Transgressive segregation for resistance to Pyrenophora teres in barley

Populations of F₁, F₂, F₁× parent 1 (BC1), F₁× parent 2 (BC2), and F₃ resulting from two crosses between four susceptible varieties of barley used in national and international breeding programmes were tested at the seedling stage for their resistance to an isolate of Pyrenophora teres which is virulent to the parental varieties. Infection type, average lesion size, and number of lesions per unit leaf area were used to assess disease reaction. In the two crosses, F₂ and especially F₃ generations (produced by self pollination of selected resistant F₂ plants) showed more resistance expressed by infection type and average lesion size than either parent. The frequency distributions of F₂ and F₃ generations derived from these crosses were continuous and showed transgressive segregation for resistance. The results indicated that the four cultivars used in this study possessed additive genes for resistance to P. teres. Thus useful resistance could be obtained following crossing of susceptible cultivars and selection in succeeding generations.     

Rapid-cycle PCR detection of Pyrenophora graminea from barley seed

Sixty RAPD primers were used to screen for a diagnostic marker that could be used to identify Pyrenophora graminea , a fungal seedborne pathogen that causes leaf stripe on barley. Primer pairs were designed to differentiate P. graminea from other Pyrenophora spp. using a sequence-characterized amplified region (SCAR) approach. A pair of P. graminea -specific primers (PG2 F/R) was obtained that amplified a single fragment from 37 isolates of P. graminea tested, but not from 29 isolates of other Pyrenophora spp. or 12 saprophytes isolated from barley seed. Rapid PCR detection was achieved using a LightCycler, in which the emission of fluorescence from the binding of SYBR Green I dye to the PCR products is measured. The P. graminea -specific product resulting from amplification with PG2 F/R can be distinguished from any nonspecific products by post-PCR melting point analysis. The PCR assay involves 40 amplification cycles of PCR, and the total PCR test including melting point analysis takes 25 min to complete. The rapidity of this test, combined with the closed 'in-tube' detection of PCR products, which reduces the potential for contamination, offers significant advantages compared with conventional laboratory and PCR analyses.     

Botryosphaeriaceae occurring on native Syzygium cordatum in South Africa and their potential threat to Eucalyptus

Eight species of the Botryosphaeriaceae (canker and dieback pathogens) were identified on native Syzygium cordatum in South Africa, based on anamorph morphology, ITS rDNA sequence data and PCR-RFLP analysis. The species identified were Neofusicoccum parvum, N. ribis, N. luteum, N. australe, N. mangiferae , Botryosphaeria dothidea, Lasiodiplodia gonubiensis and L. theobromae . Their pathogenicity on S. cordatum seedlings and a Eucalyptus grandis  ×  camaldulensis clone was determined in glasshouse inoculation trials. Isolates of all identified species, except one of N. mangiferae , were more pathogenic on the Eucalyptus clone than on S. cordatum . Some of the species that cross-infected these hosts, such as N. ribis, N. parvum and L. theobromae , were amongst the most pathogenic on the Eucalyptus clone, while B. dothidea and L. gonubiensis were the least pathogenic. Results of this study illustrate that species of the Botryosphaeriaceae from native hosts could pose a threat to introduced Eucalyptus spp., and vice versa .     

Pyrenophora teres growth and severity of net blotch on barley under salt stress

European Journal of Plant Pathology - Pyrenophora teres causes one of the most destructive barley diseases in semi-arid production areas worldwide. With the burgeoning issue of soil salinization,...     

Effects of Pyrenophora teres on dry matter production and yield components of winter barley

Inoculation of winter barley plants in a glasshouse with Pyrenophora teres at three different growth stages (GS 11, 13 or 30) greatly decreased root and shoot dry matter production and the size of healthy leaves produced subsequently, but there was no significant yield loss with a single inoculation at GS 11 or 30. Inoculation at GS 13, GS 31 or GS 39 decreased grain yield by 12,17 and 20% respectively, and also straw yield. Larger yield losses resulted from repeated inoculations on five successive occasions (GS 11, 13, 30, 31 and 39) which caused much disease on all leaves throughout the life of the plant. All components of yield measured were decreased by the cumulative effect of successive inoculations; ear number by 15%), grain sites per ear by 20%, grain yield by 48% and straw yield by 32%.     

