Eastern Filbert Blight Resistance in American and Interspecific Hybrid Hazelnuts

Eastern filbert blight (EFB), caused by the fungus Anisogramma anomala, is a primary limitation to european hazelnut (Corylus avellana) cultivation in eastern North America. American hazelnut (Corylus americana) is the endemic host of A. anomala and, despite its tiny, thick-shelled nuts, is a potentially valuable source of EFB resistance and climatic adaptation. Interspecific hybrids (Corylus americana × C. avellana) have been explored for nearly a century as a means to combine EFB resistance with wider adaptability and larger nuts. Although significant progress was made in the past, the genetic diversity of the starting material was limited and additional improvements are needed for expansion of hazelnut (Corylus sp.) production outside of Oregon, where 99% of the U.S. crop is currently produced. Our objective was to determine if C. americana can be a donor of EFB resistance. We crossed 29 diverse EFB-resistant C. americana accessions to EFB-susceptible C. avellana selections (31 total progenies) to produce 2031 F1 plants. In addition, new C. americana germplasm was procured from across the native range of the species. The new collection of 1335 plants from 122 seed lots represents 72 counties and 22 states. The interspecific hybrid progenies and a subset of the American collection (616 trees from 62 seed lots) were field planted and evaluated for EFB response following field inoculations and natural disease spread over seven growing seasons. EFB was rated on a scale of 0 (no EFB) to 5 (all stems containing cankers). Results showed that progeny means of the interspecific hybrids ranged from 0.96 to 4.72. Fourteen of the 31 progenies were composed of at least one-third EFB-free or highly tolerant offspring (i.e., ratings 0–2), transmitting a significant level of resistance/tolerance. Several corresponding C. americana accessions that imparted a greater degree of resistance to their hybrid offspring were also identified. In addition, results showed that 587 (95.3%) of the 616 C. americana plants evaluated remained completely free of EFB. These findings confirm reports that the species rarely expresses signs or symptoms of the disease and should be robustly studied and exploited in breeding.

Effects of phloem on canopy dieback, tested with manipulations and a canker pathogen in the Corylus avellana/Anisogramma anomala host/pathogen system

Canker pathogens cause necrosis of the phloem, but in many host/pathogen systems, they also cause canopy dieback, which implicates xylem, not phloem dysfunction. We hypothesize that this dieback distal to the canker is caused by water stress resulting from the lack of a phloem-to-xylem connection, which in a healthy plant would allow delivery of nonstructural carbohydrates (NSCs) and water inward to aid in xylem embolism refilling. We tested several components of this hypothesis in the host/pathogen system Corylus avellana L./Anisogramma anomala (Peck) E. Müll (Eastern filbert blight). Cankers were non-girdling and usually ≥0.1 m long. As expected, healthy controls had higher specific conductivity (Ks) than diseased stems, but unexpectedly, had similar moisture content (m.c.), showing that the lower Ks did not result from more embolisms in the diseased stems. Moreover, manipulations that removed cambium and phloem to simulate a canker, or that shaded stems to lower NSCs, did not result in lower Ks or m.c. than controls. The outer millimeter of xylem adjacent to a canker had infrequent tyloses and/or fungal hyphae in many but not all samples, and dye studies showed little xylem water transport in that region, but the incidence of these blockages was insufficient to cause the observed 19% decrease in Ks. Healthy stems had higher m.c. than diseased stems above the canker (or analogous) location and were longer for the same leaf weight, suggestive of water stress in the upper portion of diseased stems. These results suggest that dieback distal to cankers in this system results from the bottleneck in water transport in the region adjacent to a canker, but did not find evidence to support the requirement of a phloem-to-xylem connection for continued water transport.

Population Differentiation Within Anisogramma anomala in North America

Anisogramma anomala, a biotrophic ascomycete in the order Diaporthales, causes eastern filbert blight (EFB) of hazelnuts (Corylus spp.). Until recently, little has been documented on its genetic diversity and population structure. In this study, 18 simple sequence repeat markers were used to fingerprint 182 accessions of the fungus originating from across North America. Our results, based on summary statistics of the allelic data, a discriminant analysis of principal components (DAPC) scatterplot, an unweighted pair group method with arithmetic mean (UPGMA) dendrogram, and analysis of multilocus genotypes, show that A. anomala exhibits considerable genetic diversity across multiple populations. Eleven clusters were resolved from the DAPC scatterplot, five of which were validated by statistically supported clusters in the UPGMA dendrogram. The 11 DAPC clusters were statistically significant via an analysis of molecular variance. Dendrogram topology and DAPC scatterplot groups showed some correlation with collection origin; samples collected in proximity tended to cluster together and be genetically similar. However, some locations held populations that were diverse and some populations with a high degree of similarity had disparate origins, suggesting movement by humans. Overall, the results demonstrate the presence of multiple, genetically distinct populations of A. anomala in North America and serve as a reference to assist in understanding and managing EFB.

