Evidence for a new introduction of the pitch canker fungus Fusarium circinatum in South Africa

Abstract

AfricaFusarium circinatum causes pitch canker of Pinus species in many parts of the world. The fungus was first recorded in South Africa in 1990 as a pathogen of P. patula seedlings and emerged later as a pathogen of established plantation trees, especially P. radiata in the Western Cape Province (WCP). In this study the population biology of F. circinatum in the WCP was explored. The aim was to determine the possible origin and reproductive mode of the pathogen, with the ultimate intention of informing disease management strategies in the region. Vegetative compatibility assays, sexual mating studies and amplified fragment length polymorphism analyses were used. For comparative purposes, an isolate collection obtained from diseased P. radiata seedlings in a commercial nursery in the region, as well as a set of isolates from commercial seedling nurseries in the central and northern parts of South Africa, were included. The results showed that the WCP population of F. circinatum employs a predominantly asexual mode of reproduction and that it is highly differentiated from populations of the fungus elsewhere in South Africa. However, limited genetic structure was found within the respective WCP isolate collections. Overall these findings suggest that pitch canker in the WCP originates from one or more separate introductions of the pathogen and that its movement in the region is not restricted. More effective strategies are thus required to limit and manage the effects of F. circinatum in plantations in this region of South Africa.

Nursery‐linked plantation outbreaks and evidence for multiple introductions of the pitch canker pathogen Fusarium circinatum into South Africa

“…The first record of the pathogen affecting established plantation trees was in 2005, when the typical pitch canker symptoms were observed on 5‐ to 9‐year‐old Pinus radiata trees in a plantation in the Western Cape Province (WCP; Coutinho et al ., ). Pitch canker has subsequently also been reported from 12‐ to 15‐year‐old P. radiata trees in other plantations in the Province, as well as on Pinus greggii trees in the Eastern Cape Province (ECP) and the KwaZulu‐Natal Province of South Africa (Steenkamp et al ., ).…”

“…Little is known regarding the population biology of F. circinatum responsible for the pitch canker outbreaks in South African Pinus plantations. The results of a single previous study revealed significant diversity among isolates originating from diseased P. radiata trees in WCP plantations, but unlike the situation in Pinus nurseries, these populations appeared to be more structured and to reproduce clonally (Steenkamp et al ., ). This previous study also suggested that the WCP plantations originated from distinct introductions of the pathogen into the region (Steenkamp et al ., ).…”

“…The results of a single previous study revealed significant diversity among isolates originating from diseased P. radiata trees in WCP plantations, but unlike the situation in Pinus nurseries, these populations appeared to be more structured and to reproduce clonally (Steenkamp et al ., ). This previous study also suggested that the WCP plantations originated from distinct introductions of the pathogen into the region (Steenkamp et al ., ). Whether F. circinatum populations associated with pitch canker outbreaks in other parts of the South Africa are related to the WCP population and also structured similarly remains to be determined.…”

“…However, this seems unlikely given the fact that pitch canker outbreaks in South African plantations appear to be geographically and climatically diverse and that seedlings are sourced from different nurseries in the country. For example, the pitch canker‐affected P. greggii plantations in the ECP are located in the Ugie and Maclear region, which is approximately 600 and 1000 km from the respective WCP sites where isolates from P. radiata were previously examined (Coutinho et al ., ; Steenkamp et al ., ). In the WCP, these plantations usually occur near the coast at elevations not exceeding 500 m a.s.l., while those in the ECP are located in the southernmost end of the Drakensberg mountain range at altitudes higher than 1200 m. Also, the P. radiata plantations in the WCP typically experience Mediterranean to Oceanic climates with mild winters, warm summers and most rain in the winter or spring.…”

“…Most previous investigations into the population biology of F. circinatum have used dominant genetic markers to study diversity. These include vegetative compatibility group (VCG) assays (Viljoen et al ., ; Wikler & Gordon, ; Wikler et al ., ; Britz et al ., ; Iturritxa et al ., ; Steenkamp et al ., ) and amplified fragment length polymorphisms (AFLPs; Iturritxa et al ., ; Steenkamp et al ., ). However, VCG and AFLP analyses provide limited value for inferences of gene flow and reproductive biology (Majer et al ., ; Vekemans et al ., ).…”