Effects of temperature and leaf wetness on Pyrenophora teres growing on barley cv. Sonja

Conidia of Pyrenophora teres germinated only in the presence of liquid water and at temperatures above 2°C. The speed with which germination occurred was inversely proportional to temperature measured from a base of 2°C, up to the maximum temperature tested of 21°C. Once conidia on leaves had been wetted, about 40% of all infections that would eventually occur were established within 100°C-hours. Subsequent lesion extension was rapid, with area doubling times of about 1 day between 10 and 20°C.
If conidia germinated, up to 80% formed successful infections on young, susceptible leaves. On older leaves fewer spores germinated and the proportion that then infected was smaller.
The latent period, defined as the time before which sporulation did not occur under any wetness conditions, ranged from about 25 days at 5°C to 11 days at 20°C under dry conditions. Under continuously wet conditions it was about 20% shorter at all temperatures. Its inverse had a curvilinear relation to temperature.
Spores were produced after one to several days of humidity above 95%. The precise period decreased with increasing temperature, but at 25°C spores never appeared. The drier a dead leaf was, the longer the pathogen in it look to produce spores.     

Incidence and control of barley leaf stripe (Pyrenophora graminea) in Scottish barley during the period 1987–1992

The incidence of Pyrenophora graminea was determined in winter and spring barley seed harvested in each of the 6 years 1987 to 1992. The occurrence of P. graminea was highest in spring barley seed harvested in 1989 and 1990, with 69% of samples in 1989 and 82% in 1990 infected. The mean infection levels were 6·1% in 1989 and 7·5% in 1990. The introduction of a voluntary Code of Practice in January 1991 significantly reduced infection: in 1992 only 10% of spring barley samples were infected, and the mean infection level was less than 0·2%. In spring barley, there was a significant relationship between the level of seed infection and the percentage of diseased tillers in commercial crops ( r = 0·890, P < 0·001). However, tiller infection was consistently lower than seed infection. During the period of the study the mean annual leaf stripe infection on winter barley seed did not exceed 1%.     

Population structure of South African and Australian Pyrenophora teres isolates

There are two recognized forms of the disease net blotch of barley: the net form caused by Pyrenophora teres f. teres (PTT) and the spot form caused by P. teres f. maculata (PTM). In this study, amplified fragment length polymorphism analysis was used to investigate the genetic diversity and population structure of 60 PTT and 64 PTM isolates collected across Australia (66 isolates) and in the south‐western Cape of South Africa (58 isolates). For comparison, P. tritici‐repentis, Exserohilum rostratum and Bipolaris sorokiniana samples were also included in the analyses. Both distance‐ and model‐based cluster analyses separated the PTT and PTM isolates into two strongly divergent genetic groups. Significant variation was observed both among the South African and Australian populations of PTT and PTM and among sampling locations for the PTT samples. Results suggest that sexual reproduction between the two forms is unlikely and that reproduction within the PTT and PTM groups occurs mainly asexually.     

Correlation between sensitivity of barley to Pyrenophora teres toxins and susceptibility to the fungus

Detached leaves of 25 barleys, ranging from highly susceptible to highly resistant to Pyrenophora teres f. teres and Pyrenophora teres f. maculata, were tested for their reaction to three phytotoxins isolated from cultures of the fungus: toxin A [ L, L - N -(2-amino-2-carboxyethyl)aspartic acid], toxin C (aspergillomarasmine A) and toxin B (anhydroaspergillomarasmine A). 0.75 m M toxin A caused mainly dark yellow chlorotic symptoms but little necrosis, whereas leaves treated with 0.25 m M toxin C developed distinct necrotic symptoms and zones of light yellow chlorosis. Toxin B is only weakly toxic, and toxin B and control solutions containing aspartic acid in the concentration of 0.75 m M did not cause any symptoms. The best differentiation between the barleys was obtained by scoring chlorosis after 120 h, and the optimal toxin concentrations for this differentiation were 0.75 m M toxin A and 0.25 m M toxin C, respectively. Results with different toxin concentrations inducing distinct variation in symptom expression indicate that the two toxins have different potencies as phytotoxins. The reaction of the barleys to toxins A and C correlated well with their reaction to infection by P. teres f. teres and P. teres f. maculata, suggesting that toxins A and C may be used to select resistant barley lines in the early stages of a breeding programme.     

Infection experiments of Pyrenophora teres f. maculata on cultivated and wild barley indicate absence of host specificity

European Journal of Plant Pathology - It is important to investigate the possibility of pathogen transmission between cultivated and uncultivated hosts due to the role of the latter in pathogen...     

