John R. Hartman

Diversity and Biogeography of Sooty Blotch and Flyspeck Fungi on Apple in the Eastern and Midwestern United States

Sooty blotch and flyspeck (SBFS) fungi on apple fruit were sampled from nine orchards in four midwestern U.S. states during 2000 and 30 orchards in 10 eastern U.S. states during 2005 in order to estimate taxonomic diversity and discern patterns of geographic distribution. Forty apple fruit per orchard were arbitrarily sampled and colonies of each mycelial phenotype were counted on each apple. Representative colonies were isolated, cultures were purified, and DNA was extracted. For representative isolates, the internal transcribed spacer (ITS) and large subunit (LSU) regions of ribosomal DNA were amplified and sequenced. In total, 60 SBFS putative species were identified based on ITS sequences and morphological characteristics; 30 of these were discovered in the 2005 survey. Modified Kochs postulates were fulfilled for all 60 species in an Iowa orchard; colonies resulting from inoculation of apple fruit were matched to the original isolates on the basis of mycelial type and ITS sequence. Parsimony analysis for LSU sequences from both surveys revealed that 58 putative SBFS species were members of the Dothideomycetes, 52 were members of the Capnodiales, and 36 were members of the Mycosphaerellaceae. The number of SBFS species per orchard varied from 2 to 15. Number of SBFS species and values of the Margalef and Shannon indexes were significantly (P < 0.05) lower in 21 orchards that had received conventional fungicide sprays during the fruit maturation period than in 14 unsprayed orchards. Several SBFS species, including Schizothyrium pomi, Peltaster fructicola, and Pseudocercosporella sp. RH1, were nearly ubiquitous, whereas other species, such as Stomiopeltis sp. RS5.2, Phialophora sessilis, and Geastrumia polystigmatis, were found only within restricted geographic regions. The results document that the SBFS complex is far more taxonomically diverse than previously recognized and provide strong evidence that SBFS species differ in geographic distribution. To achieve more efficient management of SBFS, it may be necessary to understand the environmental biology of key SBFS species in each geographic region.

Recent Evolutionary Radiation and Host Plant Specialization in the Xylella fastidiosa Subspecies Native to the United States

ABSTRACT The bacterial pathogen, Xylella fastidiosa, infects many plant species in the Americas, making it a good model for investigating the genetics of host adaptation. We used multilocus sequence typing (MLST) to identify isolates of the native U.S. subsp. multiplex that were largely unaffected by intersubspecific homologous recombination (IHR) and to investigate how their evolutionary history influences plant host specialization. We identified 110 “non-IHR” isolates, 2 minimally recombinant “intermediate” ones (including the subspecific type), and 31 with extensive IHR. The non-IHR and intermediate isolates defined 23 sequence types (STs) which we used to identify 22 plant hosts (73% trees) characteristic of the subspecies. Except for almond, subsp. multiplex showed no host overlap with the introduced subspecies (subspecies fastidiosa and sandyi). MLST sequences revealed that subsp. multiplex underwent recent radiation (<25% of subspecies age) which included only limited intrasubspecific recombination (ρ/θ = 0.02); only one isolated lineage (ST50 from ash) was older. A total of 20 of the STs grouped into three loose phylogenetic clusters distinguished by nonoverlapping hosts (excepting purple leaf plum): “almond,” “peach,” and “oak” types. These host differences were not geographical, since all three types also occurred in California. ST designation was a good indicator of host specialization. ST09, widespread in the southeastern United States, only infected oak species, and all peach isolates were ST10 (from California, Florida, and Georgia). Only ST23 had a broad host range. Hosts of related genotypes were sometimes related, but often host groupings crossed plant family or even order, suggesting that phylogenetically plastic features of hosts affect bacterial pathogenicity.

Variability Among Forecast Models for the Apple Sooty Blotch/Flyspeck Disease Complex

Several disease forecast models have been developed to guide treatment of the sooty blotch and flyspeck (SBFS) disease complex of apple. Generally, these empirical models are based on the accumulation of hours of leaf wetness (leaf wetness duration [LWD]) from a biofix at or near the phenological growth stage petal fall, when apple flower petals senesce and drop. The models recommend timing of the initial fungicide application targeting SBFS. However, there are significant differences among SBFS forecast models in terms of biofix and the length of LWD thresholds. A comparison of models using a single input data set generated recommendations for the first SBFS fungicide application that differed by up to 5 weeks. In an attempt to improve consistency among models, potential sources for differences were examined. Leaf wetness (LW) is a particularly variable parameter among models, depending on whether on-site or remote weather data were used, the types of sensors and their placement for on-site monitors, and the models used to estimate LW remotely. When SBFS models are applied in the field, recommended treatment thresholds do not always match the method of data acquisition, leading to potential failures. Horticultural factors, such as tree size, canopy density, and cultivar, and orchard site factors such as the distance to potential inoculum sources can impact risk of SBFS and should also be considered in forecast models. The number of fungal species identified as contributors to the SBFS disease complex has expanded tremendously in recent years. A lack of understanding of key epidemiological factors for different fungi in the complex, and which fungi represent the most challenging management problems, are obvious issues in the development of improved SBFS models. If SBFS forecast models are to be adopted, researchers will need to address these issues.

INRA and Apple Disease Research in the Loire Valley Region of France

The Loire Valley Region France’s Loire Valley region frequently evokes images of châteaux, those grand homes and historic structures encompassing immense medieval fortress-like castles and extravagant renaissance mansions alike. A visitor from abroad can’t help but be impressed with each Loire Valley château: from the imposing castle of Angers, housing its immense medieval tapestry of the apocalypse; the elegant feminine mystique of château de Chenonceaux, built over a river; the sheer enormity of Chambord with its Leonardo da Vinci doublehelix staircase; Chinon, site of the historic meeting between Joan-of-Arc and Charles VII; Sleeping Beauty’s fairy tale castle, Usse; to the geometric or playful gardens found at Villandry or Chaumont sur Loire, and many, many more (Fig. 1). Mon dieu, qu’ils sont magnifiques! Although the Loire Valley is famous for its picturesque and historical châteaux, it is also a region rich in horticulture, often called the “Jardin de la France” (Garden of France). The Loire, France’s longest river, flows roughly east to west through central France past cities such as Roanne, Orleans, Blois, Tours, Angers, and Nantes before discharging into the Atlantic Ocean. Voila une tres belle region! The region is well known for its grapes and wine, pears, apples, strawberries, asparagus, lettuce, artichokes, and field and greenhouse nursery crops and ornamentals. Although apples are also grown elsewhere in France, such as in the Bordeaux region, the Rhone Valley, and Provence, the Loire region is the top producer; France produces about 25% of the apples in the European Union (3) and is a net exporter of apples. During the late 1990s, the American authors (J. R. Hartman and S. Thomson), while on sabbatical leaves, worked in the laboratories of the French authors (L. Parisi and J.-P. Paulin) on apple diseases. In this article, we report on the French apple industry and describe the national institute for agricultural research and its role in understanding apple diseases. We review results of work on apple scab and fire blight in France and provide impressions from the point of view of American plant pathologists working in France.