Oomycete & Soilborne Pathogens

The research program focuses on soilborne pathogens that limit the productivity and economic viability of apple, avocado, and macadamia crops. Our ultimate goal is to develop sustainable, integrated disease management strategies.

Apple Replant Disease (ARD) refers to the stunting of newly planted apple trees when re-established on old orchard soils. In South Africa, ARD is caused by a complex of organisms—including oomycetes (Pythium, Globisporangium, Phytopythium, Phytophthora spp.), fungi (Cylindrocarpon-like), and nematodes (Pratylenchus spp.)—with species composition varying between orchards. At present, broad-spectrum soil fumigation is the only effective management strategy. The research program aims to (i) improve the understanding and characterization of ARD pathogens, including the use of molecular detection methods; (ii) investigate inoculum sources; and (iii) explore alternative management strategies, such as the use of organic amendments (compost and mulch), semi-selective chemicals, and anaerobic soil disinfestation.

Phytophthora cinnamomi is a destructive pathogen of avocado and macadamia, causing root rot in both crops and stem cankers in macadamia. Phosphonates are currently central to its management. Our research investigates its mode of action and translocation within trees. As resistance to fungicides threatens control efficacy, we are developing cost-effective fungicide sensitivity screening methods for currently registered fungicides (mefenoxam/metalaxyl) and those under consideration (fluopicolide, oxathiapiprolin, propamocarb hydrochloride, fluoxapiprolin). Similar protocols are being developed for the Phytopythiumvexansspecies complex, which also causes root rot and stem cankers in macadamia. The fungicide research aims to establish baseline EC₅₀ values for fungicides pending registration and to determine whether resistance to mefenoxam has developed.

Using disease-free nursery material is critical to managing oomycete pathogens. The avocado, citrus, and macadamia industries have nursery certification schemes requiring regular testing for Phytophthora spp. Laboratories typically use conventional baiting and hyphal morphology for identification, which is not effective for detecting Pp. vexans. Our research focuses on developing molecular detection methods for both Pp. vexans and P. cinnamomi in nursery stock.

Despite integrated management in certified nurseries, Phytophthora spp. are still occasionally detected in nursery trees. Biological control agents, primarily Bacillus and Trichoderma spp., are used as part of an integrated management strategy, although many are not yet registered for Phytophthora management in South Africa. We are evaluating the efficacy of these commercially available agents in macadamia nurseries.

Population genetics is a minor yet valuable component of the research program, enhancing our understanding of plant pathogen epidemiology. Previous work has focused on oomycetes such as Phytophthora infestans, P. capsici, and Plasmoparaviticola. Notably, our Plant Disease Clinic Stellenbosch recently reported the first detection of the P. infestans EU23 lineage on tomatoes and potatoes in South Africa, replacing the long-standing US-1 lineage (McLeod et al., 2023).

We have also investigated the population genetics of Phyllostictacitricarpa, the causal agent of citrus black spot. This research provided key insights into the pathogen’s global genetic structure, historical spread, and reproductive biology. Contrary to previous assumptions, asexual reproduction may play a role in lemon orchards, suggesting that current management strategies, which focus mainly on the sexual phase, may need revision.

We have recently begun studying Venturia inaequalis, the apple scab pathogen, using population genetics to re-examine its epidemiology. While the sexual phase has traditionally been seen as the primary source of inoculum, recent observations of declining ascospore numbers suggest a possible shift toward asexual reproduction also being a primary inoculum source. This may be linked to climate change, as warmer, drier winters in the Western Cape reduce the environmental conditions needed for sexual spore production during fall and early spring.

SHORT BIO

I earned my BSc (Agric) and BSc (Agric) Hons degrees from the University of Pretoria and completed my MSc at Stellenbosch University (SU) while working at the ARC’s Vegetable and Ornamental Plant Institute. I received my PhD from Cornell University in 2003, followed by postdoctoral positions at Cornell (with Prof. W.E. Fry) and the University of Pretoria (with Prof. D. Berger). In 2004, I joined SU as a senior lecturer in Plant Pathology and was promoted to Associate Professor in 2015. From 2010 to 2013, I completed a sabbatical with ZZ2, South Africa’s largest tomato producer.

At SU, I established a research program on soilborne diseases, later expanding into foliar pathogen population genetics. My work initially focused on the etiology, characterization, and molecular quantification of soilborne disease complexes in tree crops and vegetables. Over the past eight years, my focus shifted to developing disease management strategies. I have supervised 12PhD, 18 MSc, and nine Honours students.

My research has been funded by the NRF, DTI, and industry bodies such as Winetech, Hortgro, SAAGA, and SAMAC, who value its practical impact on improving crop yield and quality. I maintain strong industry collaborations and have led the commercialization of SU’s Plant Disease Clinic (PDC) Stellenbosch since 2020. This included securing funding, acquiring equipment, and expanding diagnostic services. An important new diagnostic service that we established is fungicide sensitivity testing for Botrytis cinerea, Venturia inaequalisand Erysiphe necator. The PDC is South Africa’s largest plant diagnostic clinic, processing over 1,000 samples annually and employing three full-time staff.

My h-index according to Web of Science is 24. Since 1997, I have published 66 peer-reviewedarticles, which were cited 1 637 times. The NRF awarded me a C1 rating for my previous twoapplications (2009 and 2015), thus recognising my national and international standing.