High Genetic Diversity in Predominantly Clonal Populations of the Powdery Mildew Fungus Podosphaera leucotricha from U.S. Apple Orchards

Apple powdery mildew (APM), caused by Podosphaera leucotricha, is a constant threat to apple production worldwide. Very little is known about the biology and population structure of this pathogen in the USA and other growing regions, which impacts APM management. Two hundred fifty-three P. leucotricha isolates, sampled from 10 apple orchards in Washington, New York, and Virginia, were genetically characterized with novel single sequence repeat and mating type markers. Eighty-three multilocus genotypes (MLGs) were identified, most of which were unique to a given orchard. Each isolate carried either a MAT1-1 or MAT1-2 idiomorph at the mating type locus, indicating that P. leucotricha is heterothallic. Virulence tests on detached apple leaves showed that the 10 most frequent P. leucotricha MLGs were not avirulent on a line containing a major resistance gene. Analysis of molecular variance showed significant differentiation (P > 0.001) among populations, a result supported by principal coordinate analysis revealing three genetic groups, each represented by non-overlapping MLGs from Washington, New York, and Virginia. A Bayesian cluster analysis showed genetic heterogeneity between Washington populations, and a relative migration analysis indicated substantial gene flow among neighboring orchards. Random mating tests indicated that APM epidemics during the active cycle were dominated by clonal reproduction. However, the presence of sexual structures in orchards, the likelihood that five repeated MLGs resulted from sexual reproduction, and high genotypic diversity observed in some populations suggest that sexual spores play some role in APM epidemics. IMPORTANCE Understanding the population biology and epidemiology of plant pathogens is essential to develop effective strategies for controlling plant diseases. Herein, we gathered insights into the population biology of P. leucotricha populations from conventional and organic apple orchards in the United States. We showed genetic heterogeneity between P. leucotricha populations in Washington, and structure between populations from different U.S. regions, suggesting that short-distance spore dispersal plays an important role in the disease’s epidemiology. We presented evidence that P. leucotricha is heterothallic, and that populations likely result from a mixed (i.e., sexual and asexual) reproductive system, revealing that the sexual stage contributes to apple powdery mildew epidemics. We showed that the major resistance gene Pl-1 is valuable for apple breeding because virulent isolates have most likely not emerged yet in U.S. commercial orchards. These results will be important to achieve sustainability of disease management strategies and maintenance of plant health in apple orchards.

Greenhouse Evaluation of Clubroot Resistant-Brassica napus cv. Mendel and Its Efficacy Concerning Virulence and Soil Inoculum Levels of Plasmodiophora brassicae

Clubroot resistance of oilseed rape (OSR) cultivars frequently relies on a major resistance gene originating from cv. Mendel. The efficacy of this resistance was studied in greenhouse experiments using two Plasmodiophora brassicae isolates, which were either virulent (P1(+)) or avirulent (P1) on Mendel. Seeds of clubroot-susceptible cultivar Visby and clubroot-resistant cultivar Mendel were sown in soil mixtures inoculated with different concentrations of resting spores (101, 103, 105, and 107 resting spores/g soil). Clubroot severity, plant height, shoot and root weight as well as resting spore propagation were assessed for each isolate and cultivar separately at four dates after sowing. The OSR cultivars behaved significantly different in the measured parameters. The threshold of inoculum density to cause disease depended strongly on the virulence of the pathogen and susceptibility of the host plant. In Visby grown in soil infested with P1, clubroot symptoms and increases in root weight and the number of propagated resting spores occurred at inoculum levels of 101 resting spores and higher, whereas Mendel was not affected in soils under the three lowest inoculum densities. In contrast, the P1(+) isolate led to earlier and more severe symptoms, heavier galls, and a significantly higher number of new resting spores in both cultivars.

Pyramiding Quantitative Resistance with a Major Resistance Gene in Apple: From Ephemeral to Enduring Effectiveness in Controlling Scab

Genetic resistance is a useful strategy to control plant disease, but its effectiveness may be reduced over time due to the emergence of pathogens able to circumvent the defenses of the plant. However, the pyramiding of different resistance factors in the same plant can improve the effectiveness and durability of the resistance. To investigate the potential for this approach in apple to control scab disease we surveyed scab incidence in two experimental orchards located at a distance of more than 300 km planted with apple genotypes carrying quantitative resistance and major gene resistance alone or in combination. Our results showed that the effectiveness of pyramiding in controlling scab was dependent on the site and could not be completely explained by the effectiveness level of the resistances alone.

