Ganoderma boninense basidiospores in oil palm plantations: evaluation of their possible role in stem rots of Elaeis guineensis

Abstract

Basidiospores are implicated in the distribution and genetic diversity of Ganoderma boninense, cause of basal stem rot (BSR) and upper stem rot (USR) of oil palm (Elaeis guineensis). Measurement of aerial basidiospores within plantations in Sumatra showed continuous and high production over 24 h (range c. 2-11 000 spores m ) with maximum release during early evening. Basidiospores applied to cut surfaces of fronds, peduncles and stems germinated in situ. Equivalent, extensive wounds are created during plantation harvesting and management and represent potential sites for formation of infective heterokaryons following mating of haploid basidiospore germlings. Use of spore-sized micro-beads showed that basidiospores could be pulled up to 10 cm into severed xylem vessels, where they are relatively protected from dehydration, UV irradiation and competing microflora. Diversity of isolates from five locations on two plantations was assessed by RAMS fingerprinting. Isolates from within individual palms with USR were identical and represent single infections, but different USR infections had unique band patterns and revealed separate infections. Some BSR-affected trees contained more than one isolate, and thus had multiple infections. There was one example of adjacent BSR palms with the same isolate, indicating vegetative spread, but there were no identical genets from BSR infections and adjacent fallen palms. Isolate diversity was as great within a plantation as between plantations. It is evident that basidiospores play a major role in spread and genetic variability of G. boninense. Evidence for direct basidiospore infection via cut fronds, indirectly through roots via colonized debris and less frequently, infection by vegetative, clonal spread is considered.

About Ganoderma boninense in oil palm plantations of Sumatra and peninsular Malaysia: Ancient population expansion, extensive gene flow and large scale dispersion ability

“…In comparison, the white rot fungus Armillaria mellea, (order Agaricales) has a genetic diversity ranging between 0.103 and 0.600, depending on the population considered (Prospero et al 2008;Travadon et al 2012 b) and the Polyporus umbellatus (order Polyporales) has a genetic diversity of 0.544 (Liu et al 2015). The high genetic diversity observed in G. boninense, along with the small number of duplicated genotypes found in our sample, indicate that sexual reproduction plays a key role in spreading the disease, as proposed by Miller (1995), Pilotti et al (2003), and Rees et al (2012), Despite the fact that some palms could be infected by multiple genotypes (Miller 1995;Ariffin et al 1996;Pilotti et al 2003;Rees et al 2012) and that we only collected one fruiting body at random from each palm, the small number of duplicate genotypes we found (less than 2 %) suggests that disease spreading by root-toroot infection is anecdotal. Duplicate genotypes were always collected on neighbouring trees about 9 m apart, while oil palm roots are known to extend up to 30 m in general (Jacquemard 2011).…”

“…Broad-scale dispersal is common in many basidiomycetes (Hallenberg & Kuffer 2001). Pilotti et al (2003), and more recently Rees et al (2012), have shown a gene flow at the estate level, but our PCA results hint at a broader scale of gene flow embracing both Sumatra and peninsular Malaysia. Results from Bayesian assignment tests in STRUC-TURE and AMOVA support the hypothesis that isolates from both regions originate from a single population, which indicates continual gene flows between those two regions in spite of the Malacca strait.…”

“…Lastly, basidiospore dissemination can provide a genetic exchange system covering a total area of 200 000 km 2 with a maximum distance between two samples of 700 km. These basidiospores are most likely carried by wind (Rees et al 2012), but some other factors (insects, birds, animals or human activity) could play a role in spreading basidiospores across long distances. In fact, airborne dispersal may be a characteristic shared by all phytopathogenic polyporales.…”

“…The study of genetic diversity and population structure at different geographic scales, as well as the investigation of demographic history using approximate Bayesian computation (ABC) approaches, can give us clues about the influence of oil palm cultivation on G. boninense population structure and evolution. Population genetics using molecular markers is an effective tool for inferring important population parameters such as effective population size, mutation rate, gene flow and migration (Prospero et al 2008), and mating system (Rees et al 2012). As pathogens are constantly evolving and adapting to environmental changes, to predict the future evolution of a pathogen population it is important to understand its current population structure and how it has been influenced by environmental factors.…”

In vitro growth of Ganoderma boninense isolates on novel palm extract medium and virulence on oil palm (Elaeis guineensis) seedlings

“…Genetic variations among different Ganoderma isolates obtained from within same or between palms and within same or different plantations were found to be relatively high with various molecular apporaches, such as mitochondrial DNA restriction fragment length polymorphisms (RFLPs), random amplified polymorphic DNA (RAPD) and Random Amplified Microsatellite (RAMS) (Miller et al, 1999;Latiffah et al, 2005;Rees et al, 2012). Furthermore, heterogeneity in genetic backgrounds among G. boninense isolates from infected oil palms was also demonstrated through mating test with monokaryotic cultures and somatic compatibility test (Pilotti et al, 2003;Rees et al, 2012).…”