Threats

Chrysoporthe canker

Chrysoporthe canker
Chrysoporthe austroafricana Gryzenh. & M. J. Wingf.
Canker

Chrysoporthe austroafricana causes stem cankers on susceptible Eucalyptus spp. (Wingfield et al. 1989; Myburg et al. 2002). In South Africa, initial symptoms include sunken outer bark at the bases of trees (Wingfield et al. 1989). In more advanced stages, the fungus colonizes the cambium rapidly and girdles the stems leading to tree death. Young trees of susceptible species are particularly vulnerable and relatively large numbers can die in the first year of growth (Wingfield 2003). Symptoms on young trees include girdling of the stem at the root collar and apparent rapid death with leaves retained on the trees (Conradie et al. 1990). On older trees, basal cankers are obvious as cracked bark and swelling (Wingfield et al. 1989; Conradie et al. 1990). Fruiting structures of the pathogen (pycnidia) can be seen using a 10x magnification hand lens. 

The most important characteristic to identify Chr. austroafricana in the field is the presence of black fruiting structures with obvious necks on the surface of infected bark. Fruiting structures are flask-shaped black pycnidia (asexual) that are semi-immersed in the bark and occur singly or in groups with bright orange masses being exuded (Wingfield et al. 1989). Sexual structures (perithecia) have been seen but unlike in for example Chrysoporthe cubensis, are almost never seen (Gryzenhout et al. 2004). The asexual spores (conidia) are ovate, non-septate, hyaline and 3–4(–4.5) x 1.5–2mm (Gryzenhout et al. 2004).

At the time of its first discovery, the canker disease now known to be caused by Chr. austroafricana was thought to be the well-known Cryphonectria canker of Eucalyptus caused by Cryphonectria cubensis (Wingfield et al. 1989). The fungus originally treated as Cryphonectria cubensis is now known to include numerous species of which Chr. cubensis and Chr. deuterocubensis are mainly found in tropical countries of the world (Wingfield 2003; van der Merwe et al. 2010; van der Merwe et al. 2013). Chrysoporthe austroafricana is native to Southern Africa and has undergone a host-shift from native Myrtaceae such as Syzigium cordatum to infect Eucalyptus spp. (Heath et al. 2006).

Chrysoporthe austroafricana, like other species of Chrysoporthe and the Cryphonectria require wounds to infect. In South African Eucalyptus plantations, these wounds typically occur at the bases of trees during the early years of growth. They are most likely due to natural growth cracks at the soil interface but mechanical damage can also provide sites for infection.

Extensive studies have been conducted on Chr. austroafricana to better understand the importance of the pathogen and its origin (Heath et al. 2006). Screening commercially important clones for tolerance to infection has been successfully conducted using artificial inoculations. These studies have guided forestry companies in selecting clones with high levels of tolerance to infection.

1988 (Wingfield et al. 1989) as Cryphonectria cubensis. Recognised as a distinct species in 2004 by Gryzenhout et al. (2004).
KwaZulu-Natal, Mpumalanga, Northern Province
All evidence suggests that the pathogen is native to Southern Africa (Heath et al. 2006).

Gallery

Basal canker on Eucalyptus due to Chr. austroafricana
Bruce Abear alongside E. grandis clone girdled at base by Chr. austroafricana. First outbreak in SA
Pycnidia and conidial tendrils of Chr. austroafricana
Pycnidia of Chr. austroafricana on bark above stem canker on E. grandis.
Section through E. grandis clone after inoculation with Chr. austroafricana.
Six week old lesion associated with inoculation of E. grandis clone stem using Chr. austroafricana.
Asci and ascospores of Chr. austroafricana.
Pycnidia and conidial tendrils of Chr. austroafricana

Conradie E, Swart WJ, Wingfield MJ. 1990. Cryphonectria canker of Eucalyptus, an important disease in plantation forestry in South Africa. South African Forestry Journal 152:43-49.

Gryzenhout M, Myburg H, Merwe NA, Wingfield BD, Wingfield MJ. 2004. Chrysoporthe, a new genus to accommodate Cryphonectria cubensis. Studies in Mycology 50:119-142.

Heath RN, Gryzenhout M, Roux J, Wingfield MJ. 2006. Discovery of the canker pathogen Chrysoporthe austroafricana on native Syzygium spp. in South Africa. Plant Disease 90: 433-438.

Myburg H, Gryzenhout M, Heath R, Roux J, Wingfield BD, et al. 2002. Cryphonectria canker on Tibouchina in South Africa. Mycological Research 106:1299-1306.

van der Merwe NA, Gryzenhout M, Steenkamp ET, Wingfield BD, Wingfield MJ. 2010. Multigene phylogenetic and population differentiation data confirm the existence of a cryptic species within Chrysoporthe cubensis. Fungal Biology 114:966-979.

Van der Merwe NA, Steenkamp ET, Rodas C, Wingfield BD, Wingfield MJ. 2013. Host switching between native and non-native trees in a population of the canker pathogen Chrysoporthe cubensis from Colombia. Plant Pathology 62:642-648.

Wingfield MJ, Swart WJ, Abear BJ. 1989. First record of Cryphonectria canker of Eucalyptus in South Africa. Phytophylactica 21: 311-313.

Wingfield MJ. 2003. 2003 Daniel McAlpine Memorial Lecture: Increasing threat of diseases to exotic plantation forests in the Southern Hemisphere: Lessons from Cryphonectria canker. Australasian Plant Pathology 32: 133-139.