Zak Pretorius: A pathologist looks at rust

Linda McCandless
Wednesday, July 22, 2015

The host/pathogen interaction has fascinated Zak “Zakkie” Pretorius, one of the BGRI's most respected pathologists, since he started his career in 1978.

“A range of infection types occurs in the wheat/rust interaction that depends on the genetics of the host/pathogen interaction as well as on environmental factors,” said Pretorius, rust pathologist with the University of the Free State (UFS) in South Africa, speaking at the recent 2015 SAARC Wheat Surveillance training course in Kathmandu, Nepal.

Based on samples that William Wagoire sent to his laboratory from Kalyengere, Uganda in 1999, Pretorius was one of the first to help characterize Ug99 — the TTKSK race of stem rust that overcomes the Sr31 gene — thus prompting Dr. Norman Borlaug to sound the global alarm in 2005 that galvanized rust scientists and donors to fight the new race.

In his work on genetic resistance to rust diseases of field crops, particularly cereals, Pretorius is involved in the identification and genetic analyses of wheat germplasm — with an emphasis on durable resistance to leaf, stem and stripe rusts — and the establishment and refinement of evaluation and selection protocols.

In collaboration with colleagues in Minnesota, Denmark, Australia and Mexico, Dr. Pretorius has developed a research niche in assessing and utilizing adult plant resistance to cereal rusts focused on phenotyping and molecular markers.

Dr. Pretorius is an avid photographer whose skills as a macro photographer of rust using a range of camera and lens combinations were evident in his lectures in Nepal. Interestingly, he takes most of his rust pictures, especially those of infection types, with an uncomplicated Canon PowerShot SX100 IS camera. He and the students in his lab at UFS are currently focusing on the infection process of rust fungi on adult wheat plants as opposed to the more commonly studied histopathology of seedlings.

The accompanying electron micrographs that show the differentiation of infection structures of wheat rust fungi are from one of the lectures on rust fungi that Dr. Pretorius delivered to the South Asian Association for Regional Cooperation (SAARC) class for the Durable Rust Resistance in Wheat (DRRW) lecture series, “The Art and Science of Rust Pathology and Applied Plant Breeding,” in Kathmandu in March 2015, which is soon to be an on-line course.

“We have been able to study fungal development and infection structure formation in host plants in detail using light, fluorescence and scanning electron microscopy, as well as other methods of quantifying fungal colonization,” said Pretorius. “Most rust fungi enter the host plant through the stomata. They then develop a complex series of infection structures including appressoria, substomatal vesicles, primary infection hyphae and haustorial mother cells (HMCs), before invaginating host mesophyll cells by means of haustoria.”

Haustoria are the feeding structures through which host nutrients are taken up to support fungal colonization. Pretorius emphasized to the class that fungal development is greatly influenced by environmental conditions and by the genetics of the host/ pathogen interaction — topics for stem, leaf and yellow rust that he and others covered in the course.

“Comparing the visualization patterns in incompatible and compatible host/pathogen interactions provides pathologists and plant breeders with information about the extent of fungal ingress. You can tell at what stage and to what extent development is arrested with particular gene combinations,” said Pretorius.

Visualizing patterns is what this plant pathologist from South Africa does best. And it doesn’t stop in the lab or the field. Some of Pretorius’ lesser-known visualizations are with crossword puzzles about fungi and watercolors — and even those subjects are also often rust-related.

NOTE to all PATHOLOGISTS: The blog software doesn't allow us to italicize Puccinia graminis f. sp. tritici (Pgt) or P. striiformis in the captions even though we know we should!