Cryphonectria parasitica, the causal agent of chestnut blight: invasion history, population biology and disease control

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 Cryphonectriaceae (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 (Family 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) and American chinkapin (Castanea pumila).

Disease symptoms: Cryphonectria 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 is mainly focused on breeding with the goal to backcross the Chinese chestnut's blight resistance into the American chestnut genome.

Keywords: Cryphonectria hypovirus; Cryphonectria parasitica; chestnut blight; disease management; hypovirulence; review.

Figure 1

Symptoms of chestnut blight on Castanea sativa. (a) Branch wilting caused by a Cryphonectria parasitica infection. The wilted leaves typically remain hanging on the branches even after leaf fall. They produce a so‐called flag, which is the most pronounced early symptom of chestnut blight in the crown of adult trees. (b) Extended dieback after several years of infection. (c–e) Various virulent chestnut blight cankers. Cankers typically appear as sunken, reddish‐brown bark lesions. Below the cankers, trees typically react by producing epicormic shoots. (f) Cryphonectria parasitica forms pale brown mycelial fans, which advance intercellularly in the bark and cambium of the chestnut tree. (g) Grafted chestnut seedling infected by C. parasitica. (h) Blight infection (reddish discoloration) of a chestnut plant in a nursery. (i, j) Passive (healed) chestnut blight canker associated with hypovirulence of C. parasitica. In contrast with virulent cankers, hypovirus‐infected cankers typically have a swollen appearance and are superficial or callused.

Figure 2

Sporulation of Cryphonectria parasitica. (a) On the infected bark, the fungus produces masses of yellow–orange to reddish–brown pustules (stromata) harbouring sexual or asexual fruiting bodies. (b) Sexual fruiting bodies (perithecia). (c) Asexual fruiting bodies (pycnidia). The asexual spores (conidia) are extruded from the pycnidia as spore tendrils.

Figure 3

Vegetative incompatibility and hypovirus transmission in Cryphonectria parasitica. (a) Vegetative compatibility tests. For vc type determination, the unknown C. parasitica strain is paired with tester strains of known vc types. Compatible strains merge (M) into a single culture, whereas a barrage zone (B) is formed between incompatible strains. (b) Hypovirus transmission between strains of C. parasitica. Pairs of hypovirus‐infected (white) and hypovirus‐free (orange) strains are co‐cultured on potato dextrose agar. A colour change from orange to white indicates successful hypovirus transmission (left plate). Note that vegetative incompatibility prevents hypovirus transmission in the right plate, but not in the left.