Invincible Plant Pathogens – Inaugural Lecture by External Member Levente Kiss

In 2019, the Food and Agriculture Organization of the United Nations (FAO) estimated the losses caused by plant diseases in world crop production at approximately US$220 billion per year. This huge amount represents the quantitative and qualitative yield losses caused by crop diseases on a global scale, plus the cost of pesticides and other crop protection methods used to control them. Microscopic fungal species cause the majority of plant diseases worldwide; in addition, a number of bacterial species and viruses are among the plant pathogens that cause significant economic losses on a global scale.

Can we hope that the research that underpins modern crop protection techniques will make it possible to “control” the most important plant diseases in the foreseeable future? This question will be explored through the example of a significant group of plant pathogens. Certain species of powdery mildew fungi (Erysiphaceae, Helotiales, Ascomycota), such as Blumeria spp., which infects cereals, and Erysiphe necator, which infects grapes, are among the most costly plant pathogens worldwide, based on the combined costs of crop losses and the cost of plant protection procedures applied against them. A number of other powdery mildews also cause significant damage and expenditure in arable, horticultural and ornamental crops, as well as in forestry, worldwide. In total, more than 900 species are known, which together are capable of infecting more than 10,000 cultivated and wild plant species (only closed crops), of which a significant number are also considered invasive plant pathogens in different parts of the world. Australia is probably the best example of how some species can spread very rapidly across a continent and infect new host plants.

Levente Kiss
(For more pictures click on the photo) Photo: mta.hu / Tamás Szigeti

Chemical control of powdery mildew infestations is complicated by the relatively rapid development of fungicide resistance in some species, the molecular mechanisms of which are now well understood. Breeding for disease resistance in cultivated crops has led to the development of several mildew-resistant varieties, but the plastic, strikingly large transposon-laden genomes of these pathogens encode several genetic mechanisms that continually break down the mildew resistance of new varieties. Ampelomyces hyperparasites, natural enemies of powdery mildew fungi worldwide, are unique among microorganisms found on plants in their ability to specifically control the spread of these plant pathogens. However, this property has not yet been exploited in practical plant protection, although they are considered a textbook example of tritrophic relationship systems.

The use of specific small RNA molecules produced by infected host plants or even by hyperparasites for plant protection is currently at the forefront of research. Experiments have already demonstrated the anti-fungal activity of some small RNAs, but no real breakthroughs have been made yet.

This supports the somewhat provocative title of the presentation. Within the framework of integrated pest management, as a result of continuous international research and development work, the ever-changing populations of plant pathogens can usually be reduced year by year to levels below those that cause economic damage, but there is no chance of literally “defeating” them, of eliminating them from agroecosystems.