Understanding rootstock evaluation

Every stone-fruit grower knows that when it comes producing an excellent crop you have to get your rootstock right. But the true conundrum is that there’s no magic rootstock to guarantee you’ll get cash for your crop. Fresh Quarterly asked deciduous fruit-industry veteran and rootstock expert Dr Piet Stassen to guide growers in their rootstock selection. After completing 24 stone-fruit rootstock evaluation trials in the past 15 years, Stassen says it boils down to an unwavering principle: proper planning. By Esté Beerwinkel.

Q. Many people think rootstock evaluation is straightforward. Is this true?

A.  No. Many factors play a role in this process because they influence yield, fruit size, and root and tree growth. Soil is the most important factor to consider. Other factors include water tables and drainage problems, calcareous soils, plant-parasitic nematodes, and the predisposing factors for bacterial and fungal diseases.

All of these factors must be quantified accurately in order to draw up generic information about each rootstock. Each rootstock trial site is selected to get very specific information. Every site must be well-defined in terms of the above-mentioned factors and climatic conditions so one can ultimately draw up generic information about each rootstock.

In the past most rootstock trials were done on experimental farms. These farms weren’t always representative of the regions where the rootstocks would be planted, and the farm management wasn’t at the same standard as that of leading producers. With the co-operation of some producers, I planted rootstock trials within commercial orchards. This is a great system, but needs plenty of good communication — the information you get from evaluation trials is only as trustworthy as the effort and support provided by those involved.

Q. What should a researcher consider when advising growers on rootstocks?

A.  Not all soil problems can be solved by rootstocks. The necessary soil preparation, effective drainage systems and nematode treatment must be in place. Rootstocks can’t necessarily fix wrong cultivar choices and problems. But they can significantly improve horticultural traits and soil adaptability of good scion cultivars. It’s important to have all the information about the soil, climate, pests, and diseases. Researchers give better recommendations when they have a feeling for the soil, region, location, and cultivation practices of a specific farm.

Q. Is there a single rootstock that’ll do well under the various conditions South African stone-fruit growers encounter?

A.  Presently, there is no super rootstock for all conditions. An all-rounder like Atlas performs consistently well in terms of yield, fruit size, and tolerance to plan- parasitic nematodes and diseases under many conditions. Flordaguard excels in low-chill areas and well-drained sandy soil, and works well for early cultivars. GF677 is the standard rootstock for calcareous soils, but Garnem, Cadaman, Atlas and Viking provide the same results depending on the factors involved. A rootstock must compliment the scion cultivar to optimise yield and fruit quality over the lifespan of the orchard. It must be adaptable to the soil conditions in a specific orchard, as well as be resistant or tolerant to pests and diseases.

Q. What are the most common mistakes growers make when choosing rootstocks?

A.  Long-term planning is a necessity. To get good quality plant material from nurseries, planning must be done well in advance. Key tips here include growers having information on their rootstock of choice two years in advance — this almost guarantees they‘ll get the most suitable rootstock from the nursery. Skipping the planning phase can lead to growers losing out on their choice of rootstock, and having to settle for second-best.

Using the easy to propagate rootstock — to skip on bearing royalties — isn’t always the cheaper option. Only choose this option if the situation calls for it. Commercial rootstocks may greatly outperform standard rootstocks in terms of yield, fruit size, and root and tree development. It all depends on which rootstock will be more adaptable to the conditions of that specific orchard. Clonal rootstocks can provide sad results due to poor feeder-root development, trees that weren’t hardened off, or nursery trees with inadequate reserves to sustain performance in the orchard. This issue needs attention, so clonal rootstocks don’t get a bad reputation.

Q. Could growing rootstocks through tissue culture and growing nursery trees in bags improve the quality of trees and their orchard performance?