Tank Mixing Fungicides for Effectiveness Against Eastern Filbert Blight of Hazelnut

Hazelnut (Corylus avellana) production in Oregon primarily occurs on cultivars susceptible to Anisogramma anomala, the causal agent of eastern filbert blight (EFB). Management of EFB involves planting resistant cultivars, removal of cankered limbs, and the application of fungicides. Tank mixes of demethylation-inhibiting (DMI; Fungicide Resistance Action Committee [FRAC] group 3) or quinone outside inhibitor (QoI; FRAC group 11) fungicides with chlorothalonil (FRAC group M5) at full or reduced rates were evaluated for effectiveness against A. anomala. The use of chlorothalonil in a mix with a DMI or QoI fungicide was an effective treatment for EFB even if each component of the mix was at half the labeled rate. Different liquid or dry formulations of chlorothalonil were equally effective in a tank mix for EFB control. The combination of propiconazole (FRAC group 3) tank mixed with trifloxystrobin (FRAC group 11) was not effective, whereas trees treated with propiconazole tank mixed with pyraclostrobin (FRAC group 11) resulted in significantly fewer EFB cankers compared with nontreated trees. When using tank mixes for EFB management, DMI fungicides should remain at full rates while mixing with a half-rate of chlorothalonil. In contrast, QoI fungicides and chlorothalonil could both be used at half-rates and still maintain acceptable EFB control. Tank mixing chlorothalonil with fungicides at risk of resistance development can help maintain consistent EFB control and should help prevent or delay the emergence of fungicide-resistant A. anomala isolates.

Anisogramma anomala. [Distribution map].

A new distribution map is provided for Anisogramma anomala (Peck) E. Müller. Sordariomycetes: Diaporthales: Valsaceae. Host: European hazelnut (Corylus avellana). Information is given on the geographical distribution in North America (Canada, British Columbia, Manitoba, Nova Scotia, Ontario, Quebec, USA, Connecticut, Delaware, Illinois, Iowa, Maine, Maryland, Massachusetts, Michigan, Minnesota, New Jersey, New York, North Carolina, Oregon, Pennsylvania, Washington and Wisconsin).

Eastern Filbert Blight Resistance in Hazelnut Accessions ‘Culplà’, ‘Crvenje’, and OSU 495.072

European hazelnut (Corylus avellana L.) is a significant crop in Oregon, where 99% of United States hazelnuts are produced. Eastern filbert blight (EFB) caused by Anisogramma anomala (Peck) E. Müller is an important disease that infects the trees, reduces yield, and causes premature death. Managing the disease through cultural methods and fungicide applications is laborious and expensive, and genetic host resistance is considered the most viable option for control. Genetic resistance from ‘Gasaway’ has been used to develop resistant cultivars including Yamhill and Jefferson, but concern about the durability of this single resistance gene stimulated a search for additional sources of resistance. This study used three recently identified sources of EFB resistance: ‘Culplà’ from Spain, ‘Crvenje’ from Serbia, and OSU 495.072 from southern Russia. RAPD markers linked to resistance from ‘Gasaway’ were absent in all three accessions. Disease response was noted in segregating progenies following greenhouse or structure inoculation, and the resistance loci were mapped using microsatellite markers. In only four of the nine progenies did segregation for disease response fit the ratio of 1 resistant:1 susceptible expected for a single locus, a heterozygous resistant parent, and a dominant allele for resistance. Three progenies showed an excess of resistant seedlings while two showed a deficiency of resistant seedlings. The reciprocal translocations reported in several leading hazelnut cultivars may be present in the parents of the studied progenies, and affecting the segregation ratios. Microsatellite marker A614, previously mapped to linkage group (LG) 6, was closely linked to resistance from all three sources. Maps were constructed for LG6 for each resistant parent using microsatellite markers. The three resistance loci mapped to the same region on LG6 where resistance from ‘Gasaway’ and OSU 408.040 are located. The resistance alleles in all five accessions may be the same, or more likely are a cluster of different resistance genes in the same region. Markers LG628, LG610, and LG696 will be useful to breed new hazelnut cultivars with resistance from Culplà, Crvenje, and OSU 495.072.

