Host Specificity of Ascochyta spp. Infecting Legumes of the Viciae and Cicerae Tribes and Pathogenicity of an Interspecific Hybrid

ABSTRACT Ascochyta spp. (teleomorphs: Didymella spp.) infect a number of legumes, including many economically important species, and the diseases they cause represent serious limitations of legume production worldwide. Ascochyta rabiei, A. fabae, A. pisi, A. lentis, and A. viciae-villosae are pathogens of chickpea (Cicer arietinum), faba bean (Vicia faba), pea (Pisum sativum), lentil (Lens culinaris), and hairy vetch (V. villosa), respectively. Inoculations in the greenhouse and in growth chambers demonstrated that A. fabae, A. lentis, A. pisi, A. rabiei, and A. viciae-villosae were host specific. Isolates caused no visible disease symptoms on "nonhost" plants (plants other than the hosts they were originally isolated from) but were recovered consistently from inoculated, surface-disinfested, nonhost tissues. Interspecific crosses of A. pisi x A. fabae and A. viciae-villosae x A. lentis produced pseudothecia with viable ascospores, and the hybrid status of the ascospore progeny was verified by the segregation of mating type and amplified fragment length polymorphism (AFLP) markers. Interspecific progeny were morphologically normal in culture but exhibited more phenotypic variation compared with progeny from intraspecific crosses. Mating type and the majority of AFLP markers segregated in Mendelian 1:1 ratios in both intraspecific and interspecific crosses. A total of 11 and 7% of AFLP markers showed segregation distortion among progeny from interspecific crosses and intraspecific crosses, respectively; however, this difference was not significant (P = 0.90). Only 30 of 114 progeny isolates from the A. fabae x A. pisi cross inoculated in the greenhouse caused lesions on pea and only 4 caused disease on faba bean. In all, 15 of 110 progeny isolates were pathogenic to pea and none were pathogenic to faba bean under growth chamber conditions. Although no obvious postzygotic, intrinsic isolating barriers were identified in any of the interspecific crosses, it appears that host specialization may act as both a prezygotic, ecological isolating barrier and a postzygotic, extrinsic, ecological isolating barrier in these fungi. Host specificity, coupled with low pathogenic fitness of hybrids, may be an important speciation mechanism contributing to the maintenance of hostspecific, phylogenetic lineages of these fungi.     

Diagnostics,genetic diversity and pathogenic variation of ascochyta blight of cool season food and feed legumes

Molecular diagnostic techniques have been developed to differentiate the Ascochyta pathogens that infect cool season food and feed legumes, as well as to improve the sensitivity of detecting latent infection in plant tissues. A seed sampling technique was developed to detect a 1% level of infection by Ascochyta rabiei in commercial chickpea seed. The Ascochyta pathogens were shown to be genetically diverse in countries where the pathogen and host have coexisted for a long time. However, where the pathogen was recently introduced, such as A. rabiei to Australia, the level of diversity remained relatively low, even as the pathogen spread to all chickpea-growing areas. Pathogenic variability of A. rabiei and Ascochyta pinodes pathogens in chickpea and field pea respectively, appears to be quantitative, where measures of disease severity were based on aggressiveness (quantitative level of infection) rather than on true qualitative virulence. In contrast, qualitative differences in pathogenicity in lentil and faba bean genotypes indicated the existence of pathotypes of Ascochyta lentis and Ascochyta fabae. Therefore, reports of pathotype discrimination based on quantitative differences in pathogenicity in a set of specific genotypes is questionable for several of the ascochyta-legume pathosystems such as A. rabiei and A. pinodes. This is not surprising since host resistance to these pathogens has been reported to be mainly quantitative, making it difficult for the pathogen to overcome specific resistance genes and form pathotypes. For robust pathogenicity assessment, there needs to be consistency in selection of differential host genotypes, screening conditions and disease evaluation techniques for each of the Ascochyta sp. in legume-growing countries throughout the world. Nevertheless, knowledge of pathotype diversity and aggressiveness within populations is important in the selection of resistant genotypes.     

