No abstract

Groundnut is the common name for several leguminous plant species producing seeds that mature underground, including Bambara groundnut (Vigna subterranea), Hausa groundnut (Macrotyloma geocarpum) and peanut (Arachis spp.). This chapter describes the nematode parasites of groundnut, including Meloidogyne spp., Pratylenchus brachyurus, Belonolaimus longicaudatus, Aphelenchoides arachidis, Tylenchorhynchus brevilineatus [Bitylenchus brevilineatus] and Ditylenchus africanus. Information is given on their infection and histopathology, symptoms of damage, biological races, survival and means of dissemination, environmental factors affecting parasitism, disease complexes, economic importance and population damage threshold levels, and management measures, including host resistance, cultural, chemical, biological control methods.

The oxygen consumption rates of different life stages of the endoparasitic nematode, Pratylenchus zeae (Nematoda: Tylenchida) during non- and post-anhydrobiosisPratylenchus zeae, widely distributed in tropical and subtropical regions, is an endoparasite in roots of maize and other crop plants. The nematode is attracted to plant roots by CO2 and root exudates and feeds primarily on cells of the root cortex, making channels and openings where the eggs are deposited, with the result that secondary infection occurs due to bacteria and fungi. Nothing is known about the respiration physiology of this nematode and how it manages to survive during dry seasons. To measure the oxygen consumption rate (VO2 ) of individual P. zeae (less than half a millimeter long), a special measuring technique namely Cartesian diver micro-respirometry was applied. The Cartesian divers were machined from Perspex, and proved to be more accurate to measure VO2 compared with heavier glass divers used in similar experiments on free living nematodes. An accuracy of better than one nanoliter of oxygen consumed per hour was achieved with a single P. zeae inside the diver. Cartesian diver micro-respirometry measurements are based in principle on the manometric changes that occur in a fl otation tube in a manometer set-up when oxygen is consumed by P. zeae and CO2 from the animal is chemically absorbed. VO2 was measured for eggs (length: < 0.05 mm), larvae (length: 0.36 mm) and adults (length: 0.47 mm) before induction to anhydrobiosis. P. zeae from infected maize roots were extracted and exposed aseptically to in vitro maize root cultures in a grow cabinet at 50 % to 60% relative humidity at 28 ºC using eggs, larvae and adults. VO2 was also measured for post-anhydrobiotic eggs, larvae and adults by taking 50 individuals, eggs and larvae from the culture and placing them in Petri-dishes with 1% agar/water to dry out for 11 days at 28 ºC and 50% relative humidity. The VO2 was measured after the anhydrobiotic eggs, larvae and adults were re-hydrated for 12 hours in a high humidity atmosphere. The average VO2 value found for ten consecutive measurements during a 50 minute period of one adult using the diver technique was 32.8 nanoliter per hour. The differences between the ten VO2 values were less than 3.5 %, an indication of the accuracy of the diver measurements. The average VO2 values from ten measurements per life stage, expressed in nanolitres per hour per life stage of the pre-anhydro-biotes (eggs: 7.96; larva: 6.13; adult: 26.04) were compared with those of post-anhydrobiotes 12 hours after anhydrobiosis. The average VO2 values of the post-anhydrobiotes for the three life stages (egg: 19.34; larva: 14.17; adult: 32.86) were statistically signifi cantly higher in comparison with the pre-anhydrobiotes. The reasons for the difference are that high concentrations of metabolites, probably in the form of trehalose, accumulate during the anhydrobiosis stage to be utilized during the post-anhydrobiotic revival period. The oxygen consumption rate was also expressed in nanolitres per hour per microgram adult nematode after applying the following equation taken from the literature: M = a2 x b/16 x 1000 where M = mass (µg) of adult nematode; a = largest body width (µm); b = body length (µm). Using this equation it was found that one gram P. zeae uses 503 times more oxygen compared with one gram mammal the size of a cow. This high specifi c oxygen consumption rate (MO2 ) is a direct indication of the large metabolic damage this endoparasitic nematode can have on the metabolic substrates provided by the roots of the various plant crops it parasitize.

Ditylenchus africanus affects peanut quality, which leads to downgrading of consignments and economic losses for producers. This nematode is difficult to control and host-plant resistance may be the most effective way to control it. Recently, the peanut breeding line PC254K1 has been identified as resistant to a D. africanus population from Vaalharts and will be included into the peanut breeding program. The objectives of our study were to compare the reproduction potential of D. africanus geographic populations from five different areas in the peanut production area of South Africa and to assess whether PC254K1 is resistant to all five D. africanus populations. Reproduction of the D. africanus populations was evaluated on peanut callus in growth cabinets at 218C, 288C, and 358C. The peanut cv. Sellie was included in the study as the D. africanus-susceptible reference genotype in the greenhouse and microplots. Reproduction potential of all five of the D. africanus populations was similar. Resistance of PC254K1 was confirmed to all five D. africanus populations. The resistance trait of a D. africanus-resistant cultivar developed from PC254K1 should, therefore, be sustainable over the five localities tested during this study.

Ditylenchus africanus causes cellular breakdown in pod tissue of susceptible groundnut cultivars. The histopathology of this nematode on a resistant genotype was studied using light microscopy and compared with the histopathology of D. africanus on a susceptible genotype. Plants of breeding line PC254K1 and cv. Sellie were propagated in a glasshouse, inoculated with D. africanus at inoculum levels of 2000, 5000 and 7000 nematodes per plant and the pods were collected at 90, 120 and 150 days after planting. In contrast to the susceptible genotype, only a small number of nematodes were observed in restricted areas of the pod tissue of the resistant genotype. Furthermore, the resistant genotype showed neither external symptoms nor cellular breakdown in reaction to D. africanus. According to results of this study, the mechanism of resistance involved may be the inhibition of proper development, migration and reproduction of this nematode, thus preventing it building up to damaging population levels.