Thesis (M.Agric. (Plant Production)) -- University of Limpopo, 2018 / Meloidogyne incognita race 2 is internationally recognised as one of the most aggressive Meloidogyne species and it is also widely distributed in Limpopo Province, where it occurs alone or as mixed populations with other Meloidogyne species. Traditionally, Meloidogyne species had been managed using synthetic chemical nematicides, most of these products had been withdrawn from agro-chemical markets due to their environment-unfriendliness. Following the withdrawal of synthetic chemical nematicides, nematode resistance had been the most preferred strategy for managing high nematode population densities. The availability of nematode resistant genotypes in sweet potato (Ipomoea batatas) would enhance the use of resistance in managing Meloidogyne species and races in Limpopo Province. Generally, should post-infectional nematode resistance be available in the test sweet potato cultivars, the information would be relayed to plant breeders for use as source of introgression in various commercial cultivars where nematode-resistant genotypes do not exist. The objectives of the study, were to determine: (1) Host-status and host-sensitivity in sweet potato cv. ʹBopheloʹ, ʹBosbokʹ and ʹMvuvheloʹ to M. incognita race 2. (2) the existing nematode resistance mechanism in any of the test cultivars that had resistance to M. incognita race 2. For achieving Objective 1, eight treatments namely, 0, 25, 50, 125, 250, 625, 1250 and 3125 eggs and second stage-juveniles (J2) M. incognita race 2 were used under greenhouse trials for each cultivar. To achieve Objective 2, sweet potato plants were inoculated with 100 J2 with four plants harvested every other day for 30 days counting to 15 harvesting times. At 56 days after inoculation, cv. ʹBopheloʹ had reproductive factor (RF) values above unity for M. incognita race 2 and plant growth variables were reduced. Therefore, the cultivar was a susceptible host to M. incognita race 2 and mechanism trial was not conducted for this cultivar. Meloidogyne incognita race 2 failed to reproduce on cultivars ʹBosbokʹ and ʹMvuvheloʹ whereas nematode infection did not affect plant growth and therefore, the two cultivars were resistant to M. incognita race 2. Mechanisms of resistance to M. incognita race 2 on cultivars ʹBosbokʹ and ʹMvuvheloʹ demonstrated significance existence of (1) necrotic spots, (2) poorly developed giant cells, (3) formation of rootlet interferences (4) absence of root galls and (5) non-detectable J2 in roots. All these features suggested the existence of post-infectional nematode resistance in the two cultivars to M. incognita race 2. In conclusion, cultivar ʹBopheloʹ was susceptible to M. incognita race 2, whereas cultivars ʹBosbokʹ and ʹMvuvheloʹ were resistant to M. incognita race 2, with the evidence of post-infectional nematode resistance to the nematode species

Thesis (Ph.D. Agriculture (Plant Production)) -- University of Limpopo, 2020 / Worldwide, both root-knot (Meloidogyne species) and sugarcane aphid (Melanaphis sacchari Zehntner), are economic pests on sugarcane and sorghum crops. In most cases, each of the two pests is managed using host plant resistance due to the economic benefits derived from this management strategy. The highly nematode resistant sweet stem sorghum (Sorghum bicolor L.) cv. 'Ndendane-X1' used in ethanol production, is highly sensitive to sugarcane aphid, with some indication that the latter could interfere with nematode resistance in the sorghum cultivar. This study had four objectives which collectively intended to investigate the interactive effects of infection by three Meloidogyne species and infestation by aphid under different conditions on resistance to nematode in a nematode-resistant sorghum cultivar. The research objectives were achieved through four trials. In each trial a 2 × 2 factorial experiment, each with and without nematode and aphid as first and second factors, respectively, were conducted. Treatments were arranged in a randomised complete block design, with six replications, and each experiment validated in time. At 150 days, after emergence, the nematode × aphid interaction significantly reduced sucrose by 17, 74 and 42% in Meloidogyne enterolobii, Meloidogyne incognita and Meloidogyne javanica trials, respectively. Aphid infestation of sorghum significantly increased the reproductive potentials of the three respective Meloidogyne species by 196, 320 and 152%, but significantly, reduced plant growth variables from 20-44 and 48-51% in two respective trials. The mineral nutrients S and Zn were reduced in leaf tissues of the test cultivar in Trial 1, whereas Ca and Zn were respectively reduced by 24 and 51% in Trial 2 and by 52 and 51% in Trial 3. Since the reproductive potential values for Meloidogynqe species on the test sorghum cultivar were greater than unity and nematode infection reduced the plant variables, cv. 