Exploring de novo specificity: the Pyrenophora tritici‐repentis–barley interaction

Pyrenophora tritici‐repentis (Ptr) is a destructive fungal pathogen of wheat worldwide. In addition to wheat, Ptr has been isolated from various other hosts in the family Poaceae, yet the nature of its interaction with those hosts is unknown. The Ptr–barley relationship was explored and the existence of a specific interaction between Ptr and barley is described for the first time; symptom development on several barley genotypes was evaluated in bioassays and by toxin infiltration into barley leaves. Ptr ToxB‐producing isolates of the fungus were able to cause significant damage when inoculated onto certain barley genotypes, and Ptr ToxB was able to induce chlorosis in a highly selective manner when infiltrated into those same genotypes. Ptr–barley specificity is subtle and can break with slight changes in temperature after infection. To understand the infection process in barley, a cytological analysis and in planta fungal biomass estimation using quantitative PCR were performed. The fungus penetrates through the host epidermal cells and advances to colonize the mesophyll layer intercellularly, with the infection process on barley closely resembling that on wheat. Here, evidence is provided for a specific interaction between barley and Ptr, expanding understanding of Ptr host specificity and breaking the assumption that the highest level of specificity seen with Ptr is restricted to particular genotypes of the wheat host.     

Effect of barley yellow dwarf virus on susceptibility of barley cultivars to net blotch (Pyrenophora teres) and leaf blotch (Rhynchosporium secalis)

The effect of barley yellow dwarf virus (BYDV) on the development of net blotch (Pyrenophora eves) and leaf blotch (Rhynchosporium secalis) was examined on seven barley cuitivars. Seedlings were infected with BYDV at the two-leaf stage (G.S. 12). Their susceptibility to three isolates of p teres and isolates of two races (U.K. 1, U.K. 2) of R. secalis was examined at the four-leaf stsge (G.S. 14) and when plants were more mature (G.S. 33/38). At G.S. 14 numbers of lesions produced by P. teres and R. secalis were reduced, on average, by 37 and 72% respectively, and at G.S. 33/38 by 61 and 74%.     

Development of an international standard set of barley differential genotypes for Pyrenophora teres f. teres

International comparison of virulence profiles of Pyrenophora teres f . teres (Ptt), the cause of barley net blotch, is seriously restricted by inconsistencies in differential testers used among researchers. This paper reports an attempt to develop an appropriate set of differentials to standardize characterization of Ptt populations globally. Fourteen barley genotypes (Canadian Lake Shore (CLS), Harbin, c-8755, c-20019, Manchurian, Tifang, CI 9825, CI 5791, CI 9819, Beecher, CI 9214, Skiff, Prior and Corvette) were selected from among genotypes previously used as Ptt differentials. Three cultivars (Pirkka, Haruna Nijo and Harrington) were included to identify a universally susceptible control. Genotypes were inoculated with approximately 1000 Ptt isolates from Russia, Europe, Australia and Canada. The mean reaction frequency of genotypes ranged from highly resistant (CI 9819, CI 5791, c-8755 and CI 9825) to highly susceptible (Harrington, Haruna Nijo and Pirkka). The best differential abilities were demonstrated by Harbin, CLS, c-20019, Manchurian and Prior. Application of cluster analyses identified genotypes with similar reaction patterns, which supported a reduction of genotypes in the set. When combined with an algorithm comparing the ability of individual genotypes to discriminate among Ptt isolates, a further reduction of genotypes was justified. A new, concise set of barley genotypes for differentiating virulences in Ptt was formulated. It is proposed that these genotypes be adopted as the standard, international differential set to characterize and identify the virulence properties of Ptt populations across environments. The new Ptt differential set consists of the genotypes c-8755, c-20019, CI 5791, CI 9825, CLS, Harbin, Prior, Skiff and Harrington.     

The genetic diversity of Icelandic populations of two barley leaf pathogens, Rhynchosporium commune and Pyrenophora teres

Twelve species of fungi have been found growing on barley leaves in Icelandic fields. The study presented here on the population structure of two of these species, the pathogens Rhynchosporium commune and Pyrenophora teres f. teres, reveals high levels of genetic diversity, low levels of migration, and a significant differentiation from other European populations, despite the short history of continuous barley cultivation in Iceland. The gene diversity for R. commune in Iceland was 0.55 compared to a range of 0.43?C0.73 for six European populations. The gene diversity for P. teres was higher in Iceland than in populations from Russia and Finland. The two mating types were found to overlap in distribution for both fungi making sexual reproduction a possibility, supported by the few clones and the gametic equilibrium within the Icelandic populations. When the high levels of diversity, low levels of migration, and the genetic differentiation observed between Icelandic and Scandinavian populations are put into context with the short history of barley cultivation in Iceland, it raises questions regarding the origin of the Icelandic fungal populations. It also underlines the importance of proper analysis of pathogens prior to starting resistance breeding projects. The findings are an addition to the ongoing analysis of the global diversity of barley fungal pathogens in general and an important input into future barley breeding projects in Iceland in particular.     

