Dez J. Barbara

Population Variation of Apple Scab (Venturia inaequalis) Isolates from Asia and Europe

Apple scab, caused by Venturia inaequalis, is one of the most of damaging diseases worldwide on apple and currently is managed mainly by scheduled applications of fungicides. Understanding pathogen population structure is important for breeding and deployment of resistant cultivars. Isolates of V. inaequalis were sampled from a number of cultivars in China, India, and the United Kingdom to estimate differences in pathogen populations. Amplified fragment length polymorphism (AFLP) markers were used to genotype isolates, mostly from China and the United Kingdom. The AFLP data indicated that, overall, there were significant differences in V. inaequalis populations from China and the United Kingdom. Within China, there was no significant differentiation associated with their geographical or cultivar origins. In contrast, populations from four cultivars in two U.K. orchards (monoculture of Gala and a mixture orchard of Bramley, Cox, and Worcester) differed significantly. Furthermore, populations from Gala and Worcester were more homogenous than expected but those from Cox were more diverse than expected. In total, 80 isolates were selected randomly from three countries for virulence testing: 20 from the United Kingdom (10 from Gala and 10 from Cox), 30 from China (10 from Gala, 10 from Fuji, and 10 from Qingquan), and 30 from India (10 from Gala, 10 from Golden Delicious, and 10 from Black Ben Davis); of these 80 isolates, 41, 47, and 59 were inoculated against each of these cultivars in the United Kingdom, India, and China, respectively. The two local cultivars from India (Black Ben Davis) and the United Kingdom (Cox) were more resistant against non-indigenous isolates, particularly those from China, than they were against indigenous isolates; the Chinese local cultivar (Qingguan) showed a higher general level of resistance against isolates regardless of their origin.

A genetic linkage map of Venturia inaequalis, the causal agent of apple scab

BackgroundVenturia inaequalis is an economically-important disease of apple causing annual epidemics of scab worldwide. The pathogen is a heterothallic ascomycete with an annual cycle of sexual reproduction on infected apple leaf litter, followed by several cycles of asexual reproduction during the apple growing season. Current disease control is achieved mainly through scheduled applications of fungicides. Genetic linkage maps are essential for studying genome structure and organisation, and are a valuable tool for identifying the location of genes controlling important traits of interest such as avirulence, host specificity and mating type in V. inaequalis. In this study, we performed a wide cross under in vitro conditions between an isolate of V. inaequalis from China and one from the UK to obtain a genetically diverse mapping population of ascospore progeny isolates and produced a map using AFLP and microsatellite (SSR) markers.FindingsEighty-three progeny were obtained from the cross between isolates C0154 (China) × 01/213 (UK). The progeny was screened with 18 AFLP primer combinations and 31 SSRs, and scored for the mating type locus MAT. A linkage map was constructed consisting of 294 markers (283 AFLPs, ten SSRs and the MAT locus), spanning eleven linkage groups and with a total map length of 1106 cM. The length of individual linkage groups ranged from 30.4 cM (Vi-11) to 166 cM (Vi-1). The number of molecular markers per linkage group ranged from 7 on Vi-11 to 48 on Vi-3; the average distance between two loci within each group varied from 2.4 cM (Vi-4) to 7.5 cM (Vi-9). The maximum map length between two markers within a linkage group was 15.8 cM. The MAT locus was mapped to a small linkage group and was tightly linked to two AFLP markers. The map presented is over four times longer than the previously published map of V. inaequalis which had a total genetic distance of just 270 cM.ConclusionA genetic linkage map is an important tool for investigating the genetics of important traits in V. inaequalis such as virulence factors, aggressiveness and mating type. The linkage map presented here represents a significant improvement over currently published maps for studying genome structure and organisation, and for mapping genes of economic importance on the V. inaequalis genome.

