Reducing the risk of pitch canker disease (caused by Fusarium circinatum) to Pinus patula in South Africa

Abstract

The principal objective of this research has been to reduce the potential impact of Fusarium circinatum on young and mature Pinus patula trees in South Africa. The results provide new knowledge concerning the genetic variation within P. patula, and suggest alternative species and hybrids, with resistance to infection by the pitch-canker pathogen F. circinatum. The infection of nursery plants is the principal cause of dying seedlings after planting and controlling the disease in the nursery is paramount to achieving good post-planting survival of P. patula. A large number of alternative pines, which are more tolerant to the pathogen, exist. Pinus elliottii is the most versatile alternative due to the ready availability of seed and the fact that it can be planted on a wide range of sites. Although the species is more tolerant than P. patula, individual families vary in their tolerance to F. circinatum and care should be taken to eliminate the more susceptible families. Pinus maximinoi and the low elevation (LE) source of P. tecunumanii are highly tolerant and provide an excellent alternative to P. patula on sites free of frost. The high elevation (HE) source of P. tecunumanii, which is more tolerant of cold than the low elevation source, is significantly more tolerant to F. circinatum than P. patula. However, large variation in the tolerance of individual P. tecunumanii (HE) families to F. circinatum exists and tolerant families of this source need to be identified before commercial deployment. Although P. maximinoi and P. tecunumanii are sub-tropical pines and sensitive to frost, meaningful variation in the tolerance of individual families to frost has been observed which indicates that these species can be bred for improved frost tolerance. On the colder sites, P. pseudostrobus may become an important alternative to P. patula due to its excellent tolerance to F. circinatum. In all cases hybrids between P. patula and pines more tolerant to F. circinatum, are significantly more tolerant than P. patula. Of these the P. patula x P. tecunumanii hybrid is the most promising. Due to the excellent tolerance of P. tecunumanii (LE) all families of the P. patula x P. tecunumanii (LE) hybrid are tolerant of F. circinatum. On the other hand, families of the P. patula x P. tecunumanii (HE) vary greatly in their tolerance to F. circinatum and the specific combination of the parents appears to play an important role in determining tolerance. Although only 5% of the current P. patula growing stock is of similar tolerance to F. circinatum as P. elliottii heritability for tolerance to F. circinatum is high in P. patula indicating that improvements can be achieved through breeding. Good control can also be achieved by identifying specific full-sib P. patula families that are tolerant to F. circinatum, rather than the use of open pollinated seed. It is anticipated that large areas, currently well suited to P. patula, will in the future be replanted with pine hybrids and species more tolerant of F. circinatum. Pinus patula, which has been bred for improved tolerance to F. circinatum, will be limited to the most temperate regions of South Africa.