A.  Improving propagation methods requires attention. Producing nursery trees by seedlings provide good rooting and taprooted trees, which can better withstand the transplant to the orchard. Previously, virus-free Kakamas pips were abundantly available from canning factories. Kakamas isn’t a preferred peach cultivar anymore, and pips aren’t easily available. Pips can also mix with those of other cultivars. The solution is to get virus-free pips from trees planted from seed in mother blocks. When in the mother blocks, protect these trees from other trees to keep them virus free, genetically uniform, and free from bacterial and fungal diseases. Presently, Kakamas seedlings as rootstocks provide limited adaptability to medium and high potential soil where no abiotic or nematode limitations occur. Guarantee genetic uniformity of horticultural and other traits by using clonal rootstocks in the stone fruit industry.

Rootstocks like GF677 however, have a low take and rooting percentage from hardwood cuttings in the nursery, making it less economically viable for nurseries. They may also produce poor root systems without enough feeder roots, and can put the trees at risk when planted in the orchard — especially in risky soil conditions, where irrigation is ineffective, and other poor management conditions.

To produce high-quality nursery material with an effective feeder root system the industry must consider and evaluate any propagation methods (including tissue culture, and micro- or mini-propagation) and tree-management system (bags, growing mediums, effective nutrient and water systems, netting structures, and less handling) to promote fine root development, tree health and tree growth — any effective and practical solution to provide the grower with high-quality nursery material.

Most nurseries find it difficult getting ideal nursery soils every year. Using bags with mediums that promote root formation, tree health, and development may be a good solution. It is imperative for growers to plant healthy trees with effective feeder roots, and to manage those trees optimally to prevent die-back or poor tree growth that affects the orchard performance.

An orchard’s first year determines its long-term potential. Don’t spend money on soil preparation, draining systems, ridges, training, and irrigation systems, and netting, then neglect choosing the best-suited rootstock and highest quality nursery material available. Improving propagation and using top-quality rootstocks must be a priority to increase production, fruit quality, orchard health and tree development.

Q. What are the key characteristics of a good rootstock evaluator?

A.  This person must have the following:

• A good knowledge of fruit trees, and their problems and behaviour.
• A practical understanding of soils and soil-related problems.
• The skill to combine all relevant aspects into practical information to benefit the grower.
• Excellent relationships with growers and nurseries since the industry must trust the information and recommendations made available to them.

Q. What is your prediction for our stone-fruit industry 30 years from now?

A.  Focussing on rootstocks and cultivation practices. I’m optimistic about the future. The industry has a generation of young innovative growers, keen to get and use research information, and who are prepared to invest in technology to optimise their fruit businesses.

In the next few decades I see many rootstock breeding programmes run by various research or private institutions; using intraspecific and interspecific crosses, and marker-assisted selections to identify specific advantageous genes. High-potential rootstocks from current programmes will be available.

The plum industry is growing, and is a lucrative business for the committed grower. Climatic conditions and land availability may shift many of these plantings towards the Worcester, Robertson, Bonnievale and little Karoo regions.

Q. How would you like those in the stone fruit industry to remember you?

A.  My career spans over decades, and rootstock evaluation was a small part in my research career. Although the majority of my research is stone-fruit based, I’ve also done several studies on apples, and was fortunate to work with other committed researchers and fruit and soil specialists.

During the 1970s and 1980s, I worked on improving the cultivation practices of stone-fruit orchards. The importance of the autumn period in supporting the budding stage with its associated improvement in budding, flowering and fruit set of quality fruit was established. Studies on nitrogen timing and quantity of application on the spring development of the tree were conducted. The principles of achieving higher yield per unit surface area sooner through higher density plantings along with better light management by correct tree manipulation, training systems and summer pruning was developed. The importance of carbohydrate reserves and its employment for energy requirements and as carbon building blocks was studied in detail. I began the first rootstock evaluation programme for peaches in 1971, and for plums in 1974.

I would like to be remembered for all these contributions as the first steps on the path of developing a more scientific fruit industry, and for my commitment in helping fruit growers being successful. I want to pay tribute to all the researchers I’ve worked with and the support from growers and the fruit industry.