Sustainable management of root-knot nematodes in crops: Case studies with sweetpotato and ginger

Root-knot nematodes (Meloidogyne spp., RKN) are considered as the most serious nematode pest in the world and have been reported to cause up to 57-67% crop yield losses in untreated conditions in Australia. Chemical control has been the most popular method to manage RKN due to its high efficacy. However, ‘fenamiphos’ the most widely used active ingredient in nematicides, has recently been reviewed by the Australian Pesticides and Veterinary Medicines Authority (APVMA) and subsequently has been recommended for removal from the commercial market in Australia due to concerns about its potential to contribute to adverse environmental and human health impacts. With this pressure of nematicide restrictions, growers in Australia face challenges in controlling RKN in many high-value crops. Therefore, this PhD project focused on identifying alternative strategies to the restricted and currently registered chemical nematicides to control RKN in high-value crops; investigation strategies included use of chemical nematicides, novel organic formulations, organic amendments, biochar, and fungal bio-agents. As the first part of the study, a soil nematode survey was conducted covering the major sweetpotato (Ipomoea batatas (L.) Lam.) growing areas in Queensland (mainly) and New South Wales (Cudgen). Survey results demonstrated that there were ten genera of plant-parasitic nematodes in the survey regions. However, it was identified that RKN is the major pest in the area and that the industry required alternatives to replace chemicals being withdrawn from APVMA registration. The second step was to identify the damage threshold of RKN in sweetpotato, as no study has identified the damage threshold previously. A glasshouse pot trial with sweetpotato (variety Orleans) identified that the damage threshold of RKN could be as low as one RKN/ 200 g dry soil in both ferrosol (loamy) and dermasol (sandy loam) soils. Having this background information, alternative chemical and organic formulations were assessed against RKN in a sweetpotato field trial comparing with untreated control and chemical control ‘oxamyl’ (currently registered chemical for sweetpotato). Fluensulfone significantly reduced soil RKN numbers compared to other treatments, giving the highest marketable yield in this trial. Abamectin did not give significant RKN control or damage reduction. Organic nematicide treatments (garlic formulation and nutrient and microbial formulation) did not show a significant reduction of soil RKN numbers and root damage. No significant effect of nematicides was found on non-target organisms (soil bacteria and nematodes) at 24 weeks after planting. A comprehensive systematic review and meta-analysis were conducted to identify the efficacy of organic amendments against RKN. The meta-analysis identified that organic amendments can reduce RKN numbers and root galling by 63% in general compared to untreated control. A pot trial was conducted with the most effective materials identified in systematic review and meta-analysis as well as with other amendments available to Queensland growers. Amendments used were sugarcane trash, poultry manure, cattle manure based conventional compost, and fermented compost. The experiment found that sugarcane trash applied at 100 t/ ha as the most effective treatment to suppress soil RKN numbers, followed by poultry manure applied at 20 t/ ha and cow manure compost applied at both 20 and 100 t/ ha. Sugarcane trash applied at 100 t/ ha increased undamaged sweetpotato yield; however, there were no significant yield improvements observed for other treatments. The main mechanism of action of the materials probably could be enhanced natural biological suppression of RKN and release of toxic compounds (ammonia from poultry manure). A pot trial was also conducted to evaluate the performance of biochar against RKN in sweetpotato. Biochar (mineralised biochar) was produced from poultry manure and applied at 6.8 t/ ha and 9 t/ ha. Another type of biochar (Nutrient-rich biochar) was produced from mineralised biochar reacting with sweetpotato N, P, and K fertilizer mixture. The low application rate of mineralised biochar significantly reduced soil RKN numbers followed by nutrient-rich biochar. The results demonstrated that there could be a hormesis effect of mineralised biochar to give high suppression of RKN at low application rate. The effect of all biochar treatments on sweetpotato yield was not significant during ten-week experiment. A comprehensive systematic review and meta-analysis were conducted to assess the efficacy of fungal biocontrol agents against RKN in worldwide agricultural crops. This identified that fungal biocontrol generally reduces RKN numbers and root galling numbers by 45% compared to untreated control. Arthrobotrys spp. was observed as an effective genus against RKN based on the present analysis. Arthrobotrys oligospora and A. dactyloides have previously demonstrated biocontrol potential of RKN in sweetpotato; therefore, research was undertaken to produce kaolin-alginate granules incorporated with fungi separately to control RKN. The fungal granules showed 100% RKN trapping activity on agar. A laboratory microcosm system was also developed and used to demonstrate effective RKN trapping by the fungi when granules were applied in dermasol (sandy loam) soil. A pot trial was conducted to find the efficacy of these fungal granules against RKN on ginger (Zingiber officinale Roscoe) in dermasol soil. The treatments were repeated in both heat treated and untreated dermasol soil to assess the effect of natural soil microbiota on the performance of nematode-trapping fungi. Both fungi (incorporated in kaolin alginate granules) were found to be equally effective against RKN at 4 weeks after planting (early growing stages of ginger) similar to the performance of chemical control; however, the performance of fungal granules against RKN was not significant at five months after planting. In addition, the trial confirmed that there was no influence from natural soil microbiota on the performance of fungi during the five-month experiment.