Reaction of a heterozygous tomato hybrid bearing the Mi-1.2 gene to 15 Meloidogyne species

Many root-knot nematode (RKN) species (Meloidogyne spp.) are polyphagous and cultivated tomato (Solanum lycopersicum) is one of their preferential hosts, leading to significant losses. It is known that the dominant Mi-1.2 gene in tomato confers resistance to the three most important RKN species—M. incognita, M. javanica, and M. arenaria, and minor species—M. ethiopica, M. hispanica, and M. luci. However, little information is available about resistance of tomatoes carrying this gene to other tomato-infecting RKN species. In this study, resistance conferred by the Mi-1.2 gene/locus was evaluated against populations of 15 Meloidogyne species, employing tomato cultivars Santa Clara (homozygous recessive mi-1.2/mi-1.2, susceptible) and Debora Plus (heterozygous Mi-1.2/mi-1.2, resistant). Debora Plus was susceptible only to M. enterolobii and M. hapla, and was resistant to the other Brazilian populations of M. arenaria, M. ethiopica, M. exigua, M. hispanica, M. incognita, M. inornata, M. izalcoensis M. javanica, M. konaensis, M. luci, M. morocciensis, M. paranaensis, and M. petuniae. Mi-1.2 is located on tomato chromosome 6 within a cluster of seven homologous genes of the nucleotide-binding site leucine-rich repeat (NBS-LRR) family; further research is required to confirm if this multiple Meloidogyne spp. resistance phenotype is controlled exclusively by Mi-1.2 or by combined action of other closely linked genes. This evaluation of resistance of the Debora Plus cultivar to several Meloidogyne species suggests that the Mi-1.2 gene/locus may reduce losses induced by a wide range of Meloidogyne spp. Further studies using additional resistant cultivars and other populations of Meloidogyne spp. are needed to confirm these results.     

Variation in aggressiveness of Hymenoscyphus fraxineus genotypes amid the ash dieback epidemic

The ascomycete Hymenoscyphus fraxineus causes devastating damage to the European common ash (Fraxinus excelsior). The fungus originates from Asia, where it coexists with native ash species and completes its life cycle by sporulating on degrading ash leaf litter on the forest floor. Given this life cycle of the fungus, genotypes of H. fraxineus with varying degrees of aggressiveness may coexist in infected European ash forests. To test this hypothesis, we cultured 19 single-spore isolates from apothecia collected in a trial of heavily infected ash in Denmark and carried out stem inoculations on young ash seedlings. Microsatellite markers revealed that the 19 isolates were all genetically unique and did not show any genetic structure. High variation was observed among the 19 isolates in pathogenicity on young seedlings. The variation (assessed as necrosis development) was highly significant, but not correlated with the vigour of the fungal isolates when grown on culture media. The results support the hypothesis that aggressiveness of H. fraxineus may not be crucial for its fitness. In this sense, ash dieback disease may differ from other recent emerging infectious diseases on trees. We discuss the variation in aggressiveness of H. fraxineus in relation to durability of resistance and future management of the ash dieback epidemic. The findings of this study call for more research into natural variation in endophytic versus pathogenic behaviour of H. fraxineus on European ash.     

Damping-off of soybean in southern Brazil can be associated with different species of Globisporangium spp. and Pythium spp.

Damping-off of soybean is a common problem in some fields in southern Brazil. Frequently, Phytophthora sojae has been pointed out as the main causal agent. However, questions were raised as to whether other oomycetes could also be present. Hence, the objectives of this study were to isolate, identify, and determine the pathogenicity and aggressiveness of oomycetes isolated from soybean seedlings. Thirty-three isolates were obtained from 13 fields. Isolates were identified based on morphology and molecular methods. Nine species were recovered: Pythium conidiophorum (n = 14, 42.4%), Globisporangium rostratifingens (n = 7, 21.2%), G. irregulare (n = 2, 6.1%), G. spinosum (n = 1, 3.0%), G. ultimum var. sporangiiferum (n = 2, 6.1%), Pythium acanthicum (n = 2, 6.1%), P. deliense (n = 1, 3.0%), P. inflatum (n = 1, 3.0%), and P. torulosum (n = 3, 9.1%). Pathogenicity and aggressiveness were determined with two soybean cultivars (DM57i52 and NEO530) in a seed rot assay and a root rot assay. All isolates were pathogenic to soybean with variable levels of aggressiveness in both assays. The range of seed disease severity index varied from 7.5 to 100 for DM57i52 and from 23.1 to 100 for NEO530. The mean root disease severity index of the soybean cultivars ranged from 29.4 to 92.8. This is the first official report of G. irregulare, G. rostratifingens, G. spinosum, G. ultimum var. sporangiiferum, P. acanthicum, P. conidiophorum, P. deliense, P. inflatum, and P. torulosum causing damping-off of soybean in the state of Rio Grande do Sul in southern Brazil.     

