Host plant genetic control of associated fungal and insect species in a Populus hybrid cross.

Plants employ a diverse set of defense mechanisms to mediate interactions with insects and fungi. These relationships can leave lasting impacts on host plant genome structure such as rapid expansion of gene families through tandem duplication. These genomic signatures provide important clues about the complexities of plant/biotic stress interactions and evolution. We used a pseudo-backcross hybrid family to identify quantitative trait loci (QTL) controlling associations between Populus trees and several common Populus diseases and insects. Using whole-genome sequences from each parent, we identified candidate genes that may mediate these interactions. Candidates were partially validated using mass spectrometry to identify corresponding QTL for defensive compounds. We detected significant QTL for two interacting fungal pathogens and three insects. The QTL intervals contained candidate genes potentially involved in physical and chemical mechanisms of host-plant resistance and susceptibility. In particular, we identified adjoining QTLs for a phenolic glycoside and Phyllocolpa sawfly abundance. There was also significant enrichment of recent tandem duplications in the genomic intervals of the native parent, but not the exotic parent. Tandem gene duplication may be an important mechanism for rapid response to biotic stressors, enabling trees with long juvenile periods to reach maturity despite many coevolving biotic stressors.

Enhanced hypovirus transmission by engineered super donor strains of the chestnut blight fungus, Cryphonectria parasitica, into a natural population of strains exhibiting diverse vegetative compatibility genotypes.

Horizontal transmission of virulence attenuating hypoviruses of Cryphonectria parasitica is restricted by an allorecognition system termed vegetative incompatibility (vic). A super donor formulation of two engineered C. parasitica strains (SD328/SD82) with gene disruptions at four of six vic loci transmitted hypovirus to strains in the laboratory independent of vic genotype. We now report the transmission of hypovirus by the SD328/82 formulation to a diverse, natural C. parasitica population infecting American chestnut in a forest setting. Hypovirulent (HV) isolates were recovered from 94% of cankers treated with the hypovirus-infected SD328/82 formulation compared to 51% of cankers treated with a hypovirus-infected EU5/6 formulation (strains having the same vic genotypes as SD strains but lacking vic gene disruptions). Overall, the SD328/82 formulation transmitted hypovirus into more divergent vic genotypes compared to the EU5/6 formulation. These results demonstrate the SD328/82 formulation can serve as an enhanced hypovirus vector for highly divergent C. parasitica populations.

Evaluation of Two Decades of Cryphonectria parasitica Hypovirus Introduction in an American Chestnut Stand in Wisconsin.

Hypovirus-infected Cryphonectria parasitica strains were introduced in a large stand of American chestnut (>4,000 individuals) in western Wisconsin (USA) to evaluate whether hypoviruses can serve as biological control agents. They were deployed by treating cankers from 1992 to 1997 and again from 2004 to 2014. After 17 years of hypovirus introductions within an area of the stand with the longest history of disease, isolation of hypovirus-infected strains increased from 55% in 1994 to 86% in 2014 from cankers that were treated. During the same period, isolation from cankers that arose on trees with treated cankers increased from 29 to 72% and from 15 to 84% for cankers on nearby trees that received no treatment. Tree survivorship over the 23-year study period for trees with treated cankers was 51% compared with 31% for trees that were not treated. Introduction of hypovirus has resulted in the regrowth of the crowns of many large-diameter trees. Putative recovery of American chestnut in this stand provides evidence that prolonged hypovirus treatment can act as a biological control when limited numbers of vegetative compatibility types of C. parasitica exist.

The origin of Ceratocystis fagacearum, the oak wilt fungus.

The oak wilt pathogen, Ceratocystis fagacearum, may be another example of a damaging, exotic species in forest ecosystems in the United States. Though C. fagacearum has received much research attention, the origin of the fungus is unknown. The pathogen may have been endemic at a low incidence until increased disturbances, changes in land use, and forest management created conditions favorable for disease epidemics. The host genus Quercus contains some relatively resistant species native to the United States, further supporting the hypothesis that the pathogen is native in origin. However, there are also many common, highly susceptible Quercus species--a characteristic typical of introduced pathogens. Most convincingly, studies have shown that the known populations of C. fagacearum have experienced a severe genetic bottleneck that can only be explained by a single introduction. The weight of evidence indicates that C. fagacearum is an introduced pathogen, with possible origins in Central or South America, or Mexico.

Phytophthora quercetorum sp. nov., a novel species isolated from eastern and north-central USA oak forest soils.

Isolates belonging to an undescribed Phytophthora species were frequently recovered during an oak forest soil survey of Phytophthora species in eastern and north-central USA in 2004. The species was isolated using an oak leaf baiting method from rhizosphere soil samples collected from Quercus rubra, Q. macrocarpa, and Q. phellos. This species is formally described as P. quercetorum. It is homothallic and has aplerotic oogonia and paragynous antheridia. It produces papillate sporangia (occasionally bipapillate) of ovoid-elongated shapes. Its temperature optimum for growth is ca 22.5 degrees C with the upper limit of ca 32.5 degrees C. P. quercetorum differs from the morphologically related P. quercina in producing distinct submerged colony-patterns, different growth-temperature requirements, and oogonial shapes and sizes. Phylogenetic analyses using seven nuclear loci supported P. quercetorum as a novel species within clade 4, closely related to P. arecae, P. palmivora, P. megakarya, and P. quercina.

Dominating north american forest pathology issues of the 20th century.

ABSTRACT The profession of forest pathology evolved in the early decades of the 20th century from a science describing microorganisms that infect trees to a discipline that was required to deal with numerous disease outbreaks. The foundations of the science were carried from Europe to the "New World" and initially dealt with decay and the resource losses it caused. The profession was forced to shift direction quickly because it was called upon to address major diseases caused by the introduction of pathogens from other continents; notably organisms inciting chestnut blight, Dutch elm disease, and white pine blister rust. Changes in natural ecosystems that resulted from a legacy of poor forest practice, land abuse, and an increase in plantation monocultures gave rise to other disease problems when host-pathogen balances in natural ecosystems were disturbed. Further, the need for large numbers of tree seedlings resulted in numerous nursery disease problems. Although many of the principles of general plant pathology had application to the study of forest diseases, the long-term nature of forests requires varied approaches to their study and management. Today, the science continues to evolve as the complexities of forest ecosystems unfold.