Stopping the rot

What we’ve learnt about cankers in apple trees. By Grethe Bestbier.

Canker development in newly established apple trees has been increasing in the Western Cape, with stem cankers responsible for losses of 30% and 70% respectively in two affected orchards. The deciduous-fruit industry has also observed significant mortalities due to die-back. Wood-rot- and canker-related diseases pose a large risk of critical damage to young trees and threaten the establishment of new orchards.

“Cankers appear on trees young and old, but the largest impact is on young trees,” says Lizel Mostert, associate professor at the Department of Plant Pathology, Stellenbosch University. “It can cause the death of young trees.”

What are cankers?

Cankers are areas of dead or damaged tissues that show up on stems, branches, and trunks due to bacterial and fungal infections. Cankers do not have a consistent appearance and can be hard to spot. Mostert points out that young trees may have internal infections without external signs. “When those trees are confronted by stressful circumstances, the symptoms will show,” she warns.

Mostert emphasises that a whole spectrum of fungi can cause cankers, as well as other symptoms such as twig-blight and wood rot. Die-back is the premature death of trees due to infection by agents of canker and wood rot. These microbes — referred to as pathogens — enter trees through wounds and grow in the vascular tissues. Vascular tissues are essential for transport of water; obstruction leads gradually to the death of infected stems, branches and, eventually, entire trees.

Canker and wood-rot pathogens form spores that can be carried in the air. Spores transmit from tree to tree through wind or the splashing of water. Infected plant material then spreads disease from nurseries to orchards, placing farms everywhere at risk of tree damage and losses.

Stress affects how trees respond to infection. Mostert’s research has shown that orchards on suboptimal soils are more prone to cankers. She believes that die-back of newly established trees in commercial orchards may be related to the stress associated with marginal soils. Drought, waterlogging, co-infections, pests, and nutrition are all factors that may play a role in the susceptibility of trees to cankers.

“If the trees are under extremely stressed circumstances, up to 70 percent may die,” says Mostert. “It also depends on where the infection is situated: if it only occurs on the scion, it can still be pruned away. However, if it sits on the rootstock, it can potentially cause the entire tree to die.

“There are lots of different things that can cause stress and if it comes to things that need to be further researched, this is definitely it.”

Know your enemy

Mostert and her team — including postgraduate students Minette Havenga and Greg Gatsi — recently concluded a survey to determine the prevalence of organisms that cause canker and wood rot in apples. They collected samples from all phases of the tree production process, including scion and rootstock mother plants, and nursery and newly established trees.

“To work out management strategies, you must first determine where the infection occurs and which pathogenic fungi are present,” says Mostert.

Her team isolated and identified disease-causing fungi from tissues that showed signs of canker. They found potential canker and wood-rot pathogens in 55 newly established trees out of 130 that were sampled. Nearly two thirds of certified nursery apple trees were infected. Canker-causing organisms were also present on propagation material including rootstock cuttings and scion shoots used for budding. The researchers identified 44 fungal species that are associated with disease in woody plants like fruit trees.

The study linked the fungal species that were present in nursery apple trees to those infecting diseased trees in newly established orchards. This suggests that nursery trees are a significant source of infection, and that better pathogen control must be applied in all production phases.

“The young trees go through a certification scheme where inspectors classify them as visually clear, but there may still be infections internally,” cautions Mostert.

The fight continues

The survey conducted by Mostert and her team filled a gap in the field, because it focused on young trees. Whereas a lot is known about cankers and wood rot in older orchards, no one has investigated propagation material or looked at the disease status of nursery trees.

Starting an orchard with uninfected young trees is the first step in reducing losses. But this can be difficult when nursery trees have hidden infections. Mostert advocates reducing the spread of pathogens through improved nursery practices.

“One of our big control measures that needs to be applied is sanitation throughout the entire production process, especially regarding tools — pathogens spread via tools used during the budding process,” explains Mostert. “Sanitation across the whole spectrum of phases must be better applied.”

Further research on controlling cankers is already underway. One project is investigating ways to reduce infection of pruning wounds as a possible control measure. Mostert considers that many nursery practices can be improved, for example general sanitation of pruning wounds. She also recommends the removal of dead shoots and branches from nursery fields and orchards, as these can shelter disease-causing fungi.

Mostert is encouraged by the positive feedback she has received from industry. “They are very glad about the outcome, because it identifies certain areas in the plant propagation process where we can improve,” she says. “And that is really the purpose of the project — to see which areas we can improve.”

Image: This nursery tree showed no external symptoms of disease. Cross sections through the pruning wound and the bud union reveal brown discoloration due to canker.

Supplied by Lizel Mostert | Stellenbosch University.