Most Colletotrichum species associated with tree tomato (Solanum betaceum) and mango (Mangifera indica) crops are not host‐specific

An important constraint for crop production in Colombia is the high incidence of anthracnose caused by Colletotrichum species. Although several studies have focused on these fungi, the relationship between the different fungal species within the genus and their hosts and whether they display any host preference or host specificity has yet to be examined. In Colombia, diseases caused by Colletotrichum species are particularly severe in mango (Mangifera indica) and tree tomato (Solanum betaceum) crops. In a previous investigation, the Colletotrichum phylogenetic species attacking these crops were identified. The present study aimed to determine whether isolates collected from tree tomato and mango showed host preference or host specificity by assessing aggressiveness, spore density, latent period, and fitness of each strain on the two hosts. In the departments of Cundinamarca and Tolima, Colombia, isolates were collected from plants that presented typical anthracnose symptoms and were identified as C. acutatum, C. asianum, C. boninense, C. gloeosporioides, C. tamarilloi and C. theobromicola. Inoculation of conidia of each isolate onto both hosts showed isolates had no host preference and only the C. gloeosporioides isolate showed host specificity. However, in general, isolates produced a higher spore density when inoculated on the alternate host, which may indicate a difference in the degree of adaptation to each host. Statistical analyses of the assessed parameter values revealed that isolates use different infection strategies when infecting each host. In light of these results, the implications of using quantitative estimations of fitness when studying fungal pathogens are discussed.     

蚕豆和豌豆锈病病原菌的分子鉴定

为明确我国热带和亚热带地区蚕豆Vicia faba和豌豆Pisum sativum锈病的病原菌种类,通过致病性测定和ITS序列系统发育分析对来自我国云南省玉溪市的4份豌豆锈菌分离物及云南、广西、重庆和四川省(区、市)的5份蚕豆锈菌分离物进行系统鉴定。结果显示,分离自豌豆的锈菌WX1分离物对蚕豆和豌豆均具有高致病性,在侵染叶片上产生大量锈子器;分离自蚕豆的锈菌CX3分离物仅对蚕豆具有高致病性,能在叶片上产生大量夏孢子,而对豌豆的致病性相对较低,仅产生少量的夏孢子堆;分离物WX1和CX3对小扁豆和鹰嘴豆不具有致病性。基于ITS序列系统发育分析表明,所有不同寄主来源的蚕豆单胞锈菌分离物均聚类于一个系统发育组,但分离自蚕豆和豌豆的分离物分别聚类在不同的亚组。表明分离自云南省玉溪市豌豆上的蚕豆单胞锈菌Uromyces viciae-fabae应为豌豆专化型,定名为U. viciae-fabae ex P. sativaum,而来源于云南、广西、重庆和四川省(区、市)的蚕豆锈病病原菌为蚕豆专化型U. viciae-fabae ex V. faba。     

<Emphasis Type="Italic">Eutypa lata</Emphasis>, the causal agent of dieback in red currant (<Emphasis Type="Italic">Ribes rubrum</Emphasis>) and gooseberry (<Emphasis Type="Italic">R. uva-crispa</Emphasis>) in the Netherlands

Dieback of red currant (Ribes rubrum) and gooseberry (Ribes uva-crispa) is an increasing problem in commercial fields in the Netherlands. Field surveys were done in 2006–2007 and samples with dieback symptoms were analysed. In this study the causal agent was diagnosed as Eutypa lata, based on morphological characteristics and rDNA-ITS sequence data. The field surveys revealed the presence of the anamorph and teleomorph states of the fungus produced on dead infected currant wood. Eutypa lata is a vascular pathogen of many woody plants. Related fungi from the same family Diatrypaceae are difficult to distinguish from E. lata based on morphological features. The genetic variability of E. lata was compared by rDNA-ITS sequencing of isolates from different hosts and origins. Within the E. lata isolates little variability in the ITS sequences was observed. Phylogenetic analysis showed no clear subdivisions within the species. Eutypa lata strains isolated from the different hosts were closely related, indicating that there is no direct evidence for host specificity.     