'Ndendane-X1' lost resistance to xx the test Meloidogyne species. In achieving Objective 2, procedures were similar to those in Objective 1 except that the study was conducted under field conditions under mixed nematode populations of M. enterolobii, M. incognita and M. javanica. Sorghum seedlings were raised at 0.3 m × 0.3 m inter and intra row spacings. Soon after emergence, plants were thinned to one per station, randomly selected for nematode and nematode-aphid treatments. Mixed populations of Meloidogyne species (M. enterolobii, M. incognita and M. javanica) at approximately 1:1:1 (v/v) ratio were applied at 300 eggs + J2 per plants after thinning at the five plants which were used as nematode alone treatments. The latter were also infested with 20 sugarcane aphids to constitute a nematode + aphid treatments. Buffer zone plants separating the treatments were monitored for aphids and stock borer, which were sprayed when necessary. At 150 days after infestation, relative to nematode alone, nematode-aphid significantly reduced degrees Brix from 13% to 61%, but significantly increased the reproductive potential of mixed Meloidogyne species and root galls by 279 and 199%, respectively. Also, the combined effect significantly reduced plant growth variables from 35 to 55% and the mineral nutrient elements in leaf tissues of the cultivar from 33 to 73%. At 150 days after the treatment, the second and first order interaction (Nemarioc-AL × Nemafric-BL × Mordica and Nemafric-BL × Mordica) had significantly increased sucrose content from 48 to 66%, increased plant growth variables from 49 to 163%, increased accumulation of certain nutrient elements from 164 to 206%. The terpenoid-containing phytonematicides could have potential future application in the husbandry of ethanol-producing sweet stem sorghum cultivars in relation to increasing sucrose above the 16% minimum for premium delivery fees and increased plant growth. Under field conditions, in pest-free condition (Objective 3), drenched terpenoid-containing phytonematicides significantly increased sucrose content at xxi middle and bottom part of SSS cv. 'Ndendane-X1' by 66 and 48%. However, these products did not significantly increase plant variables, except tiller number, which was increased by 163 under first order interaction from Nemafric-BL and Mordica phytonematicides. Similarly, nutrient elements variables had generally not been increased by the interaction of these products, except Ca and K, which were increased by 206 and 164%. In achieving Objective 4, a 2 × 2 × 2, with the first, second and third factor being Nemarioc-AL (with and without), Nemafric-BL (with and without) and Mordica (with and without) phytonematicides, respectively. on sorghum cultivar infected with a mixture of Meloidogyne species and infested with aphids, under microplot conditions, untreated control sucrose content remained below the standard of 16 degrees Brix, whereas the second order interaction increased the variable far above the standard, along with various plant growth variables also increased. However, both nematode and aphid population densities were significantly reduced by the interactions. Findings in this thesis constituted the first report where aphid infestation broke resistance to Meloidogyne species in sweet stem sorghum cv. 'Ndendane-X1'. Therefore, the successful use of nematode resistance in the cultivar in areas with high nematode population densities would depend upon the effective management of the sugarcane aphid population densities. Also, the three terpenoid-containing phytonematicides would when combined or used alone have the potential future in the husbandry of sweet stem sorghum cultivars intended for ethanol production and suppression of nematode population densities / National Research Foundation (NRF)