Viruses and Viroids Infecting Hop: Significance, Epidemiology, and Management

The hop (Humulus lupulus) is a hardy, climbing, dioecious, perennial plant native to Europe, Asia, and North America (172). The genus Humulus belongs to the family Cannabaceae and contains three species: H. japonicus, H. lupulus, and H. yunnanensis (32,117). Hops are grown predominantly for their cones (strobiles), which contain glands producing resins, essential oils, and polyphenols (Fig. 1). These compounds are used primarily to add bitterness and aroma to beer. The most important of these compounds for brewing are the alpha acids. Alpha acids are acylsubstituted phloroglucinols, differing from each other only in the nature of the acyl R side chain. They can be separated into humulone (R = isovaleryl), cohumulone (R = isobutryl), adhumulone (R = alphamethyl butryl), prehumulone, and posthumulone (127). The main properties of alpha acids in relation to beer production are improved foam stability, suppression of gushing, and contributions to bacteriological stability (48). The bitterness of beer is related to stereoisomer formation of each major alpha acid in the brewing process (124). Beta acids are predominantly separated into lupulone, colupulone, and adlupulone. They have limited bittering power but are particularly important because of their bactericidal properties (194). More than 200 other essential oils also occur within the cones, including hydrocarbons and oxygenated and sulfur-containing compounds, which are responsible for the aroma and flavor of the final product (170). Hops are also grown for their medicinal and soporific effects, as ornamental plants, and as edible delicacies (146). The areas in which hop can be grown are limited by strict day length and temperature requirements for flowering and hence cone production. Production is generally restricted around 35° latitudes in both hemispheres (32,117). Supplementary artificial lighting has been used to produce hops in areas of lower latitudes, such as in South Africa (196). In 2005, the most significant regions of hop production (i.e., countries with >1,000 ha) were Germany, the Czech Republic, Poland, Slovenia, Ukraine, the United Kingdom, the northwestern states of the United States (Idaho, Washington, and Oregon), and China. Lesser quantities were grown in 14 other European countries. A few other countries grow 500 ha or less, most notably Japan, Argentina, Australia, South Africa, New Zealand, and India (26). The largest area of production was in Germany (17,161 ha producing 34,466.8 metric tons), while 11,956 ha were grown in the United States, producing 24,002 metric tons (26). Viruses and viroids pose significant constraints to the production of high yields of hop cultivars worldwide. In some countries, such as Australia and New Zealand, these pathogens are considered the only significant pathological problems. This is due to the absence of severe fungal diseases, such as powdery mildew (154) and downy mildew (155). Infections by five viruses and two viroids are or have been widespread and important in commercial hop yards. A further 12 viruses (of which two are poorly characterized) and 1 viroid have been reported in hop; these either have limited distributions or occur only sporadically and are not considered important. The viruses considered generally important are the three carlaviruses, Hop mosaic virus (HpMV) (3,20,94,156), Hop latent virus (HpLV) (4,21,55,150,180), and American hop latent virus (AHLV) (5,22,149); the ilarvirus Apple mosaic virus (ApMV) (27,28,46,64); and the nepovirus Arabis mosaic virus (ArMV) (6,9,50,82). The important viroids infecting hops are Hop latent viroid (HpLVd) (10,14,24,25,151) and Hop stunt viroid (HpSVd) (162–164,201). Although the problems posed by viruses and viroids for the production of hops are not unique among perennial crops, they are particularly challenging because the rates of spread are often much higher than in other crops such as top fruit. As such, the control measures used with hops present a good case study for the control of these pathogens in crops where long life (hop yards are typically kept for up to 20 years and may last 50 years) means they are repeatedly exposed to infection. The reasons for rapid spread relative to tree fruit crops are unclear, but one might speculate that the very rapid and massive annual growth (the entire aboveground part of the plants is replaced every year, and main stems generally grow more than 5 meters in only 3 to 4 months) in closely spaced plantings favors both mechanical transmission (for HpSVd, HpLVd, and ApMV) and heavy infestations with aphids (for the carlaviruses). The study of hop viruses and viroids also emphasizes the need to fully understand and quantify the effects of these pathogens on yield and/or quality of crops, which are highly dependent on cultivar, pathogen strain, and environmental conditions.

A Survey of Viruses of Alstroemeria in the UK and the Characterisation of Carlaviruses Infecting Alstroemeria

Alstroemeria samples collected in the UK were tested for a range of viruses using ELISA. Alstroemeria mosaic virus (AlMV), alstroemeria carlavirus (AlCV), lily symptomless virus (LSV), cucumber mosaic virus (CMV) and tobacco rattle virus (TRV) were detected either singly or in combination in 67.5% of 203 samples. AlCV and LSV isolates from Alstroemeria and lily were studied and characterised serologically using existing antisera, and by PCR, using primers to an 11 kDa open reading frame (ORF) unique to carlaviruses and to the coat protein gene of LSV. Sequences of isolates of AlCV and LSV from the coat protein gene were 94–99% similar and were 99% similar in the 11 kDa ORF, supporting the view that these are strains of the same virus.

Host specificity, but not high-temperature tolerance, is associated with recent outbreaks of Verticillium dahliae in chrysanthemum in the Netherlands

Two hypotheses which might explain a recent increase in the incidence of verticillium wilt of chrysanthemums in glasshouses in the Netherlands were investigated, viz whether selection for increased resistance to elevated temperatures has occurred due to frequent steaming of soils in the glasshouses, or whether the strains of Verticillium dahliae occurring in chrysanthemum glasshouses are particularly virulent towards this host. Following artificial inoculation, five isolates of V. dahliae from chrysanthemum were pathogenic on chrysanthemum but five isolates from potato were non-pathogenic for this host. When inoculated onto potato plants, all isolates caused early senescence with no significant difference between the two groups of isolates. In amplified fragment length polymorphism analysis, the potato isolates formed a cluster distinct from all other isolates. As a group the chrysanthemum isolates were no more diverse than the potato isolates but did not form a cluster distinct from 12 other isolates tested. This suggests that high pathogenicity to chrysanthemum has developed on several occasions but that the group of potato isolates were possibly monophyletic. Microsclerotia produced in vitro from the chrysanthemum isolates had significantly lower average lethal temperature tolerance than those from the five potato isolates suggesting that being able to resist the effects of soil sterilisation by steam is not a factor in wilt of chrysanthemums in the Netherlands.