Australian Uromyces viciae-fabae: Host and nonhost interaction among cultivated grain legumes

Uromyces viciae-fabae, rust of faba bean, parasitizes other legume crops such as lentils (Lens culinaris) and field peas (Pisum sativum) in some environments. In this study we examined the host range of two Australian isolates of U. viciae-fabae collected and purified from a faba bean crop and classified as U. viciae-fabae ex V. faba. Field pea (P. sativum), chickpea (Cicer arientinum), lupin (Lupinus spp.), lentil (L. culinaris), and mung bean (Vigna radiata) genotypes were tested with these isolates, as well as resistant and susceptible genotypes of the faba bean host. Race specificity for these two pathogen isolates was observed on Vicia faba, with two faba bean genotypes showing partial resistance. Both U. viciae-fabae isolates also colonized field pea seedlings and successfully produced uredinia under glasshouse conditions, despite this fungus not being known as a pathogen of Australian field pea crops. No sporulation of either isolate of U. viciae-fabae ex V. faba was observed on any of the remaining legume species tested. However, obvious differences in fungal growth were observed, ranging from small infection sites with very rare haustorium formation in mung bean to more extensive growth and the development of potential uredinial structures in chickpea. These observations are discussed in relation to the phylogenetic relationship of these host and nonhost species.     

Control of seed-borne pathogens on legumes by microbial and other alternative seed treatments

Greenhouse trials were carried out in order to test the efficacy of different seed treatments as alternatives to chemicals against Colletotrichum lindemuthianum cause of anthracnose on bean and Ascochyta spp. cause of Ascochyta blights on pea, respectively. Resistance inducers, commercially formulated microorganisms, non-formulated selected strains of different microorganisms (fungi, bacteria and yeasts) and plant extracts were applied as dry or liquid seed treatments on naturally infested seeds. Seedling emergence and disease incidence and/or severity were recorded. Almost all seed treatments turned out to be ineffective in controlling the Ascochyta infections, which is in line with the literature stating that these pathogens are difficult to control. The only alternative treatments that gave some control of Ascochyta spp. were thyme oil and a strain of Clonostachys rosea. The resistance inducers tested successfully controlled infections of bean by C. lindemuthianum. Among the formulated microorganisms, Bacillus subtilis-based formulations provided the best protection from anthracnose. Some strains of Pseudomonas putida, a disease-suppressive, saprophytic strain of Fusarium oxysporum and the mustard powder-based product Tillecur also proved to be effective against bean anthracnose. However, among the resistance inducers as well as among the other groups, certain agents caused a significant reduction of plant emergence. Different alternative seed treatments can therefore be used for the control of C. lindemuthianum on bean, while on pea only thyme oil and a strain of Clonostachys rosea showed some effectiveness against Ascochyta spp.     

Detoxification of α-tomatine by tomato pathogens Alternaria alternata tomato pathotype and Corynespora cassiicola and its role in infection

