Plant Pathology
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Oomycete & Soilborne Pathogens


Programme leader: Dr Adele McLeod

(CV: Dr A. McLeod)

Soilborne Fruit Tree Diseases

Soil-borne pathogens that are a constraint in the production and economic viability of apple, avocado and citrus are investigated. Apple replant disease (ARD) is the phenomena where newly planted apple trees are stunted when replanted onto old apple orchard soils.  In South Africa, ARD is caused by multiple agents, including oomycetes (Pythium and Phytophthora), fungi (Cylindrocarpon) and nematodes (mainly Pratylenchus), of which the composition varies from orchard to orchard. Expensive broad spectrum soil fumigation is the only effective management option currently available for ARD. The research program aims to (i) develop methods to better understand and characterizing the causative agents of ARD, including molecular detection methods, (ii) investigate inoculum sources of ARD and (iii) alternative management options with an emphasis on the use of phosphonates.

Root rot of avocado is caused by the oomycete Phytophthora cinnamomi, which is a devastating disease if not controlled. Phosphonates form an important component of preventative and curative root rot management. The program aims to better understand the mode of action of phosphonates and their translocation in trees. The development of cost effective methods for quantifying phosphites (breakdown products of phosphonates) in roots is also a priority.

Citrus decline is a major challenge for citrus growers worldwide, especially in orchards older than 13 years. The decline symptoms associated with soil-related decline include sparse foliage, nutrient deficiencies, twig die-back, root rot and reduced growth, yield and fruit size. The program aims to investigate abiotic and biotic factors associated with citrus decline, with an emphasis on Pythium species and nematode community profiling.

Onion Pathology

Fusarium basal rot is caused by Fusarium oxysporum f.sp. cepae that mostly causes postharvest losses, but it can also affect plants at the seedling stage. The program aims to develop molecular markers for identification of the pathogen and to investigate relationships between the ability of populations to cause seedling damage and mature bulb rot. The only viable management option is plant resistance, which is also under investigation.  

Onion production is negatively affected by several postharvest pathogens including fungi and bacteria. Little is known about the environmental factors that influence these diseases. The project aims to identify some of these factors using long term data and multivariate statistical analyses in order to predict seasons and farms that will have high postharvest losses.