Jiarong Yang

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.

Threshold microsclerotial inoculum for cotton verticillium wilt determined through wet-sieving and real-time quantitative PCR.

Quantification of Verticillium dahliae microsclerotia is an important component of wilt management on a range of crops. Estimation of microsclerotia by dry or wet sieving and plating of soil samples on semiselective medium is a commonly used technique but this method is resource-intensive. We developed a new molecular quantification method based on Synergy Brands (SYBR) Green real-time quantitative polymerase chain reaction of wet-sieving samples (wet-sieving qPCR). This method can detect V. dahliae microsclerotia as low as 0.5 CFU g(-1) of soil. There was a high correlation (r=0.98) between the estimates of conventional plating analysis and the new wet-sieving qPCR method for 40 soil samples. To estimate the inoculum threshold for cotton wilt, >400 soil samples were taken from the rhizosphere of individual plants with or without visual wilt symptoms in experimental and commercial cotton fields at the boll-forming stage. Wilt inoculum was estimated using the wet-sieving qPCR method and related to wilt development. The estimated inoculum threshold varied with cultivar, ranging from 4.0 and 7.0 CFU g(-1) of soil for susceptible and resistant cultivars, respectively. In addition, there was an overall relationship of wilt incidence with inoculum density across 31 commercial fields where a single composite soil sample was taken at each field, with an estimated inoculum threshold of 11 CFU g(-1) of soil. These results suggest that wilt risk can be predicted from the estimated soil inoculum density using the new wet-sieving qPCR method. We recommend the use of 4.0 and 7.0 CFU g(-1) as an inoculum threshold on susceptible and resistant cultivars, respectively, in practical risk prediction schemes.

Within‐ and between‐orchard variability in the sensitivity of Venturia inaequalis to myclobutanil, a DMI fungicide, in the UK

BACKGROUND Myclobutanil, a demethylation inhibitor (DMI) fungicide, is an important fungicide for controlling apple scab and powdery mildew. Overuse of this fungicide has led to establishment of scab isolates with reduced sensitivity to this fungicide in several countries. Experiments were conducted to determine the sensitivity of the causal agent of apple scab, Venturia inaequalis (Cooke) Winter, to myclobutanil in the UK, in order to assess whether there is a relationship between fungal insensitivity and the number of DMI applications, and establishing whether fungal sensitivity varied greatly within an orchard. RESULTS Reduced sensitivity of V. inaequalis to myclobutanil was positively related linearly to the number of DMI applications. ED(50) values ranged from 0.028 to 1.017 mg L(-1) (average = 0.292) for the baseline population, whereas isolates from two other orchards had much greater ED(50) values, ranging from 0.085 to 5.213 mg L(-1) (average = 1.852). There was significant variation in fungal sensitivity to myclobutanil among fungal isolates from different locations within a single orchard. CONCLUSIONS Spatial spread of insensitive isolates of V. inaequalis to myclobutanil is likely to be limited in distance. Conidia may be an important source of primary inoculum. Myclobutanil should still be effective for most field isolates, but its use should be strategically integrated with other groups of fungicides.

Transcriptomic Analysis Reveal the Molecular Mechanisms of Wheat Higher-Temperature Seedling-Plant Resistance to Puccinia striiformis f. sp. tritici

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a destructive disease of wheat worldwide. The disease is preferably controlled by growing resistant cultivars. Wheat cultivar Xiaoyan 6 (XY 6) has been resistant to stripe rust since its release. In the previous studies, XY 6 was found to have higher-temperature seedling-plant (HTSP) resistance. However, the molecular mechanisms of HTSP resistance were not clear. To identify differentially expressed genes (DEGs) involved in HTSP resistance, we sequenced 30 cDNA libraries constructed from XY 6 seedlings exposed to several temperature treatments. Compared to the constant normal (15°C) and higher (20°C) temperature treatments, 1395 DEGs were identified in seedlings exposed to 20°C for 24 h (to activate HTSP resistance) and then kept at 15°C. These DEGs were located on all 21 chromosomes, with 29.2% on A, 41.1% on B and 29.7% on D genomes, by mapping to the Chinese Spring wheat genome. The 1395 DEGs were enriched in ribosome, plant-pathogen interaction and glycerolipid metabolism pathways, and some of them were identified as hub proteins (phosphatase 2C10), resistance protein homologs, WRKY transcription factors and protein kinases. The majority of these genes were up-regulated in HTSP resistance. Based on the differential expression, we found that phosphatase 2C10 and LRR receptor-like serine/threonine protein kinases are particularly interesting as they may be important for HTSP resistance through interacting with different resistance proteins, leading to a hypersensitive response.

Spatial Pattern of Verticillium dahliae Microsclerotia and Cotton Plants with Wilt Symptoms in Commercial Plantations.

Spatial patterns of pathogen inoculum in field soils and the resulting patterns of disease may reflect the underlying mechanisms of pathogen dispersal. This knowledge can be used to design more efficient sampling schemes for assessing diseases. Spatial patterns of Verticillium dahliae microsclerotia were characterized in commercial cotton fields through quadrat and point sampling in 1994 and 2013, respectively. Furthermore, cotton plants with wilt symptoms, caused by V. dahliae, were assessed in six commercial cotton fields in 2013. Soil samples were assayed for the density of microsclerotia (expressed as CFU g-1 of soil) using a wet-sieving plating method and a real-time quantitative PCR method for the 1994 and 2013 study, respectively. The estimated inoculum threshold for causing wilt development on individual plants varied with the three fields: ca. 1.6 CFU g-1 of soil for one field, and 7.2 CFU g-1 of soil for the other two. Both quadrat and point sampling spatial analyses showed that aggregation of V. dahliae inoculum in soils was usually not detected beyond 1.0 m. Similarly, the spatial patterns of wilted cotton plants indicated that spatial aggregation of diseased plants were only observed below the scale of 1.0 m in six commercial cotton plantations. Therefore, spatial aggregation of both V. dahliae inoculum and cotton plants with wilt symptoms is not likely to be detected above the scale of 1.0 m for most commercial cotton plantations. When designing schemes for assessing wilt inoculum and wilt development, this scale needs to be taken into consideration.