Apple replant disease impacts growers in all major apple-producing countries. Fresh Quarterly spoke with Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University about apple replant disease in South Africa. By Anna Mouton.
What is replant disease?
Specific replant disease is a worldwide problem affecting certain species of plants when they are grown on soils that were previously occupied by the same or related species.
Trees develop replant disease when their roots are damaged by the population of soil organisms that accumulated during the lifetime of the trees previously planted on the site. “These harmful soil organisms destroy the feeder roots of the young tree,” explains McLeod.
In South Africa, replant disease is associated with apples. Pears are not affected.
Specific replant disease can also occur in stone fruit — Prunus replant disease is an example of an economically important condition of stone fruit and almonds in California. There are no research projects on replant disease in stone fruit in South Africa at present.
Apple replant is essentially a childhood ailment. “As trees become older, let’s say from around their third year, these soil organisms naturally proliferate on their roots, but the trees are no longer susceptible to them,” says McLeod. “So those older trees are fine. The problems start when you remove those old trees, because young trees are susceptible to these soil organisms.”
McLeod stresses that growers have to protect their trees for the first two years. “If you don’t manage replant in those two years, it’s a lost cause — the tree will never catch up. You will never make up for the economic losses due to reduced yields.”
Replant disease manifests as a failure to thrive. It stunts the growth of the young trees without increasing tree mortality. Growers may not realise they have a replant problem, says McLeod, because the symptoms can sometimes be inconspicuous, and because the grower doesn’t see a side-by-side comparison of trees with and without replant disease.
The underground movement
So what are these soil organisms that cause all the problems? “There are fungi, and fungi-like organisms called oomycetes, and nematodes,” says McLeod. “But not bacteria.”
Fungi are common in soil where most make a valuable contribution by breaking down organic material such as dead plant matter. Some fungi that grow in association with roots are beneficial to the plant. Others cause damage and disease.
Several species of Cylindrocarpon-like fungi are involved in apple replant disease in South African soils, but research suggests that they are less important here than in some other parts of the world. Whereas fungi in the genus Rhizoctonia are virulent agents of replant disease abroad, they don’t play the same leading role locally.
Oomycetes superficially resemble fungi — both have thread-like growth and make spores — but the two groups are about as closely related as people and plants. Oomycetes include Phytophthora infestans, which is arguably the most notorious plant pathogen in history, having caused the Irish potato famine which led to the starvation of an estimated 1 million people in 1845–1852.
A single species of Phytophthora — P. cactorum — and species of two other oomycete genera called Pythium and Phytopythium are linked to apple replant disease in South Africa. A survey found these organisms to be especially prominent in South African apple orchards, and the researchers speculated that this may be due to mild, wet autumns and winters, which favour oomycetes.
The other soil dwellers that cause replant disease are nematodes. Soil-living nematodes are generally tiny — less than 1 millimetre long — animals with worm-like bodies. Like most fungi, most nematodes are beneficial, and contribute to soil health. But a few bad actors pose a significant threat to plant health.
Lesion nematodes of the genus Pratylenchus are agents of apple replant disease. They occur in South African systems, but seem to be less widespread than in some other apple-producing parts of the world.
Risk factors for replant disease
The biggest risk factor for apple replant disease is replanting apples on the same site where apples were previously planted — obviously. Even so, as McLeod points out, risk isn’t the same as certainty. “Not all soils have replant disease, and the degree of disease severity also differs between different soils.”
McLeod has looked at the effect of soil characteristics such as the ratio of carbon to nitrogen on the occurrence of replant disease, without finding any correlations. “We don’t understand why one soil is a problem and another isn’t. We don’t have a way of predicting replant, so we recommend fumigating all replanted orchards.”
Soil fumigation is the standard method for controlling replant disease. The idea is to kill all potential disease agents before introducing susceptible young apple trees. Fumigation isn’t always effective, and one possible reason is that the young trees are already infected in the nursery. A survey of five South African apple nurseries found Cylindrocarpon-like fungi, oomycetes in the genus Pythium, and lesion nematodes, in tree roots.
Oomycetes that cause apple replant disease have also been detected in irrigation water, in both Elgin and the Koue Bokkeveld. Further investigation is necessary to clarify whether microbes in nursery trees and irrigation water will eventually lead to replant disease in the orchard.
The organisms that cause apple replant disease are naturally present in soils. Their populations increase when apple trees are present, but the soil organisms don’t have an absolute requirement for the trees. Agents of replant disease can survive for years in fallow soils, or soils planted to other crops.
“We conducted a trial in an orchard where lucerne had been grown for ten years, but it still had serious replant disease,” notes McLeod.
Land that is suitable for growing apples will always be in short supply, so apple replant disease will remain a challenge for producers. Fumigation remains the only real option for control — for now. Our next article in this series examines control measures and asks, what are the alternatives to fumigation?