Nature of wheat resistance to Fusarium head blight and the role of deoxynivalenol for breeding

Twenty (1990-93) and 25 (1994-96) wheat genotypes with different degrees of resistance and origins were tested with seven and eight isolates, respectively, of Fusarium graminearum and four Fusarium culmorum isolates of diverse origin in Europe. Infection severity depended largely on the genotypes and the isolates used. Head blight values, yield response and kernel infection values revealed close but varying relationships with deoxynivalenol (DON) content. This variability is explained by the presence of tolerance mechanisms which affect the relationship between Fusarium head blight severity and yield response. Kernel infection resistance accounted for decreasing Fusarium head blight values. Genotypes were found with lower infection severity and higher DON contamination and vice versa. Evidently, the cultivar has a significant influence on DON production in the infected tissue, i.e. highly susceptible genotypes may have moderate or low accumulation of DON. However, in the most resistant genotypes showing no infection to any of the isolates or only sporadic symptom development, no or very low accumulation of DON was detected. Resistant genotypes gave a stable reaction with b-values close to zero for all traits tested. Susceptible genotypes were unstable under different epidemic conditions and their stability was different for the traits investigated. Therefore, the mean of b-values is suggested to better describe the stability of the wheat genotypes. Significant positive relationships were found between aggressiveness of the isolates and their production of DON in the infected grain. The correlation improved significantly for the nivalenol-producing isolate (F89.4 from France) when the sum of DON and nivalenol contents were considered. This indicates that the total trichothecene toxin-producing capacity of the isolates may be a decisive component of pathogenicity. Since the tests included isolates from different European countries the results provide further proof that no host specificity exists within these pathogens in Europe. This was also valid for kernel infection, yield response and DON accumulation. Therefore, the nature of resistance is horizontal. The results also support the view that there is no difference between the resistance of the host plant to F. graminearum and to F. culmorum.     

Susceptibility of Douglas fir (Pseudotsuga menziesii) to pitch canker, caused by Gibberella circinata (anamorph = Fusarium circinatum)

For better characterization of the risk of pitch canker (caused by Gibberella circinata , anamorph =  Fusarium circinatum ) to Douglas fir ( Pseudotsuga menziesii ), Californian isolates, selected exotic isolates, and ascospore progeny of a cross between wild-type Californian isolates were tested for aggressiveness to this host species. In addition, seedlings from representative provenances of P. menziesii in California were tested for susceptibility to pitch canker. The results revealed only minor differences between isolates, but differences in susceptibility between trees were often significant. The majority of the tested trees were relatively resistant as indicated by the development of only very short lesions, but some were clearly susceptible.     

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.     

Phenotypic variability of Leptosphaeria lindquistii (anamorph: Phoma macdonaldii), a fungal pathogen of sunflower

Growth of 17 isolates of Phoma macdonaldii , the causal agent of sunflower black stem, was investigated for response to pH and temperature, and for morphology and asexual morphogenesis (pycnidiogenesis and pycnidium size). For all isolates, the optimum pH for growth was between 4 and 5, and the optimum temperature varied between 20 and 30°C and radial growth was slowest at 5 and 35°C. Significant differences in the number and size of pycnidia were observed between isolates. Pycniospore germination was investigated under various conditions in five isolates chosen for their geographical origins, pigmentation, optimum growth temperature and pycnidiogenesis. Increasing the concentration from 10⁶ to 10⁷ pycniospores per mL decreased the germination rate. The optimum temperature for pycniospore germination varied between 15 and 30°C, depending on the isolate, and the optimum and maximum pH values were 5 and 7, respectively. The optimum and minimum relative humidities allowing pycniospore germination were 100 and 95%, respectively. Pycniospore germination was photo-independent. An artificial inoculation method was developed and the aggressiveness of the pathogen was assessed on a susceptible sunflower cultivar, using a 1–9 scale that integrated the percentage of necrotic area on the cotyledon petiole at the stage when the first pair of leaves was fully developed. Significant differences in aggressiveness were observed among the 17 isolates. The parameters investigated clearly suggest the occurrence of a wide phenotypic variability in Phoma macdonaldii .     

Genetic variation in Fusarium avenaceum causing root rot on field pea

Isolates of Fusarium spp. were recovered from the roots of field pea (Pisum sativum) collected from 15 commercial fields in Alberta, Canada. Most of the isolates (75 out of 96) were identified as F. avenaceum, based on morphology, phylogeny and species‐specific PCR amplification. Molecular differences in the F. avenaceum isolates were detected based on putative mating type, and on ITS and CPN60 sequences. MAT‐1 and MAT‐2 were equally distributed among the isolates. Phylogenetic analysis based on ITS and CPN60 sequences clustered most of the F. avenaceum isolates into a single group. In some cases, isolates with low aggressiveness clustered together in additional groups. There was no correlation between phylogenetic profile and either mating type or geographic origin. This population of F. avenaceum has a low level of genetic variation and consists of isolates derived from the two mating types. Isolates with low aggressiveness are also retained in the population.     

The return ofPhytophthora infestans, a potato pathogen that just won't quit

Summary Since the early 1980s and continuing through the end of the 20^th century, potato late blight caused byPhytophthora infestans has become noticeably more problematic throughout the world. Exotic strains of the pathogen are largely responsible for the increased problems. The new strains were initially detected in Europe, but subsequently they have been detected in Asia, the Middle East, South America and North America. Populations in the Middle East and South America may represent secondary migrations from Europe. Several independent migrations have occurred such that migrant populations in North America are very different from those in Europe. Also, population structures ofP. infestans are different in different worldwide locations. In some locations the migrant strains are resistant to metalaxyl, but in other locations either migrant or indigenous strains may be resistant. In most locations, the migrant strains are more aggressive than the previous indigenous strains.     

四川地区嗜蚧镰刀菌的培养和侵染研究

本文研究了嗜蚧镰刀菌Fusarium coccophilum 培养与培养基、温度和ph 值的关系及寄生致病力。