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Hypovirulence of plant pathogens:

the control of chestnut blight

Chestnut blight, caused by the fungus Cryphonectria
parasitica(Ascomycota), has destroyed many millions
of trees in North America, following the first report
of its occurrence in the New York Zoological Garden
in 1904 (Fig. 9.13). Its progressive spread across the
eastern half of the USA reduced the native American
chestnut (Castanea dentata) almost to the status of an
understorey shrub, whereas it was once a magnificent
“high forest” tree. However, recent studies raise the
prospect that chestnut blight might be controlled by
genetic manipulation of hypovirus-associated dsRNA,
discussed below.
Cryphonectriainfects through wounds in the bark
of chestnut trees, and then spreads in the cambium,
progressively girdling the stem and killing the plant
above the infection point (Fig. 9.14). This disease was
also a significant problem in European sweet chestnut
trees (Castanea sativa) in the 1940s, but in Italy in
the early 1950s some heavily infested sweet chestnut
plantations began to recover spontaneously. The
lesions in these trees stopped spreading round the
trunks, and strains of C. parasiticaisolated from them
showed abnormal features. They grew slowly and
erratically on agar, were white rather than the normal

orange color, produced significantly fewer conidia, and

showed only low virulence when wound-inoculated

into trees. This low virulence (hypovirulence) could

be transmitted to other strains during hyphal anasto-

mosis on agar, so it was coined transmissible

hypovirulence. None of the hypovirulent strains

174 CHAPTER 9

Fig. 9.13Recorded spread of chestnut blight

caused by Cryphonectria parasiticaafter it was first

recorded in New York in 1904.

Fig. 9.14Diagram of a cross-section of a trunk with a

spreading canker caused by Cryphonectria parasitica. This

fungus produces airborne spores that infect through

wounds, then progressively girdles the trunk by growing

in the cambium.