Improved farming systems for managing soil-borne pathogens of ginger in Fiji and Australia

Disease and pest surveys were carried out in Fiji during May and September 2006 with a focus on soil-borne pathogens of ginger. Farm inspections during May revealed crop damage caused by Pythium myriotylum and a suspected outbreak of Rhizoctonia in ginger growing on old pasture land in the Navua district. Ginger rhizomes were also collected during September from the intake area of Fiji's major ginger processor and followed up with on-farm inspections. Factory rejects had significant levels of damage caused by nematodes (Meloidogyne and Radopholus spp.) and unspecified rots. Farm inspections revealed significant levels of Fusarium and Erwinia rots present on rhizomes that were rejected on-farm for planting or sale during September. Samples were collected for culture, isolation and identification. A further survey is planned for February 2007 to coincide with early harvest ginger.

Quality of planting material is a significant problem for both the Fijian and Australian ginger industries. Tissue cultured plants are being multiplied and several farms in Australia have been established as 'clean seed' producers. Their planting material is in demand. Fiji has also experimented with 'clean seed' schemes in the past and is keen to reintroduce such a program again. The project is supporting these industry led initiatives by provision of tissue cultured stock material.

Three field trials (2 on growers' farms and one at Koronivia Research Station) were established in Fiji during the planting season in September. The effect of poultry manure applied at 4 rates was examined to determine its effect on the incidence of Pythium myriotylum and its effect on crop losses as a result of this disease. Preliminary results have indicated that at the highest rate used (1.7 t/ha) there was an increased incidence of disease following a period of above average rainfall in early December 2006, however the symptoms have abated at the time of the reporting period. The next two trials were established to investigate the effect of drainage and soil water-logging on disease incidence. All trials will be harvested in September 2007 and will measure yield and rhizome quality as affected by soil-borne pathogens.

In Australia, 3 field trials have been established on growers' properties. The first trial was planted in December 2005 with a range of 6 cover crops before being planted to ginger in September 2006. Beds were minimum tilled and one bed was cultivated as per normal farm practice. An experimental double-disc planter developed for minimum tillage in the sugar industry was modified for planting ginger. The ginger crop established well in the minimum tilled beds and will be harvested in March 2007. A major field trial at Kandanga covers approximately a quarter of a hectare and will examine the impact of minimum tillage and rotational cropping (conservation tillage) on farm profitability and reduction of losses caused by soil-borne pathogens. The land was prepared in May 2006 and has been cover cropped with brassica during winter and forage sorghum during summer. The block will be planted to ginger in September 2007 with a modified planter. A third trial was established to compare the impact of tillage and cover cropping on the suppression of Fusarium and root-knot nematode (Meloidogyne incognita). The treatment that has been bare fallowed for 12 months has shown a significant decline in microbial activity and consequently has lost much of its suppressiveness to these two pathogens. Those beds that have been continuously cropped, and particularly those that have been amended with organic matter, were suppressive to root-knot nematode.

Planning meetings have been held during the two project visits to Fiji in 2006 and work plans agreed to and implemented. Visits and meetings with ginger processors and exporters were also conducted to obtain their views and elicit their support for the project. Frespac allowed the project leader to examine factory records and determine the impact of soil-borne pathogens on factory rejects of ginger taken for processing during the 2005 and 2006 season. The project team were then able to follow through with some on-farm surveys. Finally an Australian ginger growers' field day is planned at two of the field trial sites in January 2007.

After conducting disease and pest surveys in Fiji during the first phase of the project, studies are currently concentrating on the two most serious soil-borne pathogens of ginger in Fiji, Pythium and Radopholus. The 2006-07 and 2007-08 ginger growing seasons were characterised by fluctuating weather conditions where heavy rainfall events were followed by dry conditions. For instance on December 18 2006 Suva established its highest one-day rainfall record (272 mm) but was followed by its driest January on record. Again, above average rainfall was recorded during January 2008, but was followed by two drier than average months with March 2008 being the driest on record. Under these conditions Pythium on ginger does not reach epidemic proportions and our surveys conducted over the reporting period found low levels of infection on ginger harvested for the immature market. The low incidence of Pythium also confounded field experiments established during this period.

However the summer of 2007-08 in the ginger growing area of Australia was one of the coolest and wettest for many years; similar La Nia events were recorded for the 1974-75 and 1975-76 summers. Crops were in excellent condition in early December but higher than normal rainfall and long periods of cool, overcast conditions over the next two months left the soils saturated and Pythium spread very rapidly on some farms. Fields were totally destroyed or severely affected and Australia's immature ginger harvest was reduced by 15%.

Pythium has been isolated from ginger in both Fiji and Australia and found to be highly pathogenic in glasshouse tests. The Australian isolate has been identified as P. myriotylum and the Fijian isolates are currently held in quarantine at the University of Queensland and will be identified later this year. In laboratory tests conducted at Koronivia Research Station both high temperature (capable of growth up to 38oC) and low temperature (grew at 25oC, but not at 35oC) isolates were recovered and of the high temperature isolates, KRS14 damaged plants more quickly and more severely than KRS17. High temperature isolates of Pythium are likely to be P. myriotylum and/or P. aphanidermatum. Glasshouse experiments showed that neither Radopholus nor Meloidygyne need to be present to cause damage to ginger roots or rhizome before Pythium infection occurred.

Further studies with Radopholus in Fiji have demonstrated that nematode damage is minimal in the immature ginger crop but can be devastating when ginger is being carried over for seed. On one farm in the Veikoba district, seed ginger losses of 70% were recorded in one field. The numbers of nematodes on rhizomes varied considerably but were extremely high in some cases (5,200 Radopholus extracted from a 25 g piece of rhizome). Furthermore Radopholus populations are being maintained during crop rotations on volunteer ginger and also on common weeds such as crow's foot (Eluesine indica). Results from field and glasshouse studies have shown that when nematode-infested seed is planted, Radopholus can multiply to relatively high population densities within a few months. Likewise nematode-free seed when planted in infested soils is readily susceptible to infection.

Growers and exporters recognise that lack of good quality seed for planting material is a major problem with the spread of soil-borne pathogens. A recent industry survey has also shown that hot-water treatment equipment provided by the extension service is seldom used and even when hot-water treatment of seed is practiced, it is being done incorrectly and at temperatures that are insufficient to kill nematodes.

Strategies to control soil-borne pathogens of ginger are under investigation. We believe central to control of nematodes, Pythium and Fusarium is increasing soil microbial activity and diversity, while improving soil nutrition, to create conditions that suppress pests and diseases of ginger. Preliminary results from Australian field and glasshouse experiments have shown that carbon inputs from plants and amendments improve the biological status of soils while excessive tillage and fallowing have a negative impact. Suppressiveness to root-knot nematode, and to a lesser extent Fusarium, was enhanced by amending soil with poultry manure/sawdust, and by reducing tillage. Research into control strategies continues.

Finally two project staff from Fiji have undertaken training activities in Australia. The project's pathologist has undertaken studies for a Master of Plant Protection from the University of Queensland with support from a John Allwright Fellowship during 2007-08. The project's nematologist completed a training program in practical nematology with Biological Crop Protection in 2007 and plans to undertake further studies. Results have been communicated at grower field days and will be presented at two international scientific symposia later in the year.