TABLE CONTENTS

First Report of Groundnut Genotypes Resistant to the Groundnut Pod
Nematode Ditylenchus africanus
Steenkamp, S. (1), A.H. McDonald (1) & D. De Waele (2)
(1) ARC-Grain Crops Institute, Private Bag X1251, Potchefstroom 2520, South Africa; (2) Laboratory of
Tropical Crop Improvement, Catholic University of Leuven, Kasteelpark Arenburg 13, 3001, Heverlee, Belgium
The economically important, seed-borne, groundnut pod nematode Ditylenchus africanus, is
omnipresent in the groundnut-producing areas of South Africa. Resistant genotypes could
play an important role in reducing these parasites to levels below the damage threshold.
Selected genotypes, including the cultivars Sellie (susceptible) and Kwarts (tolerant) were
first screened for host suitability against this nematode in a microplot trial. Genotypes that
showed resistance were subsequently tested again in a microplot and two field trials under
natural infestations. In both microplot trials each seed was artificially inoculated with + 3 000
D. africanus at planting. Significantly lower D. africanus numbers were present in pods of
PC254K1, PC287K5 and CG7 compared to both reference cultivars. Differential levels of
resistance that was identified in PC254K1 and PC287K5 was determined by inoculating each
seed of both genotypes, Sellie and Kwarts with a range of initial populations (Pi’s), viz. + 0,
50, 250, 1 000, 2 500, 5 000, 10 000 and 20 000 nematodes. In this study PC254K1 and
PC287K5 had reproduction factor (RF) values < 1 at all Pi levels, while Sellie and Kwarts
showed RF > 1 except at the highest Pi. In terms of kernel quality PC254K1 produced choice
grade kernels at Pi < 1 000 and PC287K5 at Pi < 10 000. Possible differences in reproduction
and damage potential of D. africanus isolated from five different localities were evaluated on
PC254K1 and Sellie in a greenhouse and microplot trial, with each seed having been
inoculated at planting with + 2 000 D. africanus. Although no significant differences existed
among the five nematode populations in terms of reproduction and damage potential,
PC254K1 persistently maintained significantly smaller nematode population levels than
Sellie. Preliminary studies to identify molecular marker(s) associated with the resistance trait
were done. Results showed that resistance to D. africanus is probably inherited by a single,
recessive gene.
The seed-borne groundnut pod nematode Ditylenchus africanus is omnipresent in the
groundnut-producing areas of South Africa and remains one of the most economically
important pests of this crop. Resistant genotypes could play an important role to reduce D.
africanus populations below damage threshold levels. Various evaluations of local
germplasm, selected elite breeding lines as well as hybrids have been done previously. In this
selected accessions were evaluated against this pathogen in microplots and resistant host
responses were verified under natural infestations in the fields. The cultivars Sellie and
Kwarts were used as susceptible and tolerant reference cultivars, respectively. In the
microplot trials each seed was artificially inoculated with ± 3 000 D. africanus at planting.
Significantly lower D. africanus numbers were observed in seeds and hulls of PC 254K1,
PC287K5 and CG7 compared to both reference cultivars and the other accessions.
Sustainability of resistance of PC254K1 and PC287K5 was evaluated in microplots. Each
seed was artificially inoculated with initial population densities (Pi) of 0, 50, 250, 1 000, 2
500, 5 000, 10 000 or 20 000 D. africanus at planting. The reproduction factors (RF values)
of PC254K1 and PC287K5 were < 1 at all Pi’s. For reference cultivars Sellie and Kwarts the
RF remained > 1 except at the highest Pi. PC254K1 produced choice grade kernels at Pi of up
to 1 000, PC287K5 up to 10 000 and Kwarts only up to 250 nematodes. A search for
molecular markers associated with the observed resistance of PC254K1 to D. africanus is
currently in progress.
5 th International Congress of Nematology, 2008 32