. On cv. Saladin isolates in cluster IIB were more common and were pathogenic to this cultivar. However, c. 30% of the single zoospores from field isolates from Saladin were avirulent on this cultivar and belonged to cluster I. Hyphal-tip and single-zoospore cultures from selected field isolates in cluster IIB did not always have the same virulence phenotype as the parent isolates. One hyphal-tip culture from a field isolate in cluster IIB had a virulence phenotype (IID) which had not been recorded before in Europe, and it attacked some cultivars not previously affected by red core. When cultivars such as Saladin were inoculated with mixtures of zoospores from two isolates from different clusters, with and without the corresponding virulence factors, the isolate with the corresponding virulence factor was selected. However, on the universally susceptible cv. Favourite the results depended on the relative competitiveness of the isolates and not on virulence factors.     

Detection and identification of Phytophthora fragariae Hickman by the polymerase chain reaction

Phytophthora fragariae Hickman, which causes strawberry red stele and raspberry root rot, is a quarantine organism for which specific and sensitive detection methods are required to test the health of planting material. Sequences of the internal transcribed spacer regions of the ribosomal gene repeat (rDNA) were used to develop primers for P. fragariae in a nested Polymerase Chain Reaction (PCR). The fungus was readily detected in infected but symptomless roots by nested, but not single-round, PCR. It was also detected in infested water samples obtained from the Dutch General Inspection Service by nested PCR. Detection of PCR products was at least 10-fold more sensitive by PCR-ELISA than by conventional visualisation on agarose gels.     

Cross-reactivity of antiserum raised against Phytophthora fragariae with other Phytophthora species and its evaluation as a genus-detecting antiserum

Antiserum (anti-PfM) raised against mycelial suspensions of Phytophthora fragariae isolates reacted strongly with antigens from several Phytophthora species. Some cross-reactions with antigens from Pythium species were decreased by fractionating on an affinity column of Sepharose 4B bound to extracts of Fragaria vesca roots infected with P. fragariae. The affinity-purified anti-PfM retained its high cross-reactivity with the various Phytophthora species tested. It also detected infection of raspberry and strawberry roots by some Phytophthora species. This antiserum could, therefore, prove useful as a broad-spectrum Phytophthora-detecting antiserum.
Anti-PfM could not be made specific for P. fragariae because it was raised against components shown to be antigenically similar in all Phytophthora species tested. However, immunoblotting with the affinity-purified anti-PfM produced distinct patterns for P. fragariae, P. erythroseptica and P. cactorum: three serotypes were identified for the latter species. This antiserum might therefore prove useful in classifying Phytophthora species.     

Variability in aggressiveness of Quambalaria pitereka isolates

Quambalaria shoot blight, caused by the fungal pathogen Quambalaria pitereka, is a serious disease of eucalypt plantations in Australia. The aggressiveness of four Q. pitereka isolates was compared on a range of host genera, species, provenances and clones. Isolates differed substantially in their aggressiveness, with two consistently showing higher levels of aggressiveness based on incidence and severity of disease and lesion size. Isolates derived from Corymbia citriodora subsp. variegata (Ccv) and C. torelliana were shown to have a relatively restricted host range, with lesions but no sporulation found on Eucalyptus species, Angophora species other than A. costata and Corymbia species other than Ccv, the host of origin. The level of aggressiveness toward the different provenances of spotted gum and C. torelliana varied between isolates and there was evidence of some isolate × host interaction within provenances of Ccv. The two methods of inoculation used in this study, spray and spot inoculation, gave similar results. However, the fact that the spot inoculation method was labour‐intensive was a disadvantage limiting the numbers of isolates and hosts that can be tested.     

Containment and Eradication of Phytophthora fragariae in Sweden1

Sweden is the only Nordic country in which Phytophthora fragariae is locally established. Growers and nurserymen have suffered severe financial losses in the last 5 years. Further spread is being limited by an intensification of the official compulsory certification scheme for strawberry planting material and by information, surveys and supervision of control. Imported material is subject to post-entry quarantine.     

Survival,distribution and genetic variability of inoculum of the strawberry red core pathogen,Phytophthora fragariae var. fragariae,in soil

An experimental field infested with Phytophthora fragariae var. fragariae (Pff) and used for strawberry red core fungicide and cultivar resistance trials until 1981 was surveyed for the presence of inoculum of the pathogen 11 and 12 years later. Alpine strawberries, highly susceptible to all races of Pff, were grown from true seed and planted as a bait crop on a 0·5 m‐spaced grid. Rapid and widespread red core infection was observed, which provided good evidence that oospores had survived in soil for this extended period. Site elevation and the distribution of red core infected plants showed a strong correlation, with a higher frequency of infected and dead plants in the lowest areas of the field. The race designation of 18 recovered isolates were determined and AFLP fingerprint patterns of some of these and their single‐spore derivatives were analysed. The isolates differed little in race type, and the majority were genetically identical at 433 AFLP loci. Races used to inoculate the site in the 1970s were recovered. The fingerprints of the single variant isolate matched that of an isolation made by Hickman in the 1950s, originally used to inoculate the site. Clearly Pff is a very stable and long‐lived pathogen able to retain its genetic integrity and lie dormant in soil for many years, ensuring its survival between epidemiologically favourable conditions which occur erratically.     

