Keep your trees virus-free

Virus infections often fly under the radar while growers look elsewhere to explain underperforming orchards. By Anna Mouton.

“All the literature suggests that the Scheme viruses have significant economic impacts,” said plant virologist Prof. Gerhard Pietersen. He has studied plant viruses for over four decades and is the research director of specialised plant-pathology company Patho Solutions.

The Scheme viruses to which he refers are those included in the Deciduous Fruit Plant Certification Scheme: apple chlorotic leaf spot, apple mosaic, apple stem pitting, apple stem grooving, prune dwarf, and Prunus necrotic ringspot viruses.

Plum viroid 1 — the cause of plum marbling — is not a virus but is subject to similar control measures.

Examples of the Scheme viruses’ effects on susceptible cultivars include yield reductions of 12%–30% in infections with the group that includes apple chlorotic leaf spot, apple stem pitting, and apple stem grooving viruses. Yield reductions can be as much as 46% in apple mosaic virus infections.

“If you have high infection levels, you’re starting an orchard with fully infected trees, but you wouldn’t even know it,” said Pietersen, “because the symptoms are difficult to see.”

He presented the results of a survey testing 1 008 apple trees in 168 orchards in five major production areas. Nearly three-quarters of orchards were infected with apple stem pitting virus, the most common virus. Almost a quarter were simultaneously infected with apple chlorotic leaf spot, apple stem pitting, and apple stem grooving viruses.

“We did a similar, slightly smaller, survey in pears, with pretty much the same results,” reported Pietersen. “These results are quite shocking.”

The disease triangle

Virus control requires understanding the disease triangle: the virus, its vector, and the plant, within the context of their environment. This allows researchers to discover and exploit the virus’ weaknesses.

“For example, if you see that there’s only one vector, you could break the triangle by controlling the vector, thereby controlling the virus,” said Pietersen.

The transmission mechanisms for the Scheme viruses are well-established. Pietersen emphasised that all these viruses are spread by vegetative propagation. “They are all graft transmissible — it’s the one quality they all have in common,” he said.

“If your scion is infected, it will infect the rootstock. If the rootstock is infected, it will infect the scion. So, both of those components must be free of viruses.”

None of the Scheme viruses are transmitted by vectors. Of the other routes — seed, pollen, and mechanical — only pollen transmission of prune dwarf and Prunus necrotic ringspot viruses in stone fruit is significant in an orchard scenario. However, pollen transfer requires the initial introduction of infected plant material.

“Field spread of pome-fruit viruses only occurs through natural root grafting,” said Pietersen. “That is also a plant material issue. If there weren’t any infected plants, they could not infect those next to them.”

Multiplication of viruses

Because most viruses aren’t seed-transmitted, plant sexual reproduction helps eliminate viruses. Vegetative propagation sidesteps this natural process, creating innumerable infected plants and promoting the accumulation of viruses.

“In vegetatively propagated crops, you tend to have multiple viruses in any particular plant,” said Pietersen. Witness the results of his apple and pear survey mentioned above.

“In the case of vegetatively propagated crops, including pome and stone fruit, the plant material has an outsized importance in the virus’ epidemiology,” said Pietersen. “If you use vegetative propagation, most or all of the progeny of the infected plant contain the virus.”

He has little doubt that infected plant material explains the high prevalence of viruses in so many South African apple and pear orchards.

A single infected plant in starting or nuclear material can become thousands of infected plants. “What’s more, those thousands of infected plants land up distributed throughout your orchard,” warned Pietersen.

This is one reason viruses have the potential to inflict grave economic harm. Although a virus might cause a relatively small performance reduction in an individual tree, each erodes profitability—hundreds or thousands of infected trees contribute to a substantial cumulative loss.

Nonetheless, vegetative propagation of pome- and stone-fruit trees has been well-established for over 150 years. It allows growers to take advantage of specific clones’ superior genetics and establish orchards of genetically uniform trees.

A safe start

The absence of vectors implies that orchards established with healthy plant material will likely remain free of the Scheme viruses, especially in the case of pome fruit, which do not have pollen-transmitted viruses.

Pome-fruit orchards that start clean should require no further testing. Stone-fruit orchards may need testing if pollen-transmitted viruses are suspected, or their absence must be established.

One function of the Deciduous Fruit Plant Certification Scheme is ensuring that growers have access to so-called clean trees — plant material that has been tested free of the viruses specified in the Scheme. So why are there so many infected orchards?

Pietersen was quick to point out that growers shouldn’t blame the Scheme. “During the last six or so years, certified material only represented about 30% of the material being planted. The uptake of certified plant material is very poor.”

Certification schemes are pivotal to virus control in most countries with active pome- and stone-fruit industries, he said. There is considerable overlap in the viruses included in the various schemes.

Pietersen advocates for strengthening and expanding the South African Deciduous Fruit Plant Certification Scheme by including apple rubbery wood viruses. There are two, one of which has been found in South Africa.

“In the United Kingdom, they’ve largely eliminated apple rubbery wood disease through certification, leading to planting healthy material,” he said.

Locally, Pietersen has worked extensively on grapevine viruses, especially grapevine leafroll-associated viruses. He has seen the benefits of clean plant material and downstream virus control first-hand in the wine industry.

“I hope I’ve convinced you that we need to clean up the plant material of the Scheme viruses,” he concluded. “It’s a small intervention at the beginning of the process and doesn’t involve grower participation — except for buying certified plant material.”

Watch Pietersen’s presentation on the Hortgro YouTube channel.