Hypovirus

Abstract: ▪ Abstract Most hypovirulence in the chestnut blight fungus, Cryphonectria parasitica, is associated with infection by fungal viruses in the family Hypoviridae. Hypovirulence has controlled chestnut blight well in some locations in Europe and in Michigan in the United States. In contrast, with few exceptions, biological control has failed almost completely in eastern North America. Therapeutic treatment of individual cankers is successful in most cases, but the success of hypovirulence at the population level depends on the natural spread of viruses. Characteristics of three interacting trophic levels (virus, fungus, and tree), plus the environment, determine the success or failure of hypovirulence. Vegetative incompatibility restricts virus transmission, but this factor alone is a poor predictor of biological control. Any factor reducing the rate of chestnut blight epidemics enhances hypovirus invasion. Overall, however, not enough is understood about the epidemiological dynamics of this system to determ...

Abstract: After 77 years of being attacked by the chestnut blight fungus, American chestnut trees continue to sprout from gradually declining root systems. The blight fungus in Italy is now associated with virus-like agents that limit its pathogenicity, and attempts have been made to introduce these controlling agents into the blight fungus in the United States. If a way can be found to help the spread here of strains of the fungus with controlling agents, it may be possible to save the American chestnut trees in our eastern forests.

Abstract: Cryphonectria parasitica (MUff.) Barr. (Syn. Endothia parasitica (MUff.) And, & And.), the causal agent of chestnut blight, is a fungal pathogen of Castanea and Quercus species. After its introduction at the beginning of this century. C. parasitica virtually eliminated C. dentata Borlch. as an important forest tree species in the eastern United States. A similar result was anticipated when chestnut blight was found in Europe, but the unexpected appearance of trans­ missible hypovirulence-a unique natural biocontrol phenomenon-prevented the European chestnut (c. sativa Mill.) from succumbing to the blight (for reviews see 5,51, 69,92, 101, 103, 130). Castanea sativa was introduced by the Romans from Minor Asia (79). This tree species was formerly of major economic importance in the mountainous areas of Southern Europe, in the southern foothills of the Alps from Italy into Hungary, and along the Black Sea. It provided timber, firewood, tannin, and litter bed on marginal land, and produced nuts for human consumption and animal forage. Chestnuts are traditionally grown as coppices with a rotation period of 15 to 30 years, as coppices with standards, in high forests, and in orchards. The latter are often formed of old-growth giant trees (100 years is not uncommon), grafted with the local varieties (Figure 1). Socioeconomic changes and the introduction of the devastating chestnut blight (c. parasitica) caused a rapid decline in chestnut cultivation in many regions after the second world war (54, 76).

Abstract: Chestnut blight, caused by Cryphonectria parasitica, is a devastating disease infecting American and European chestnut trees. The pathogen is native to East Asia and was spread to other continents via infected chestnut plants. This review summarizes the current state of research on this pathogen with a special emphasis on its interaction with a hyperparasitic mycovirus that acts as a biological control agent of chestnut blight. Taxonomy: Cryphonectria parasitica (Murr.) Barr. is a Sordariomycete (ascomycete) fungus in the family Cryphonetriaceae (Order Diaporthales). Closely related species that can also be found on chestnut include Cryphonectria radicalis, Cryphonectria naterciae, and Cryphonectria japonica. Host range: Major hosts are species in the genus Castanea (Fam. Fagaceae), particularly the American chestnut (C. dentata), the European chestnut (C. sativa), the Chinese chestnut (C. mollissima), and the Japanese chestnut (C. crenata). Minor, incidental hosts include oaks (Quercus spp.), maples (Acer spp.), European hornbeam (Carpinus betulus L.), and American chinkapin (Castanea pumila). Disease symptoms: C. parasitica causes perennial necrotic lesions (so-called cankers) on the bark of stems and branches of susceptible host trees, eventually leading to wilting of the plant part distal to the infection. Chestnut blight cankers are characterized by the presence of mycelial fans and fruiting bodies of the pathogen. Below the canker the tree may react by producing epicormic shoots. Non-lethal, superficial or callusing cankers on susceptible host trees are usually associated with mycovirus-induced hypovirulence. Disease control: After the introduction of C. parasitica into a new area, eradication efforts by cutting and burning the infected plants/trees have mostly failed. In Europe, the mycovirus Cryphonectria hypovirus 1 (CHV-1) acts as a successful biological control agent of chestnut blight by causing so-called hypovirulence. CHV-1 infects C. parasitica and reduces its parasitic growth and sporulation capacity. Individual cankers can be therapeutically treated with hypovirus-infected C. parasitica strains. The hypovirus may subsequently spread to untreated cankers and become established in the C. parasitica population. Hypovirulence is present in many chestnut growing regions of Europe, either resulting naturally or after biological control treatments. In North America, disease management of chestnut blight mainly focuses on breeding with the goal to backcross the Chinese chestnut's blight resistance into the American chestnut genome. This article is protected by copyright. All rights reserved.