Characterization of Eastern Filbert Blight-resistant Hazelnut Germplasm Using Microsatellite Markers

The development of new cultivars resistant to the disease eastern filbert blight (EFB), caused by Anisogramma anomala, is of primary importance to hazelnut (Corylus sp.) breeders in North America. Recently, a large number of EFB-resistant cultivars, grower selections, and seedlings from foreign germplasm collections were identified. However, for a significant number of these, little is known of their origin, relationships, or genetic background. In this study, 17 microsatellite markers were used to investigate the genetic diversity and population structure of 323 unique accessions, including EFB-resistant and tolerant germplasm of uncertain origins, in comparison with a panel of known reference accessions representing a wide diversity of Corylus cultivars, breeding selections, and interspecific hybrids. The resulting allelic data were used to construct an unweighted pair group method using arithmetic averages (UPGMA) dendrogram and STRUCTURE diagram to elucidate relationships among the accessions. Results showed 11 consensus groups with EFB-resistant or tolerant accessions in all, providing strong evidence that EFB resistance is relatively widespread across the genus Corylus. Furthermore, open-pollinated seedlings tended to group together with reference accessions of similar geographic origins, providing insight into their genetic backgrounds. The results of this study add to the growing body of knowledge of hazelnut genetic resources and highlight recently introduced EFB-resistant seedling germplasm as new, unrelated genetic pools of resistance.

Disease Incidence and Ascospore Dispersal from Cut Hazelnut Branches Colonized by Anisogramma anomala

Hazelnut branches bearing stromata of Anisogramma anomala cut in December (2009 and 2010) were compared with branches cut prior to bud break in March to investigate these sources of inoculum. Branches were placed into brush piles (sources). Spore traps and potted hazelnut trees were placed adjacent to each source, 6.4 m upwind and downwind, and 20 m downwind from each source. Significantly more ascospores were detected near sources of branches cut in March compared with December in 2010 however, no differences were detected between pruning treatments in 2011. Ascospore viability, as assessed by trypan blue stain, averaged 50% for both pruning times each season. Significantly more ascospores were detected 6.4 m downwind compared with 6.4 m upwind or 20 m downwind of a source both years. All potted trees exposed to branches from both pruning treatments within sources became diseased both years. The proportion of potted trees that became infected was greater for the downwind group than the upwind for both years, suggesting that ascospores were dispersed beyond the rain splash dispersal range of sources. Ascospores from diseased branches pruned in December or March remained viable, infectious and were dispersed at least 20 m downwind.

Flowering Phenology of Eastern Filbert Blight-resistant Hazelnut Accessions in New Jersey

Hazelnuts (Corylus sp.) are monoecious and wind-pollinated with reproduction limited by a sporophytic self-incompatibility system. They flower during the winter and are dichogamous with the dates of flowering ranging from December to March in New Jersey depending on the genotype, geographic location, and year. Successful, consistent nut production depends on both genetic compatibility and the appropriate timing of flowering between pollinizing and nut-producing cultivars. While the disease eastern filbert blight (EFB), caused by Anisogramma anomala, has severely limited past hazelnut production in the eastern United States, resistant and tolerant genotypes are now available for testing. However, little is known of their flowering phenology in this region. In this study, the flower and budbreak phenology of 19 different EFB-resistant and EFB-tolerant hazelnut accessions was evaluated over 4 years, and the results compared with air temperature data collected during bloom. Results showed that the accessions followed a similar progression of bloom each year (both staminate and pistillate flowers), which allowed their placement into early-, mid-, and late-flowering groups. However, the date of bloom and duration of bloom, especially for pollen shed, differed each year, largely corresponding to average air temperature trends. Confirming previous reports from other cold regions, it was shown that consistently colder average temperatures delayed bloom until later in the winter, which then led to a compressed period of flowering once temperatures warmed. In contrast, relatively warm temperatures over the season led to earlier flowering as well as a significant lengthening of the duration of bloom, similar to responses reported in Mediterranean climates. Our study documents hazelnut flowering phenology under New Jersey’s variable winter climate, and the results provide a benchmark for selecting suitable pollenizers and breeding parents for future nut production, flowering research, and/or genetic improvement in this region.