Trial shows good growth from old apple ground treatmentsResearch & Extension
In 2017 a Future Orchards® trial was set up to assess options for managing apple replant disease (ARD) by observing the growth rate of a new planting of Fiero® Fuji apples on MM106 rootstock into old apple ground under five different pre-plant treatments.
Two biofumigation mixes, beneficial bacteria and chloropicrin were trialled alongside the standard practice of Metham Sodium fumigation at Trevor and Carmel Fontanini’s orchard at Manjimup in South West WA. This trial has been monitored every autumn since planting and is now in the 3rd leaf.
Two treatments stand out at the end of the third season and trees planted with these treatments have performed exceptionally well. They are:
- Treating the soil with the chemical fumigant chloropicrin. Although a well-known ARD treatment elsewhere, chloropicrin treatment has not been a standard practice in the South West of WA due to the expense of hiring certified contractors from outside the area to apply the fumigant, and
- The relatively new approach of application of beneficial bacteria onto the root system during planting to improve soil health.
Old apple ground can harbour apple replant disease which reduces new tree growth and production by around 20 per cent per annum for each year of the tree’s life. The demonstration site tested:
- biofumigant seed mixes of mustard and rocket (David Grays’ Green Fume), and
- mustard and Ethiopian cabbage (PGG Wrightson Seeds BQ Mulch);
- a beneficial bacteria product (Serenade® Prime from Bayer); and
- the chemical fumigant chloropicrin (Rural Telone C-60 applied by an accredited contractor), against:
- the standard grower practice treatment of Metham Sodium.
A block of Fiero Fuji was planted on 12 October 2017 onto ground that in the previous years had grown Fuji apples at 5m row spacing and 3m tree spacing on MM105 rootstock. The new planting was planted at 3.5m x 0.8m spacing on MM106 rootstock under sprinkler irrigation. Each treatment strip is 20m wide across the block and five trees in the centre rows of each treatment have been monitored throughout the project.
Biofumigation is the practice of using specialised plants from the Brassicaceae and Sorghum families that naturally produce chemicals that suppress soil-borne pathogens and break the life cycle of pests and diseases. The Brassica crop is mulched at early flowering stage. The maceration process breaks down sulphur-containing glucosinolates in the plant to release isothiocyanates that are toxic to soil-borne pests and pathogens. Biofumigation has been shown to have some control of soil-borne pathogens including Sclerotinia, Rhizoctonia, Alternaria, Colletotrichum, Fusarium, Phytophthora, Pythium, Verticillium wilt, as well as Root-knot nematode.
The demonstration trial tested two different biofumigation seed mixes.
- David Grays’ Green Fume which consisted of 90 per cent mustard and 10 per cent rocket planted at 20 kg/ha on 22 May 2017.
- PGG Wrightson Seeds BQ Mulch which consisted of 75 per cent Ethiopian cabbage and 25 per cent mustard planted at 10 kg/ha on 22 May 2017.
Both mustard mixes were seeded and grown until they reached flowering. They were then cut, mulched and ploughed in and left for three weeks so that the biofumigation activity could occur and the plant material could break down. The site received over 100mm of rainfall in this time which assisted breaking down plant material and helping release the biofumigant gas.
Serenade® Prime from Bayer was applied via a soil drench to each tree at a rate of 35ml per tree at the time of planting. Serenade® Prime contains viable spores of the highly active QST 713 strain of Bacillus subtilis. These beneficial bacteria live on the plant root surfaces and in the soil around the plant roots. The beneficial relationship between the plant roots and the bacteria can enhance plant growth and productivity. If applied at planting and during root flushes it is reported to play a role in improved nutrient exchange, crop vigour and uniformity.
Rural Telone C-60 has a high chloropicrin content and is better for use in heavier, wetter, colder soils where rapid volatilisation will allow quicker release of the product from the soil to prevent the need for extended plant-back periods. It targets soil-borne diseases including those caused by Fusarium, Verticillium, Rhizoctonia and Pythium, as well as helping to control parasitic nematodes, Symphylans (garden centipedes) and wireworms and suppress weeds.
An accredited contractor (A-Gas® Rural) applied the Rural Telone C-60 at a rate of 300L/ha. This needed to be applied at least three weeks prior to planting to avoid any phytotoxic effects. The trees were planted six weeks after treatment.
Trunk diameter, stem elongation and fruit counts were measured each autumn for three years after planting. The results after the third growing season clearly show which treatments have given the trees the best start and enabled good growth into the third leaf stage. May 2020 (3rd leaf) measurements show the trees on the chloropicrin-treated ground had a trunk diameter 8mm larger than the closest-performing treatment, the beneficial bacteria (Figure 1).
Stem elongation at 3rd leaf shows that the trees planted into soil treated with chloropicrin have produced longer leaders, followed by the trees planted into soil with the beneficial bacteria treatment and then those that underwent the standard practice treatment of Metham Sodium. The trees to which beneficial bacteria were applied at planting had more leaders than trees in the other treatments.
The shoot mass was calculated for each treatment by multiplying the average number of shoots and average shoot length for the monitored trees. For the 3rd leaf results the trees treated with beneficial bacteria had the largest mass closely followed by the trees in the chloropicrin-treated soil (Figure 2).
This year was the first in which apples were harvested from the block. Fruit counts were undertaken on the five monitoring trees in each treatment. The chloropicrin and beneficial bacteria treatments had the equal highest number of apples at 24 apples per tree, followed by the biofumigation treatment of mustard and rocket (Figure 3), which yielded an average 20 apples.
Biological treatments vs chemical treatments
It is interesting to note that there was a big difference between the two biofumigants after the second leaf, but at third leaf there is now little difference between these two in shoot length and number of shoots. The tree diameter is, however, 10mm less under the cabbage and mustard biofumigant treatment (Figure 1). The cabbage and mustard also had the least apples per tree and the highest root lesion nematode population after planting.
Apple replant disease is a complex interaction of the soil-borne nematodes, fungi, bacteria and microfauna that build up in the soil as a result of continuous planting of apple trees in the same area, slowing root growth and function, and negatively affecting tree growth, vigour and productivity.
The trial has highlighted that growers have good chemical and non-chemical options available to them, with both chloropicrin and the beneficial bacteria performing well, showing impressive growth in stem elongation, trunk diameter and yield.
Access to alternative biological treatment methods for apple replant disease is important due to the uncertainties around the future availability and acceptability of some chemical treatments, and their environmental impacts.
Adding beneficial bacteria onto the roots enabling the trees to unlock more of the nutrients in the soil and improve the tree growth and therefore the yield has performed very well in this trial and this biological treatment offers an effective chemical-free method of treating apple replant disease prone sites.
Since this trial has been undertaken, treating old apple ground with chloropicrin has now become more widely used in WA apple orchards, as the benefits of improved growth rates and yield are now more widely understood.
Acknowledgements: Thank you to our suppliers of the products used in this demonstration; David Grays, PGG Wrightson Seeds, Bayer and A-Gas Rural.
Thank you to Pomewest for funding the soil and nematode tests, to the levy-funded Future Orchards® project delivered by APAL, to Steve Spark, AgFirst NZ, and to Trevor, Carmel, Joe and Lucy Fontanini for generously allowing the trial to be conducted on their orchard.