NZPPI's former Fruit Tree Sector has recently completed a project with Plant and Food Research and MPI's Sustainable Farming fund to seek alternatives to fumigation to mitigate the impact of "apple replant disease".
Apple replant disease causes poor plant growth as a result of trees being replanted on sites where the same (e.g. apple) or a closely related species (e.g. pear) had been grown. The symptoms are not clearly identified but appear to be mostly biotic (eg changes in the microbial community). Fumigation has been shown to be the most effective tool in overcoming apple replant disease, but it is expensive (>NZ$10,000/ha), hazardous to health, damaging to the ozone layer and often difficult to integrate into the high-density-planted nursery apple beds.
Key objectives of the project were to implement biological control techniques based on research from around the world. The original SFF project was designed to assess a variety of new biocontrol soil treatment methods as alternatives to fumigation with the industry standard chloropicrin. A new treatment Ag lime/Nemacur was included in second year trials after a positive result from the initial trials.
Industry work with researchers developed consensus on trial rootstock selection, treatments and methodology. Soil treatments in two pipfruit nurseries affected by apple replant disease started at the beginning of planting season with industry recommended rootstocks and variety. Sites were monitored and results recorded during growing season and best treatments to minimise apple replant disease identified
Rootstocks were grown on ground treated with Ag lime/Nemacur and compared to the industry standard fumigation method and untreated control. The rootstock “shoot extension growth” was measured over the growing season At the end of the growing season the tree shoot was “headed back” to approximately 25cm from the ground in preparation for “budding” which involved grafting of the desired variety of scion wood onto the rootstock. The next growing season the scion “shoot extension growth” was measured.
Early results showed that the chloropicrin-treated plot plants were taller, but the proportion of healthy plants was much the same throughout the randomised trial plots of both sites. However, the advantage seen for fumigation has not been sustained.
The extended trials using the Ag lime/Nemacur treatment method appeared anecdotally promising. This treatment was adopted by the trail nurseries based on confidence gained from observations after the first year. They have grown in scale and tree numbers over the last three growing seasons and the treatment is also now being tested by an orchardist.
Results from the final growing season show that Ag lime/Nemacur treatments provide an additional management tool; one that will likely reduce the need for fumigation or shorten rotation periods. Further benefit may accrue from additional biomass amendments alongside tthe Ag lime/Nemacur technique, and this may be the subject of further investigation.