Bt抗虫棉田龟纹瓢虫发生规律及保护利用研究初报

通过系统调查和观察,初步明确了Bt抗虫棉田龟纹瓢虫的发生规律、控害效果,并总结了Bt抗虫棉田龟纹瓢虫保护与利用的有效途径。     

我国大麦条纹病菌(Pyrenophora graminea)群体遗传多样性AFLP分析

条纹病是重要的种传真菌病害之一,能对大麦生产造成严重产量损失。本研究利用AFLP方法对来自青海、河南等10省份大麦种植区的条纹病菌菌株进行遗传多样性分析,从分子水平上揭示我国不同大麦产区的条纹病菌群体遗传差异。结果显示,利用8对AFLP选择性引物组合对19份大麦条纹病菌进行了全基因组多态性扩增,共获得144条可统计的条带,其中112条具有多态性,多态性条带占77.8%。在相似系数为0.83时,可将19份大麦条纹病菌菌株划分为4个类群,多数菌株聚类在类群Ⅰ和类群Ⅱ中,类群Ⅲ和类群Ⅳ中仅各包含1个菌株,且与其他2个类群中的成员亲缘关系较远。因此,我国大麦条纹病菌群体中存在一定程度的遗传变异。另外,发现菌株间亲缘关系远近与其分布地域无明显规律。     

Ceratocystis species infecting stem wounds on Eucalyptus grandis in South Africa

Reports of Ceratocystis spp. causing disease of exotic plantation hardwood species have increased in recent years. Ceratocystis fimbriata causes wilt and canker on Eucalyptus spp. in Africa and South America, and C. albofundus results in wilt and death of Acacia mearnsii in Africa. Ceratocystis spp. generally infect wounds on trees, and artificial stem wounding can thus be used to determine the presence of these fungi. The aim of this study was to identify Ceratocystis spp. infecting wounds on Eucalyptus grandis in South Africa. Isolated Ceratocystis spp. were identified using morphological characteristics and comparisons of DNA sequence data for the ITS and 5·8S regions of the rRNA operon. Pathogenicity trials were conducted in the greenhouse to determine the possible role that these Ceratocystis spp. could have in disease development. These trials were also conducted under field conditions. Three Ceratocystis spp. were collected: C. fimbriata , C. moniliformis and C. pirilliformis . This is the first report of C. fimbriata and C. pirilliformis from Eucalyptus spp. in South Africa, and the first report of the latter fungus outside Australia. Both C. fimbriata and C. pirilliformis caused significant lesions on inoculated E. grandis trees. This is the first evidence that C. pirilliformis is a pathogen of Eucalyptus spp. From the results of both greenhouse and field trials, it has the potential to cause serious disease problems in Eucalyptus plantations.     

Population Structure of Phytophthora cinnamomi in South Africa

ABSTRACT Phytophthora cinnamomi isolates collected from 1977 to 1986 and 1991 to 1993 in two regions in South Africa were analyzed using isozymes. A total of 135 isolates was analyzed for 14 enzymes representing 20 putative loci, of which four were polymorphic. This led to the identification of nine different multilocus isozyme genotypes. Both mating types of P. cinnamomi occurred commonly in the Cape region, whereas, predominantly, the A2 mating type occurred in the Mpumalanga region of South Africa. A2 mating type isolates could be resolved into seven multilocus isozyme genotypes, compared with only two multilocus isozyme genotypes for the A1 mating type isolates. Low levels of gene (0.115) and genotypic (2.4%) diversity and a low number of alleles per locus (1.43) were observed for the South African P. cinnamomi population. The genetic distance between the Cape and Mpumalanga P. cinnamomi populations was relatively low (D(m) = 0.165), and no specific pattern in regional distribution of multilocus isozyme genotypes could be observed. The genetic distance between the "old" (isolated between 1977 and 1986) and "new" (isolated between 1991 and 1993) P. cinnamomi populations from the Cape was low (D(m) = 0.164), indicating a stable population over time. Three of the nine multilocus isozyme genotypes were specific to the "old" population, and only one multilocus isozyme genotype was specific to the "new" population. Significant differences in allele frequencies, a high genetic distance (D(m) = 0.581) between the Cape A1 and A2 mating type isolates, significant deviations from Hardy-Weinberg equilibrium, a low overall level of heterozygosity, and a high fixation index (0.71) all indicate that sexual reproduction occurs rarely, if at all, in the South African P. cinnamomi population.     

Rhizoctonia leaf spot of tobacco in South Africa

Leaf spot of tobacco caused by Rhizoctonia solani isreported forthe first time from South Africa. All the leaf spot isolates anastomosed with the AG-3 tester strain.