First report of Neocosmospora vasinfecta associated with the root rot complex of peanuts in Vietnam

Diagnostic studies were undertaken on the fungal pathogens associated with the root, crown and stem rot complex of peanuts in Nghe An, Quang Nam and Thua Thien Hue Provinces, Vietnam, in the growing seasons of 2006 and 2007. These involved ad hoc surveys to document symptoms and collect samples of root, crown and stem rots for isolation and identification of putative fungal pathogens, using procedures described in Burgess et al. (2008).

Root rot samples were washed in tap water, surface sterilised in 70% ethyl alcohol for 5 s, immediately rinsed in sterile water and damp-dried on sterile paper tissues. Sections of root (2–3 mm) were removed from the margin of diseased and symptomless tissue and plated on water agar containing streptomycin sulfate (1.0 g/L) and neomycin sulfate (0.12 g/L). Colonies that developed from the sections were transferred to carnation leaf-piece agar (CLA), and then purified by hyphal tipping (Burgess et al. 2008) to CLA and potato dextrose agar (PDA). Some cultures, putatively identified as Neocosmospora vasinfecta, produced abundant orange-red perithecia in the mycelium in colonies on PDA (Fig. 1) and on leaf pieces in CLA (Fig. 2). The perithecia contained thin-walled cylindrical asci. The eight globose to ellipsoidal ascospores were uniseriately arranged in the ascus (Fig. 3). The anamorph stage, small cylindrical to oblong-ellipsoidal, hyaline, one-celled conidia were formed in false-heads on long, thin, sometimes branched, conidiophores (Fig. 4) arising from vegetative mycelium, on both media. These morphological features corresponded to the description of N. vasinfecta in Cannon and Hawksworth (1984).

Fig. 1.  Abundant orange-red perithecia of Neocosmospora vasinfecta forming in a colony on potato dextrose agar.

Fig. 2.  Cluster of perithecia of Neocosmospora vasinfecta on edge of leaf piece from a colony on carnation leaf-piece agar.

Fig. 3.  Asci and ascospores of Neocosmospora vasinfecta.

Fig. 4.  Small one-celled conidia of the anamorph of Neocosmospora vasinfecta formed in a false-head on a conidiophore in situ in a colony on carnation leaf-piece agar.

Nine putative cultures of N. vasinfecta from Nghe An Province were forwarded to the Centraalbureau voor Schimmelcultures (CBS) where the identification of eight cultures was confirmed by sequencing. They were identical to each other and related to AY381142. The Nghe An Plant Protection Sub-Department accession numbers (NA prefix), and the corresponding CBS reference numbers are: NA61 = CPCI4197; NA62 = CPC14198; NA63 = CPC14199; NA64 = CPC14200; NA65 = CPC14201; NA66 = CPC14203; NA67 = CPC14204; NA68 = CPC14205. One culture, NA69 = CPC14202, differed slightly from the other cultures but was still closely related to AY381142.

Bright orange-red perithecia were only observed on diseased peanut taproots in the field in Thua Thien Hue Province. N. vasinfecta was not isolated from peanut root rot samples, and perithecia were not observed on diseased roots from Quang Nam Province. Although the fungus was commonly isolated from diseased roots in Nghe An Province, perithecia were not observed on these roots. More extensive taxonomic research is needed on the cultures listed above to determine their varietal status.

Further studies are also needed to clarify the role of N. vasinfecta in the root rot complex in peanuts in Vietnam. However, it should be noted that Fuhlbohm et al. (2007) have clearly shown N. vasinfecta to be pathogenic to peanuts in Australia. It has also been reported as a pathogen of peanuts (syn. groundnuts) in South Africa (Baard and van Wyk 1985) and Taiwan (Huang et al. 1992). Additional studies are needed to clarify the distribution and host range of this fungus in Vietnam given that it has a wide distribution in tropical and temperate regions, is commonly associated with roots of leguminous crops, and has been isolated from soil (Cannon and Hawksworth 1984).