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202012 Fresh Quarterly Issue 11 02 To Your Trees Health
Issue ElevenDecember 2020

Here’s to your tree’s health

Start with pest- and disease-free nursery trees. By Grethe Bestbier.

It is vital to establish a new orchard with healthy trees. Pests and diseases pose a threat to young nursery trees and can cause serious damage or even death. Bacteria, viruses, fungi, and nematodes are just some of the pathogens that may affect tree survival and orchard performance. A clean start is therefore of utmost importance, and a good external and internal inspection could save you thousands.

Make a clean start

“When you talk to producers, one of their biggest concerns is the poor quality of nursery trees,” says Prof. Wiehann Steyn, assistant general manager at Hortgro Science. “If nursery trees aren’t healthy from the start, they may experience stunted growth, deliver poor fruit quality and low yields, or even die.”

The financial repercussions of a bad start are potentially devastating, according to Prof. Lizel Mostert of the Department of Plant Pathology at Stellenbosch University.

“It’s all about replant costs,” she explains. “If you plant a young tree that is already disease infected in suboptimal or stressed conditions, it will start to die back, or growth will be stunted. In some circumstances, trees might have to be completely removed and replanted. This becomes problematic, as you can’t always use the same planting holes, adding even more expenditures.”

Replant costs are definitely something to avoid, considering that orchard establishment carries a price tag of R450 000–R800 000 per hectare for apples.

“Starting with plant material that is as clean as possible is crucial,” says Mostert. “But it is also important to ensure that your soil is well prepared and that the correct planting practices are followed. It is not only the propagation material’s disease status which has an influence. The way in which the producer plants and cares for the trees is also very important, especially in the case of opportunistic pathogens that are triggered by host stress.”

The path to improvement

Over time, the quality of plant material deteriorates due to the accumulation of pathogens and pests. The solution is a plant improvement scheme that provides assurances of the quality and disease status of plant material.

“One of the most important benefits of plant improvement is ridding plant material of viruses,” says Mostert. “This is absolutely instrumental to the process of plant propagation, and without plant improvement we’d have many bigger problems in the future.”

Plant improvement for the South African deciduous fruit industry is guided by the Deciduous Fruit Plant Improvement Scheme, which includes inspection and testing of plant material and determines whether material is suitable for further propagation.

“Having a scheme and central bodies responsible for the coordination of plant-material testing and improvement not only simplifies things for industry, but also ensures that standards are upheld, and no shortcuts are taken,” says Mostert.

“This system enables us to provide a nursery tree of a certain standard that underwent the right tests in terms of virus-free assurance. We are ensuring that industry as a whole receives better quality material.”

Hidden threats

Plant material produced under the Scheme’s control in registered blocks must be free of specified pests and diseases, but some threats to nursery tree health can escape detection. Mostert’s recent study on plum and nectarine orchards showed that latent infections are just as serious, and also challenging to detect.

On the lookout for stem canker pathogens, Mostert made isolations from scion and rootstock propagation material from mother blocks, as well as from a total of 1 080 trees from three certified nurseries.

“All the materials were visually clean and did not look infected or sick. We wanted to find out which infections were dormant within the material and how clean it was in a phytosanitary sense,” says Mostert.

The research showed that stone-fruit nursery trees and propagation material can harbour latent infections. In the propagation material, 6.2% of dormant rootstock shoots carried canker pathogens before planting in the nursery fields and this increased to 11.1% after planting. Of 1 080 nursery trees, 21.8% were infected with fungi.

When a tree with a latent infection experiences stress after planting, a visible stem canker may develop. Preventing these infections through correct management practices is crucial to ensuring healthier trees. For more on this, read our article on stem cankers in the March 2019 issue of Fresh Quarterly.

Apple replant disease

Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University found that visually healthy apple nursery trees contained several apple replant disease pathogens. Investigating replant disease pathogens in five apple nurseries, McLeod found that up to 47% of the trees contained fungal-like pathogens and 29% contained lesion nematodes in their roots.

“The association of apple replant disease pathogens with nursery trees is of concern since this complex of pathogens has the potential to cause tree stunting and ultimately yield losses. The extent of damage will depend on the level of infection and abiotic stress factors that trees are exposed to during their first two years of growth,” explains McLeod.

Planting apple replant disease contaminated trees may also defeat expensive soil fumigation by growers, which is the only management strategy available against apple replant disease in orchards. Growers can consider applying a semi-selective drench treatment at planting followed up with phosphonate sprays as a safeguard against nursery trees infected with apple replant disease.

Managing apple replant disease in apple-tree nurseries is often ineffective, due to the open-field production systems and mother layering-blocks employed. According to McLeod, the best solution would be to move to production of trees in bags.

Tissue culture is another option for producing cleaner trees, adds Steyn. Tissue culture also has the advantage that it facilitates production of clonal rootstocks that are difficult to root.

You can read more about the potential benefits of bags in our article To Pot or Not.

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