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Rural Industries Research & Development Corporation
Methodology and viability of re-establishing commercial Boronia megastigma plantations
by Doris Blaesing & Lee Peterson, Serve-Ag Pty Ltd April 2002
RIRDC Publication No 02/007 RIRDC Project No: SAG-1A
The boronia industry became concerned about the viability of replanting their current plantations following unsuccessful efforts to re-establish plants in small areas. There was concern that boronia may be prone to ‘specific replant problems’ (soil sickness) as known from orchards and nurseries.
Development and management of soil sickness are not yet fully understood.
The research on the methodology and viability of re-establishing commercial Boronia megastigma plantations concentrated on the following areas: 1. Confirmation of specific replant disease in a pot trial (soil sickness test).
2. Isolation of soil-borne fungi from current boronia plantations.
3. In-vitro screening of soil pathogenic fungi suppressive soil amendments.
4. Replant field trial, testing soil amendments, effects of fire, nutrients simulating an ‘ash bed effect’, and fungicides known to be active against Phytophthora spp.
5. Survey of Tasmanian boronia plantations and growers to investigate site factors and plant establishment, management and longevity.
The soil sickness test was conducted using soil taken from the replant field site. The test showed that the soil did not suffer from specific replant problems. The grower had reported that all boronia plants on the trial site had been removed due to the high plant mortality in that part of the plantation.
The soil was also tested for the presence of Phytophthora. Soil samples from other plantations in the area were tested at the same time. Tests showed that either Phytophthora or Pythium spp. were present in soil from most sites.
Following these findings, the replant field trial was designed to include organic and fungicide treatments that might control or suppress soil-borne diseases, including mulches. Further treatments were selected to simulate conditions that encourage revegetation of boronia in its natural habitat.
Revegetation was reported to be best after scrub fires (J. Plummer and D. Ward, personal communication). The assumption was that a scrub fire would smoke-sterilise the top centimetres of soil without heating it, and provide nutrients, especially calcium (CaO), to the plants. Therefore, further treatments included ‘cool’ fire by burning dry boronia plants, and a fertiliser mix that was designed to provide a nutrient combination similar to ash, without adding chloride or carbonate.
Treatments were compared to the commercial standard nutrition and an untreated control. All plants were replanted into previously used rows, apart from one that looked at the effect of inter-row planting. Apart from the untreated control, all treatments that did not include nutrients were fertilised as per the commercial standard.
Severe plant losses (<50%) occurred in some trial treatments during the first year. They were highest in plots with the commercial standard nutrition, the untreated control, inter-row planting, and in plots treated with straw mulch and Ridomil granules. Results suggested that nutrition programs should be re-evaluated considering that treatments like fire, a worm casting product and the ‘ash bed fertiliser mix’ produced better results than the commercial standard. Best results in survival, growth and vigour were achieved using pine bark mulch. Even though the cost of mulching may seem prohibitive to most growers, it appears worthwhile to carefully consider possible financial benefits of mulching on plant survival and vigour, over the life of the planting on a per hectare basis (e.g.
protection of young plants from injury through wind, yield increase, return on management inputs, longevity, etc).
Older boronia plantations had experienced decline problems that often started during establishment.
Growers were able to explain some plant losses. In a lot of cases, however, single boronia or groups of plants had died suddenly for no obvious reason. A field survey was conducted to identify the causes of plant losses to assure that replanted boronia would not suffer from the same problems. If problems were site or management related they could be addressed prior to or after replanting.
The survey results showed that previous site use had an effect on plant vigour. Plantations thrived on land that was not used for grazing or plant production prior to planting boronia. Most sites had experienced plant losses in water-logged areas. Even though boronia grows on wet heath land sites in its natural environment, the soil there is not water-logged for extended periods, and always maintains a certain proportion of large air filled pore space, supplying roots with oxygen (D, Ward, personal communication).
A number of plants were lost due to the use of pot bound or poorly rooted transplants. The transplants developed weak, sometimes self-strangling root systems. They were prone to wind and harvester damage (poor anchorage), as well as water and nutrient deficiency.
A relatively large number of boronia in each plantation showed deformations and swellings of the crown (stem area just above and below soil surface). The symptoms often resembled those of the bacterial crown gall disease, sometimes suggested nematode damage, but could also be a reaction to strong wind in some cases. The crown of dying plants often carried fungal pathogens that could have entered through roots or wounds in the crown area caused by machinery or wind. The origin and causal organisms of crown diseases could not be further investigated within the scope of this study.
Soil from one plantation was tested for plant parasitic nematodes. Two species, Rotylenchus sp. and Hemicycliophora sp. were found at relatively high numbers.
Based on the results of the trials and surveys, 'Replant Problems of Boronia' were defined as follows: "All factors that influence the successful re-establishment of boronia plantations on sites previously used for boronia production." The following recommendations are made to assist with the successful re-establishment of boronia plantations that will not suffer from premature decline: . Boronia plantations in Tasmania do not suffer from classical ‘soil sickness’ problems. This means that, with adequate management, sites can be replanted.
. Replanted sites may experience establishment, growth or early decline problems due to one or a combination of the following factors: 1. Transplant quality (e.g. pot bound plants), 2. Mechanical damage to young plants or plants with poor root systems by wind and harvesters.
3. Soil heath/quality (decline in soil structure or organic matter content, soil borne diseases, e.g.
Phytophthora, Pythium or nematodes), 4. Nutrition (nutrient balance, e.g. N / Ca, K & Mg, fertiliser types, amounts & timing), 5. Pests and diseases of shoots, foliage, crown and roots.
. Several soil treatments and nutritional inputs can improve plant establishment and may improve longevity on replanted sites. These are: 1. Pine bark mulch with no added nitrogen during composting.
2. Controlled burning of old plantation about four weeks prior to replanting creating a ‘cool fire’.
3. Use of a fertiliser mix that is high in cations but does not contain chloride (e.g. sulfate forms of Ca, K, Mg).
4. Use of ammonium nitrogen rather than nitrate nitrogen.
It is considered important that growers clearly identify the cause for decline in their current plantation to avoid the problem repeating itself when replanting. Possible causes for early decline:
. Wind stress, causing injury to the crown area when plants swirl and rub against soil or are blasted by sand. The resulting injuries are entry sites for pathogens.
. Pot bound roots due to planting later then the ‘ready date’ given to the nursery.
. Drip irrigation not providing sufficient lateral water distribution in sandy soils to establish plants with a healthy root system.
. Water-logging leading to a lack of oxygen in the root zone and root rots.
. Phytophthora, Pythium or other soil-borne fungal diseases attacking weak, stressed plants. The importance of Phytophthora and Pythium as pathogens of cultivated boronia requires further investigation.
. Crown deformation possibly caused by wind or harvester damage and subsequent pathogen invasion. The crown gall pathogen Agrobacterium tumefaciens should also be investigated as a possible cause of crown deformations in plantations.
. Imbalanced fertiliser programs. Cation (Ca, K, Mg NH4) and trace element nutrition need to be better understood and monitored in relation to plant health.
. Nematodes which may play a role, and need further investigation.
Fungicides that may control Pythium and/or Phytophthora and other agricultural chemicals must not be used without confirming their efficacy and lack of phytotoxicity and residues in flowers.
Appropriate trials would have to be conducted under GLP (Good Laboratory Practice) protocols to obtain use permits from the National Registration Authority (NRA) for the boronia industry.
The industry may benefit from producing a comprehensive boronia management guide and/or introducing a quality management system using HCCP (hazard audit, critical control point) principles.
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