Effect of temperature and host genotype on the production of inoculum by Phytophthora fragariae var. fragariae from the roots of infected strawberry plants

A bioassay was used to monitor the release of inoculum in drainage water from strawberry plants inoculated with zoospores of Phytophthora fragariae var. fragariae. The fungus was detected in drainage water from plants that had been held at temperatures between 2 and 20 C. but not from plants held at 26°C. The lag phase before secondary inoculum was first released, the maximum and total amounts of inoculum released, and the length of time over which inoculum was released were all greater at the lower temperature regimes, especially those below 10 C. The results were consistent with observations on the effect of temperature on zoospore production from agar discs and on zoospore motility: more zoospores were produced at lower temperatures and they remained motile for longer. From this it is concluded that the inoculum detected consists mainly of motile zoospores. In most experiments with standardized suspensions c. 10-15 were sufficient to initiate infection of the plants in the bioassay. In general, more inoculum was produced by host genotype/fungal isolate combinations in which there were marked root rot symptoms than in combinations in which the host was resistant.     

Variability in Virulence Among Asexual Progenies of Phytophthora infestans

ABSTRACT One hundred two single zoospore isolates of Phytophthora infestans, derived asexually from four parental isolates of US-8 genotype and one isolate of US-1 genotype, were characterized for their virulence phenotypes to determine changes in virulence during asexual reproduction. Potato differentials, each containing a major gene for resistance to P. infestans (R1 to R11), were used to characterize the virulence patterns. Isolates were also characterized for mating type, glucose-6-phosphate isomerase (Gpi) banding pattern, and DNA fingerprints using probe RG57 to determine any genotypic changes in the single zoospore isolates. A subset of these single zoospore isolates was tested for response to mefenoxam to determine any shifts in sensitivity. Results showed that single zoospore isolates derived from parent PI-1 (US-8, 11 isolates) were identical to their parental virulence. Isolates derived from parent PI-191 (US-8, 29 isolates) showed some differences in virulence, mainly toward R8 and R9. Isolates derived from parent PI-126 (US-8, 14 isolates) demonstrated a higher level of virulence diversity. Isolates derived from parents PI-52 (US-1, 28 isolates) and PI-105 (US-8, 20 isolates) showed the highest level of virulence variability among the single zoospore isolates. Mating type, Gpi banding pattern, and DNA fingerprints for the single zoospore isolates were, in most cases, identical to the parental isolates. Single zoospore isolates showed different levels of sensitivity to mefenoxam. Virulence and other genetic changes during asexual reproduction are likely to play a major role in changing the race structure of P. infestans populations. This continuous change in the race structure is a serious problem and now poses a new challenge for utilization of race-specific resistance to manage late blight of potato.     

The role of aggressiveness and competition in the selection of Phytophthora infestans populations

Selection within populations of Phytophthora infestans was investigated by comparing the aggressiveness of single‐lesion isolates on detached leaflets of four potato cultivars with differing levels of race‐nonspecific resistance to P. infestans. The isolates included 23 representative of Northern Ireland genotypes from the early 2000s, used to inoculate previously reported field trials on competitive selection (2003–2005), plus 12 isolates recovered from the 2003 trial. The cultivars were those planted in the previous trials: Atlantic (blight‐susceptible) and Santé, Milagro and Stirling (partially resistant). Very highly significant variation for latent period, infection frequency and lesion area was found between genotypes and cultivars; differences between genotypes were more marked on the more resistant cultivars, but no one genotype was the most aggressive across all. Detached leaflets were also inoculated with mixtures of isolates from each genotype group at three sporangial concentrations: differences in aggressiveness between genotypes were more apparent at lower concentrations and on the more resistant cultivars. Genotype groups that were the most aggressive on the more resistant cultivars tended to be those selected by the same cultivars in the field. A mixture of all isolates of all genotypes was used to inoculate detached leaflets of the same cultivars. With one exception, single spore isolates recovered from any one leaflet belonged to a single genotype, but different genotypes were recovered from different cultivars. Phytophthora infestans isolates from Northern Ireland showed significant variation for foliar aggressiveness, and pathogen genotypes exhibited differential aggressiveness to partially resistant cultivars and interacted competitively in genotype selection.