Pyramiding for Resistance Durability: Theory and Practice

Durable disease resistance is a key component of global food security, and combining resistance genes into “pyramids” is an important way to increase durability of resistance. The mechanisms by which pyramids impart durability are not well known. The traditional view of resistance pyramids considers the use of major resistance gene (R-gene) combinations deployed against pathogens that are primarily asexual. Interestingly, published examples of the successful use of pyramids in the traditional sense are rare. In contrast, most published descriptions of durable pyramids in practice are for cereal rusts, and tend to indicate an association between durability and cultivars combining major R-genes with incompletely expressed, adult plant resistance genes. Pyramids have been investigated experimentally for a diversity of pathogens, and many reduce disease levels below that of the single best gene. Resistance gene combinations have been identified through phenotypic reactions, molecular markers, and challenge against effector genes. As resistance genes do not express equally in all genetic backgrounds, however, a combination of genetic information and phenotypic analyses provide the ideal scenario for testing of putative pyramids. Not all resistance genes contribute equally to pyramids, and approaches have been suggested to identify the best genes and combinations of genes for inclusion. Combining multiple resistance genes into a single plant genotype quickly is a challenge that is being addressed through alternative breeding approaches, as well as through genomics tools such as resistance gene cassettes and gene editing. Experimental and modeling tests of pyramid durability are in their infancy, but have promise to help direct future studies of pyramids. Several areas for further work on resistance gene pyramids are suggested.

Virulence Characterization of Venturia inaequalis Reference Isolates on the Differential Set of Malus Hosts

A set of differential hosts has recently been identified for 17 apple scab resistance genes in an updated system for defining gene-for-gene (GfG) relationships in the Venturia inaequalis-Malus pathosystem. However, a set of reference isolates characterized for their complementary avirulence alleles is not yet available. In this paper, we report on improving the set of differential hosts for h(7) and propose the apple genotype LPG3-29 as carrying the single major resistance gene Rvi7. We characterized a reference set of 23 V. inaequalis isolates on 14 differential apple hosts carrying major resistance genes under controlled conditions. We identified isolates that were virulent on at least one of the following defined resistance gene hosts: h(1), h(2), h(3), h(4), h(5), h(6), h(7), h(8), h(9), h(10), and h(13). Sixteen different virulence patterns were observed. In general, the isolates carried one to three virulences, but some of them were more complex, with up to six virulences. This set of well-characterized isolates will be helpful for the identification of additional apple scab resistance genes in apple germplasm and the characterization of new GfG relationships to help improve our understanding of the host-pathogen interactions in the V. inaequalis-Malus pathosystem.

Resistance of Spring Barley Varieties to Powdery Mildew in theCzechRepublicin 1971–2000

Resistance to powdery mildew of 127 spring barley varieties was evaluated in 702 official variety trials, using scores from 1 to 9. Trials with sufficient disease severity were only analysed. Varieties possessing the resistance genes Mla7 (Elgina), Ml(Kr) (BR-1519), Mla13 (Koral) and mlo (Forum) were among the most resistant ones. The varieties Diamant, HE-3527, HE-3631, II/61-FUDII and Zefir showed high susceptibility. Fifteen varieties, carrying the genes Mla1, Mla3, Mla6, Mla9, Mla13, Ml(Kr) and Ml(Sc), were in the first years of testing highly resistant, but became susceptible later. The score of the most resistant variety ranged each year from 8.05 to 9.00, only in 1987–1991 it was lower. Since the pathogen population rapidly adapted to most resistance genes in the mid eighties, no resistant variety was found in 1987–1989. From 1986 to 1995 on average only 6.6% of the tested varieties were resistant, but there was no variety with resistance score above 8.50. High resistance was typical for varieties possessing an effective major resistance gene. Since the resistance of such varieties was not durable, we recommend breeding of varieties with at least two fully effective resistance genes, using molecular markers.