Soilborne fungal pathogens cause serious crop losses in both tropical and temperate regions, with each climatic zone tending to favour a different suite of species. The fungi can build up in the soil slowly and insidiously over many years. Diagnosing soilborne pathogens, identifying them to species level, and testing for pathogenicity is generally much harder than for fungi causing leaf infections. Soilborne fungal infection may cause very general symptoms to the parts of the plant above ground - such as reduced yield, wilting or leaf fall - which may not be obvious to inexperienced observers as signs of infection.
The picture is further complicated by the fact that many of these observable symptoms may be caused by other plant stresses - mineral deficiencies or toxicities, climatic stress or nematodes - which may be operating at the same time. Moreover, such stresses may in themselves exacerbate the fungal infection or predispose to it. Environmental factors may also affect the pathogen. In northern Sulawesi, cloves, vanilla, sago and maize, all important domestic or export crops, suffer what appear to be soilborne fungal diseases, severely reducing their productivity.
The project involved Australian experts who demonstrated conventional and molecular technologies to diagnose a range of soilborne fungal diseases of important crops and determine possible controls, while working with local scientists to train and prepare them to continue the work.
This project focused on the management of two diseases, vanilla stem and root rot, and clove decline. Vanilla stem and root rot is caused by Fusarium oxysporum f.sp. vanillae (Fov), a soilborne vascular pathogen, and to a lesser extent Colletotrichum gleoesporioides, borne on rain splash and by insects. The causal agent of clove decline is Ceratocystis polychroma, associated with the clove trunk borer Hexamitodera semivelutina.
The first project component involved work to isolate and identify pathogens in the key target crops, as well as surveying minor crops. In the lab, the scientists assessed the pathogenicity of the cultures. Agronomic information and data on the incidence and severity of each disease were collated and analysed to provide a picture of its epidemiology and an overall assessment of its economic importance. Using information on the pathogens the scientists then worked in close consultation with growers and farmers to develop management strategies for minimising losses.
The second component focused on training, both through formal attendance for study at the University of Sydney and, in Sulawesi, in the form of collaboration between Australian and Indonesian scientists to enhance postgraduate research training and teaching at Sam Ratulangi University. A diagnostic laboratory was established there.
Disease management strategies for vanilla stem and root rot In light of earlier evidence that Fov inoculum potential was very low in natural farm soil, an initial experiment determined the appropriate concentration of inoculum for use in control experiments. Subsequent greenhouse experiments established that the incorporation of cinnamon powder or shredded clove leaves into the soil effectively lowered Fov inoculum in the soil, thus reducing disease incidence. Clove leaves contain eugenol while cinnamon powder contains cinnamaldehyde, both of which have antimicrobial properties. Clove leaves, however, are more economical and more readily available throughout North Sulawesi. Experiments also showed that fungicides Thiaram and Mancozeb did not effectively reduce disease incidence. Two of four vanilla clones obtained from the Indonesian Institute for Spice and Medicinal Crops, Bogor, exhibited promising levels of resistance. Management strategies involving farm sanitation (removal of all infected plant tissue, removal of weeds and planting of disease-free material), soil amendment (incorporation of shredded clove leaves) and fertiliser application (chicken and cow manure) were established in farm trials that eventually became demonstration farms for vanilla growers.
Epidemiology of clove decline Experimental results revealed that the clove decline pathogen, C. polychroma, was not soilborne but was occasionally detected in soil due to the presence of infected host debris or spore-carrying insects. The clove trunk borer H. semivelutina was associated with the fungal pathogen by providing a favourable micro-environment in borer galleries for the proliferation of the pathogen. Masses of C. polychroma spores produced on fruiting structures were observed within borer galleries. The association between the fungal pathogen and the trunk borer was not found to be an intimate one. While borer larvae were not shown to be disease vectors, other invertebrates inhabiting the borer galleries (nematodes, mites as well as a myriad of other insects) were observed to carry spores of the pathogen. Insects trapped outside the trunk galleries on clove trees, in particular Nitudulids, were shown to carry propagules of the pathogen which were infective on cloves. A comprehensive inventory of insects associated with clove farms and trunk borer galleries was established over two seasons (rainy and dry), gathered from insect nets, traps and sweeping Malaise traps, and also hand-picked from borer gallery samples. The wet season inventory comprised seven orders and 38 families, while that of the dry season comprised seven orders and 45 families.
Disease management strategies of clove decline A series of control experiments tested the efficacy of injecting insecticides into clove tree trunks to eliminate trunk borers and other invertebrates associated with borer galleries. Liquid injections of Carbosulfan, Dimethoate, Emamectin and Thiamethoxam were all ineffective. Solid naphthalene, however, as a slow-release volatile insect fumigant used as plugs for borer gallery holes along tree trunks was highly effective in controlling trunk borers and other associated invertebrates. The naphthalene plugs also served as physical barriers preventing invertebrate passage in or out of the gallery holes, thus keeping the spread of the fungal pathogen in check. In addition to its effectiveness, naphthalene has the advantage over other insecticides in terms of lower cost, easy availability, low toxicity and ease of application. A demonstration farm using these naphthalene plugs was set up and monitored for insect control. Management strategies also include the removal and burning of severely infected or dead branches to control disease spread.
Farmer extension Three brochures on vanilla and clove diseases and disease management information were produced and distributed to farmers. Four farmer extension workshops (two on vanilla diseases and two on clove decline) took place in August 2007 at Sam Ratulangi University. Fifty-four vanilla farmers from 16 villages attended the workshops. Ongoing discussion and extension now takes place between farmers and staff from the AIAT, North Sulawesi and Sam Ratulangi University.
Links:
[1] http://www.aciar.gov.au/country/Indonesia
[2] http://www.aciar.gov.au/programarea/Crop Protection