Canker pathogens
PhD student Reshika Kallideen introduced a new project to determine the risk of spreading cankers with wood-chip mulches. Her supervisors are Profs Lizél Mostert and Francois Halleen, as well as Dr Lindy Rose of the Department of Plant Pathology at Stellenbosch University.
Chipping grubbed trees for mulch has become a standard practice on many apple farms. As this mulch is frequently applied to young orchards, it could place them at risk of infection with canker pathogens.
Previous research has shown that Diplodia seriata, a canker pathogen, can remain viable on apple wood for up to 18 months, generating spore-producing structures on 87% of the wood pieces.
To investigate spore dispersal, Kallideen sampled six apple orchards annually. Mulches were usually applied in the spring of the previous year.
In the first year, three orchards were mulched with apple-wood chips, one with pine chips, and two with straw. In the second year, three orchards were mulched with apple-wood chips, two with pine chips, and one with straw.
Kallideen trapped spores every week from June to October over two years to determine the average number of D. seriata spores, and to correlate spore release with rainfall.
In both years, there were more weeks with spore release in orchards mulched with apple-wood chips than in orchards mulched with pine chips or straw.
Kallideen also sampled apple pruning debris, apple-wood chips and pine chips for visual inspection and DNA analysis. D. seriata was present on apple and pine material. She also occasionally found several other canker pathogens, including Cytospora, Eutypa lata, and the wood-rot species Schizophyllum commune.
“Chipped apple and pine wood can harbour canker pathogens and release spores, which can infect young trees,” concluded Kallideen.
White root rot
Prof. Adéle McLeod of the Department of Plant Pathology at Stellenbosch University presented the work of her MSc student, Anika Keuck, who graduated in 2024. Keuck investigated the effect of organic soil amendments on white root rot, caused by Rosellinia (Dermatophora) necatrix.
Experts disagree on whether soil organic matter increases or reduces the risk of white root rot.
“Since the application of mulches is so widely used in our apple industry, it was important to determine whether the application of a mulch, or a mulch combined with compost, will promote or suppress disease development,” said McLeod.
Keuck obtained soil from three commercial apple orchards on different production units – Paardekloof, Nooitgedacht, and Tandfontein – at Dutoit Agri. Each orchard had rows where either no amendments, mulch, or a combination of mulch and compost had been applied for 6–9 years. Keuck sampled all of these.
In the laboratory, she planted Golden Delicious seedlings in the soil samples and inoculated the soil with Rosellinia. She planted the control seedlings in unamended soil samples that were not inoculated with the fungus. Then Keuck evaluated the disease severity 148 days after inoculation.
The carbon and nitrogen content of the soils was also analysed. On all sites, the carbon to nitrogen ratio tended to increase from the lowest in unamended soils, to intermediate in soils amended with compost and mulch, to the highest in soils amended with mulch only.
The Paardekloof site had a silt loam soil. When amended with compost and a straw mulch, this soil had the highest carbon content and was also associated with the lowest disease severity in the pot trials.
When amended with mulch only, the Paardekloof soil had the second-highest carbon content, while the unamended soils had the lowest. But the disease severity was similar in both soils.
The Nooitgedacht site had a sandy loam soil. The carbon content of this soil amended with wood chips alone, or with wood chips and compost combined, was significantly higher than that of the unamended soil.
However, at Nooitgedacht, the disease severity was significantly higher in the soils amended with organic matter than in the unamended soil.
The Tandfontein site had a loamy sand soil amended with a straw mulch and compost. Unsurprisingly for a sandy soil, organic amendments had little impact on the soil carbon levels. The disease severity did not differ significantly in the amended and unamended soils.
“Based on these results, there isn’t a clear association between soil organic amendments, percentage carbon, and disease development,” concluded McLeod. “Our current recommendation is to continue using mulches and compost because we know these practices benefit apple production.”
“Our current recommendation is to continue using mulches and compost.”
As is so often the case, more research is needed. “One of the enigmas of this disease is why it develops in some orchards and not in others,” she reflected. “I think the key in the end may lie in the soil microbiome that differs in different soils.”
Nematodes and woolly apple aphids
MSc student Erin Bond discussed the preliminary results of her work on the potential for organic soil amendments to suppress plant-pathogenic nematodes and woolly apple aphids. Her supervisor is Dr Noma Stokwe of the Department of Conservation Biology and Entomology at Stellenbosch University.
Bond’s co-supervisors are Dr Rinus Knoetze of the ARC Infruitec-Nietvoorbij, and Matthew Addison, previous crop-protection programme manager at Hortgro Science.
“Many of the control methods for plant-parasitic nematodes and woolly apple aphids are environmentally harmful, and some are quite ineffective,” said Bond. “My project aims to explore the use of soil microbes for the management of these pests.”
Bond is comparing the effect of straw, wood chips, a combination of wood chips and compost, a cover crop, and an unamended control on populations of woolly apple aphids and plant-parasitic nematodes in two commercial apple orchards on different sites.
The amendments were first applied in 2024, with mixed results. But monitoring is ongoing.
Another part of the project focuses on fungi that suppress nematodes. Bond has extracted and analysed DNA from soil samples to see whether these are present in her treatments. She has found fungi that trap nematodes, parasitise nematode eggs and juveniles, and produce natural nematicides.
Nematode biodiversity is an established measure of soil health. Hence, Bond is also characterising the nematode populations for each treatment at each site, quantifying the abundance of different feeding types.
So far, she has seen large populations of nematodes that feed on bacteria and fungi at both trial sites. “These preliminary results may indicate that organic amendments have had a positive impact on the microbial health of the soil,” notes Bond. “That’s exciting.”
