Seminar – Incidence and pathogenicity of plant-parasitic nematodes on coffee in Australia and the potential of organic amendments as a management tactic
Khoa Le, PhD Candidate, SOLES, University of Sydney
Coffee is the world’s most traded commodity and a source of employment for millions of people, but one of the many challenges facing those who grow coffee is to minimise the yield losses caused by plant-parasitic nematodes. Numerous nematode pests are associated with coffee worldwide but members of three genera (Pratylenchus, Meloidogyne and Radopholus) have a global distribution and are estimated to reduce yields by 10 -20 %. However, minimising these losses is a difficult task due to the wide range of nematode species that are pathogenic to coffee; the longevity of coffee plants; the de-registration of effective but environmentally harmful soil fumigants such as ethylene dibromide and methyl bromide; and the environmental and human health risks associated with the use of non-volatile nematicides.
The research described in my thesis aimed to identify the plant-parasitic nematodes associated with coffee in Australia and investigate the pathogenicity of nematodes that were either present in coffee plantations or associated with crops growing nearby. However, the major focus was to determine whether organic amendments were a useful management tool, as they are known to enhance natural mechanisms of biological control in other crops. The target pest was Pratylenchus coffeae, one of the most damaging nematodes worldwide, and the specific aim was to determine whether organic amendments improved the growth of coffee and reduced populations of this important pest. The host specificity of Pasteuria thornei was also investigated, as this bacterium has the capacity to regulate populations of the many Pratylenchus species known to attack coffee.
Six nematode genera (namely Pratylenchus, Meloidogyne, Rotylenchulus, Helicotylenchus, Paratylenchus and Xiphinema) were found to be associated with coffee in the tropics of Queensland and the subtropics of New South Wales. Meloidogyne hapla, Pratylenchus brachyurus and Rotylenchulus reniformis were identified using molecular methods and found to be the main species parasitic on coffee in Australia. Although Pratylenchus coffeae and Radopholus similis were not found on coffee, they were isolated from banana roots in north Queensland and caused severe damage to seedlings of Coffea arabica cv. K7 (the main coffee cultivar in Australia) in a pathogenicity test in the greenhouse. Moreover, the findings of this trial indicated that coffee did not host Pratylenchus zeae, a species that is widespread on sugarcane in Queensland and New South Wales. Thus, Australian isolates of M. hapla, P. brachyurus, R. reniformis, P. coffeae, and R. similis can now be added to a recently compiled global list of nematodes that cause damage to coffee, while P. zeae can be removed from that list.
Amending soil with organic materials, including sugarcane trash, cow manure, lucerne hay and chicken manure, at a rate equivalent to 20 t/ha (i.e. 10 g organic amendment/kg soil) produced positive changes in soil chemical and biological properties and stimulated the growth of coffee plants for up to 12 months. Evidence was obtained to indicate that amending soil with these materials increased numbers of nematode-trapping fungi (e.g. Arthrobotrys musiformis and Arthrobotrys oligospora) and nematode predators (mainly Mononchus papillatus species), and this in turn enhanced natural mechanisms of suppression and reduced the population of P. coffeae. Sugarcane trash reduced the population of P. coffeae by 91% and was more effective than lucerne hay (84%), cow manure (82%) and chicken manure (74%) amendments. Laboratory tests showed A. musiformis is an obligate parasite of nematodes whereas A. oligospora seems to be naturally saprophytic in soil. Also, A. musiformis trapped Pratylenchus coffeae much more efficiently than A. oligospora.
With regard to the work on Pasteuria thornei, an in-vitro test showed that this nematophagous bacterium was only parasitic to Pratylenchus zeae (its original host) and not to other root lesion nematodes (e.g. P. coffeae and P. brachyurus). Thus, future work on this host-specific parasite will have to focus on finding isolates capable of parasitising the Pratylenchus species that are economically important in each coffee-producing country.