In tomato plants, α-tomatine, a steroidal glycoalkaloid saponin, inhibits fungal growth. Tomato pathogens that produce host-specific toxins, Alternaria alternata tomato pathotype causing Alternaria stem canker and Corynespora cassiicola causing Corynespora target spot, were investigated for sensitivity to α-tomatine. Although spore germination of A. alternata pathogenic and nonpathogenic to tomato and of C. cassiicola pathogenic to tomato was not affected by 0.1 mM α-tomatine, spore germination of C. cassiicola nonpathogenic to tomato was significantly inhibited. This result showed that A. alternata, regardless of its pathogenicity, and only the C. cassiicola pathogenic to tomato are resistant to α-tomatine. Germinating spores of A. alternata and C. cassiicola resistant to α-tomatine detoxified α-tomatine by degrading it to a less polar product. After inoculation of tomato leaves, spores of A. alternata and C. cassiicola nonpathogenic to tomato germinated and formed appressoria, but did not form infection hyphae in host tissues. When a host-specific toxin (CCT-toxin) produced by C. cassiicola pathogenic to tomato was added to nonpathogenic spores, colonization within leaves was observed in A. alternata, but not in C. cassiicola. On the other hand, when spores of C. cassiicola nonpathogenic to tomato were suspended in spore germination fluid of nonpathogenic A. alternata with α-tomatine detoxification activity, the fungus could be induced to colonize leaves in the presence of CCT-toxin. These results indicate that A. alternata tomato pathotype and C. cassiicola pathogenic to tomato detoxify α-tomatine during infection and that this detoxification is essential for host colonization by pathogens that produce host-specific toxins.     

Direct and host‐mediated interactions between Fusarium pathogens and herbivorous arthropods in cereals

Fusarium head blight and fusarium ear rot diseases of cereal crops are significant global problems, causing yield and grain quality losses and accumulation of harmful mycotoxins. Safety limits have been set by the European Commission for several Fusarium‐produced mycotoxins; mitigating the risk of breaching these limits is of great importance to crop producers as part of an integrated approach to disease management. This review examines current knowledge regarding the role of arthropods in disease epidemiology. In the field, diseased host plants are likely to interact with arthropods that may substantially impact the disease by influencing spread or condition of the shared host. For example, disease progress by Fusarium graminearum can be doubled if wheat plants are aphid‐infested. Arthropods have been implicated in disease epidemiology in several cases and the evidence ranges from observed correlations between arthropod infestation and increased disease severity and mycotoxin accumulation, to experimental evidence for arthropod infestation causing heightened pathogen prevalence in hosts. Fusarium pathogens differ in spore production and impact on host volatile chemistry, which influences their suitability for arthropod dispersal. Herbivores may allow secondary fungal infection after wounding a plant or they may alter host susceptibility by inducing changes in plant defence pathways. Post‐harvest, during storage, arthropods may also interact with Fusarium pathogens, with instances of fungivory and altered behaviour by arthropods towards volatile chemicals from infected grain. Host‐mediated indirect pathogen–arthropod interactions are discussed alongside a comprehensive review of evidence for direct interactions where arthropods act as vectors for inoculum.     

Ultrastructural analysis of responses of host and fungal cells during plant infection

Cellular responses in fungi and in susceptible or resistant hosts during fungus–plant interactions have been studied ultrastructurally to examine their role in pathogenicity. Pathogenicity is determined in some saprophytic fungi by various factors: the production of disease determinants such as the production of host-specific toxins (HSTs) or the extracellular matrix (ECM) by fungal infection structures and H₂O₂ generation from penetration pegs. Three different target sites for HSTs have been identified in host cells in many ultrastructural studies: plasma membranes, chloroplasts, and mitochondria. The mode of action of HSTs is characterized by the partial destruction of the target structures only in susceptible genotypes of host plants, with the result that the fungus can colonize the host. The infection structures of most fungal pathogen secrete ECM on plant surfaces during fungal differentiation, while the penetration pegs of some pathogens produce reactive oxygen species (ROS) in the cell walls and plasma membranes. The pathological roles of ECM and H₂O₂ generation are discussed here in light of ultrastructural evidence. Host and fungal characteristics in the incompatible interactions include the rapid formation of lignin in host epidermal cell walls, failure of penetration pegs to invade lignin-fortified pectin layers, the inhibition of subcuticular hyphal proliferation and the collapse of hyphae that have degraded cell walls within pectin layers of the host. Apoptosis-like host resistant mechanism is also discussed.     

