At the recent Hortgro Technical symposium, Gerhard Pietersen from Patho Solutions spoke to delegates about the importance of keeping deciduous trees free from scheme viruses.
Gert Pietersen founded Patho Solutions in 2016 and has established labs in Pretoria and Wellington that perform research, diagnostics and control of plant pathogens. Before starting the company, he completed his B.Sc Honours degree in Microbiology with distinction. Based in Wellington, the Laboratories provide important services to producers in the Winelands and the Deciduous fruit industry in the Western Cape.
Pietersen said that it is crucial to understand the epidemiology to control a virus disease. Using the disease triangle can help you determine weaknesses amongst the components interacting in the disease. The “scheme” viruses are Apple stem pitting virus (ASPV), Apple stem grooving virus (ASGV), Apple chlorotic spot virus (ACLSV), Apple mosaic virus (ApMV).
Some control strategies include vector control, resistant cultivars, the elimination/reduction of the virus’s sources, and modifying the environment.
Pietersen said that in vegetatively propagated crops (including stone and pome fruit), viruses in planting material play a significant role in the epidemiology of the diseases they cause. Most, if not all, the progeny plants are infected by the virus.
Vegetative propagation of apples and pears is more than 150 years old. Propagation of pome and stone fruit is almost exclusively done by vegetative propagation in modern times.
Hugh Campbell, Hortgro technical and plant improvement manager, recently shared some of the most pertinent questions about plant improvement and its necessity for sustainable growth in an article on the Hortgro website.
Cambell said that plant improvement is a continuous process that aims to produce better fruit trees. New and improved genetics is one of the cornerstones of plant improvement. New cultivars provide better yields, colour, texture, storability, etc. Plants are also improved by removing economically important pests and diseases. A plant improvement scheme’s primary focus is to ensure plant material is free of economically important viruses.
There is no cure or spray for a virus or viroid. The only option is to remove and destroy a virus or viroid infected tree. A virus is like a parasite. It does not kill the tree as it uses the cellular machinery of the tree to produce more viruses. The only way of managing viruses is to ensure that producers don’t have them in new trees before it is planted.
- Viruses are spread through propagation material – i.e., if budwood is cut from an infected tree, it will spread to all the trees made from that budwood.
- Viruses reduce your bud-take.
- Economically important viruses can reduce tree growth and yield by 10 to 40%, depending on the virus.
- Viruses can reduce tree survival and increase management costs.
According to Cambell, the plant certification scheme is designed to provide planting material that has been tested to be clear of selected economically important viruses. Uncertified plant material requires that the trees be visually clean. You cannot easily see viruses on planting material. No virus tests are required for uncertified planting material.
Pietersen also reiterated that it is vital to have a “virus elimination step” (seed or other) or start with clean material because if the producer has an accumulation of viruses in cultivated apples and pears it will also often result in mixed infections of multiple viruses.
Pietersen said that the irony is that because most pome fruit viruses lack a vector, and if producers control it by planting healthy plant material, it will eliminate these viruses from the industry (as done for apple rubbery wood disease in the UK). To ensure you have virus-free nuclear material of scions and rootstocks, select virus-free material or subject starting material to virus-elimination processes. There will be no need for further downstream tests if the grafted material is traceably derived from nuclear material.
Most countries with an active Pome or Stone fruit industry have certification schemes (e.g. Canada, USA, Australia, EU, Netherlands, France, Germany, UK, Italy, Spain, Hungary).
Cambell also added that in the context of plant improvement, certification is the process of providing growers with evidence that plant material of official varieties has been tested and certified for trueness to type (i.e. a Kakamas is a Kakamas), health status (free of scheme viruses etc.) and physical quality (size, thickness etc.).
The Deciduous Fruit Plant Improvement Association (DPA) is the delegated authority for plant improvement for stone and pome fruit in terms of the Plant Improvement Act 53 of 1976.
PlantSA is a non-profit company that manages the voluntary certification scheme for deciduous fruit (DPA) and grapevines (VIA). Only registered plant improvement organisations (PIO’s) may apply for the certification of propagation material.
The classic approach for plant improvement of ‘start clean – end clean’ is to start with a nucleus plant that is placed in a bio-secure facility, tested clean, and kept clean through regular inspection and testing. Plant material from the nucleus plant is used to make foundation plants. These foundation plants are used to provide material for mother plants, which are ideally placed in bud parks but are currently mostly in commercial orchards. The mother plants are inspected and regularly tested and are used to cut budwood that is provided to the registered nurseries to make nursery trees.
PlantSA oversees, inspects and audits this process throughout the year (spring, summer, autumn, and winter), providing a certified blue label for plant material that meets the scheme’s requirements.
The inspection of uncertified plant material is a DALRRD (Dept of Agriculture Land Reform and Rural Development) function. The DPA can refuse to certify trees, but only DALRRD has the power to prohibit the sale of non-compliant trees. Currently, DALRRD has a capacity challenge and cannot fully meet this mandate.
What is the difference between the DPA and a PIO?
The DPA is the authorised authority that manages and inspects certified material. They, therefore, have an oversight role.
There are currently three PIO’s (SAPO, Topfruit and Stargrow) that operate under the DPA.
The role of a PIO is to import, breed or select new or improved cultivars or clones. Currently, only PIO’s may apply for certification of plant material. The PIO’s are responsible for the multiplication of the certified plant material (buds) that they then provide to registered nurseries.