Importance of the life cycle in sympatric host race formation and speciation of pathogens

ABSTRACT Numerous morphological species of pathogenic fungi have been shown to actually encompass several genetically isolated lineages, often specialized on different hosts and, thus, constituting host races or sibling species. In this article, we explore theoretically the importance of some aspects of the life cycle on the conditions of sympatric divergence of host races, particularly in fungal plant pathogens. Because the life cycles classically modeled by theoreticians of sympatric speciation correspond to those of free-living animals, sympatric divergence of host races requires the evolution of active assortative mating or of active host preference if mating takes place on the hosts. With some particular life cycles with restricted dispersal between selection on the host and mating, we show that divergence can occur in sympatry and lead to host race formation, or even speciation, by a mere process of specialization, with strong divergent adaptive selection. Neither active assortative mating nor active habitat choice is required in these cases, and this may explain why the phylo-genetic species concept seems more appropriate than the biological species concept in these organisms.     

Biocontrol traits of plant growth suppressive arbuscular mycorrhizal fungi against root rot in tomato caused by <Emphasis Type="Italic">Pythium aphanidermatum</Emphasis>

Arbuscular mycorrhizal (AM) fungi known to cause plant growth depressions in tomato were examined for their biocontrol effects against root rot caused by Pythium aphanidermatum. The main hypothesis was that plant growth suppressive AM fungi would elicit a defence response in the host plant reducing Pythium root rot development. To test this hypothesis a fully factorial experiment was performed with AM fungi (Glomus intraradices, G. mosseae, G. claroideum or nonmycorrhizal), Pythium (± P. aphanidermatum) and harvest (7 and 14 days after pathogen inoculation (dapi)) as the main factors. Two weeks after AM fungi inoculation, roots were challenged with P. aphanidermatum. Variables evaluated at each harvest were root colonization levels of the interacting fungi, plant growth responses, and expression of a plant pathogenesis related protein gene (PR-1). All of the tested AM fungi caused marked growth suppressions, but did not affect PR-1 gene expression or the phosphorous concentration in the host plant. Plants singly inoculated with P. aphanidermatum had an increased PR-1 expression and phosphorous concentration. Among the AM fungi included in the study only G. intraradices reduced the pathogen root infection level, measured both in terms of Pythium ELISA and by recovery on selective media and only at the first harvest. Likewise, P. aphanidermatum root infection reduced colonization levels of G. intraradices, but not that of the two other AM fungi. In conclusion, plant growth suppressive AM fungi may offer plant beneficial traits in terms of biocontrol of root cortical pathogens.     

Investigating the effector suite profile of Australian Fusarium oxysporum isolates from agricultural and natural ecosystems

Pathogenic and putatively nonpathogenic isolates of Fusarium oxysporum are ubiquitously present in soils. Pathogenic isolates designated as formae speciales are very host specific. The genes that determine host-specific pathogenicity may be expected to be similar between strains within a forma specialis. Three different pathways were used for mining putative effectors from 85 assembled genome sequences representing isolates from the agricultural and natural ecosystems. A total of 436 putative effectors were identified, of which 115 were present in all the isolates. The presence–absence phylogeny showed some clustering of isolates based on the ecosystems, while some clustered with isolates from different ecosystems. Isolates from the agricultural and natural ecosystems shared 358 putative effectors. Agricultural isolates had 78 unique putative effectors, while isolates from the natural ecosystems had no unique effectors. Isolates from the natural ecosystems had effectors that have previously been confirmed with virulence functions in other filamentous fungi, suggesting that the natural ecosystems may be acting as a reservoir for pathogenic isolates awaiting compatible hosts. They also possessed putative effectors with domains involved in polysaccharide degradation. The putative effectors in the natural ecosystem isolates may be important for both parasitic and ecological fitness. Further analysis of the three characterized ff. sp.—lycopersici, niveum, and pisi—revealed high conservation in the pathogenicity mechanism in F. oxysporum. However, each strain had unique putative effectors, which may be responsible for host specificity.     

Phylogenetic investigations in the downy mildew genus <Emphasis Type="Italic">Bremia</Emphasis> reveal several distinct lineages and a species with a presumably exceptional wide host range

Bremia lactucae is one of the most devastating and widespread pathogens in lettuce production worldwide. Despite its economical importance, uncertainty prevails about the species delimitation in the genus Bremia. Commonly, Bremia is considered to be monotypic, containing only Bremia lactucae, while taxonomists have described additional species, and molecular phylogenetic studies have shown significant sequence divergence between accessions from different hosts. Here, we report that several previously described species are genetically highly distinct from Bremia lactucae parasitic to Lactuca sativa. These include Bremia lapsanae, Bremia sonchicola, and Bremia taraxaci. In addition to these host-specific species, a plurivorous species is revealed, which infects hosts from three different tribes in the Asteraceae subfamilies Asteroideae and Carduoideae. The broad host range of clade 1 is exceptional for downy mildews and only paralleled by Pseudoperonospora cubensis, which infects a broad range of Cucurbitaceae. The taxonomic status of Bremia cirsii and of Bremia centaureae remains unresolved, as the accessions from Cirsium and Centaurea, respectively, did not form a monophylum but were partly contained in the plurivorous clade 1. Bremia lactucae was found to be restricted to Lactuca sativa and Lactuca serriola. Thus, it can be assumed that Bremia infections on weeds apart from Lactuca species do not pose a significant risk for lettuce production. However, it is unlikely that breeding resistance genes from Lactuca serriola into Lactuca sativa will result in durable resistance of lettuce to downy mildew disease, because the current study provides additional evidence that Bremia accessions from both hosts form a population continuum.     

A mild strain of broad bean mottle virus from faba bean (Vicia faba L.) in the Sudan

A new strain of broad bean mottle virus, isolated from faba bean (Vicia faba L.) in the Sudan, is described. It differs considerably from known isolates by its nearly symptomless infection of faba bean in spite of high concentrations of the virus in infected plants. It does not differ from regular isolates in gel-diffusion serology, light and electron microscopy, host range and symptoms in major hosts other than faba bean. It may constitute a potential threat to other food legumes in the region.     

Host matrix has major impact on the morphology of <Emphasis Type="Italic">Pseudoperonospora cubensis</Emphasis>

Oomycetes contain some of the economically most important pathogens of flowering plants. Most have a rather narrow host range, often being restricted to single host species. In downy mildews and other obligate biotrophic plant parasites, like powdery mildews and rusts, delimitating species on grounds of morphological characteristics is often hardly possible and thus often based on only subtle differences. This has led to the widespread application of a broad species concept for these organisms. Consequently, despite the fact that morphological differences were reported for Pseudoperonospora cubensis from different host species, the corresponding new pathogen species were not accepted as being independent, and the host range of Pseudoperonospora cubensis is reported to encompass more than 50 host species in the Cucurbitaceae in temperate to tropical climates. However, recent studies have reported narrow host ranges for other downy mildew genera and advocated a narrow species concept. Here, we report successful colonisation of five different tribes of the Cucurbitaceae by a strain of Pseudoperonospora cubensis and demonstrate that the host matrix has a major impact on the morphology of the pathogen. On the basis of five morphological criteria significant differences could be found for all hosts. These differences were more pronounced in phylogenetically unrelated than in related hosts. Our results provide evidence for a broad host range of Pseudoperonospora cubensis and demonstrate that species delimitation based on morphological characters is not feasible in Pseudoperonospora on Cucurbitaceae. Also in other biotrophic plant pathogens, the situation could be similar, thus necessitating thorough morphological, molecular phylogenetic and cross inoculation experiments for species recognition.     

Local maladaptation of the anther-smut fungus parasitizing Dianthus carthusianorum

Pathogens are generally expected to evolve faster than their hosts and are therefore likely to be locally adapted. However, some pathogens might lag behind in the co-evolutionary arms race because they do not have some of the advantages shared by most other pathogens (e.g., high mutation or recombination rates, short generation time, high dispersal ability). This is the case of Microbotryum fungi that cause the anther smut disease in plants of the family Caryophyllaceae. We investigated the patterns of local adaptation and maladaptation in Microbotryum carthusianorum and its host plant Dianthus carthusianorum. We performed a full cross-inoculation experiment using half-sib plant families and fungal samples originating from three naturally infected populations in the Czech Republic. We specifically asked, which components of pathogen fitness (i.e., infectivity and host manipulation) are affected by local (mal)adaptation. The pathogen was on average 1.6 times more successful in infecting plants from foreign populations compared to plants from its home population. Once the infection was successful, the pathogen accelerated the plant’s flowering and thus increased the opportunity for transmission to new hosts. However, the level of manipulation of host flowering did not differ between home and foreign populations. This study showed that the pathogen’s infectivity followed a clear pattern of local maladaptation, whereas the host manipulation did not. Our study taken together with previous studies of a related anther smut species reveals the pervasiveness of local maladaptation in this group of pathogens that arises as the result of their restricted gene flow and reduced recombination rates.

     

Molecular characterization,comparison of screening methods,and evaluation of cross‐pathogenicity of black rot (Xanthomonas campestris pv. campestris) strains from cabbage,choy sum,leafy mustard and pak choi from Taiwan

Choy sum (Brassica rapa var. parachinensis), leafy mustard (Brassica juncea) and pak choi (B. rapa var. chinensis) are highly nutritious components of diets in Taiwan and other Asian countries, and bacterial black rot caused by Xanthomonas campestris pv. campestris (Xcc) is a major biotic constraint in these crops. As very little was known about the Xcc strains from these crops in these regions, including their cross‐pathogenicity and aggressiveness on different hosts, Xcc strains were obtained from cabbage (Brassica oleracea var. capitata), choy sum, leafy mustard and pak choi crops in Taiwan. Two previously published PCR‐based assays reliably distinguished the Xcc strains from other Xanthomonas species and subspecies. Phylogenetic analysis based on repetitive sequence‐based PCR assays placed the Xcc strains in a clade distinct from other Xanthomonas species, and also showed host specificity. Although all of the Xcc strains from the different host species were pathogenic on all five Brassica test species in both a detached leaf assay and an intact plant assay, in the intact plant assay they showed differences in virulence or aggression on the different test hosts. The Xcc strains from leafy mustard and pak choi were consistently highly aggressive on all the test host genotypes, but the strains from choy sum and cabbage were less aggressive on leafy mustard and choy sum. The intact plant assay proved more discriminating and reliable than the detached leaf assay for comparing the aggressiveness of Xcc strains on different host genotypes, and so, with the new Xcc strains isolated in this study, will be useful for screening leafy brassica germplasm accessions for resistance to black rot.     

Black Spot of Peach Caused by <Emphasis Type="Italic">Alternaria alternata</Emphasis> (Fr.) Keissler

A new disease of peach (Prunus persica Batsch var. vulgaris Maxim.), causing brown, sunken lesions and brownish to blackish brown spots with cracks on peach fruits, was found in Okayama prefecture, Japan, in 1995. The disease was observed not only on peach fruits but also on twigs and leaves. An Alternaria sp. was consistently isolated from these diseased fruits, twigs and leaves. The isolates were pathogenic to peach fruits and leaves. Based on the morphological characteristics, the causal fungus was identified as Alternaria alternata (Fr.) Keissler. After cross-inoculation with isolates from peach, Japanese pear and apple, the isolates were found to be pathogenic only to their original host. This is the first report on a peach disease caused by a host-specific A. alternata; therefore, the common name of black spot (`Kokuhanbyo' in Japanese) was proposed. Received 25 June 1999/ Accepted in revised form 12 October 1999