Classic: The 3 different lifestyles of fungal pathogens

Fungal pathogens are a heterogeneous group of organisms which differ in many important traits such as mode of nutrition, type of reproduction, dispersal mechanisms and survival. Traditional classification of fungal pathogens has relied on their mode of nutrition to classify them into 3 broad categories, biotrophs, necrotrophs, and hemibiotrophs (figure 1 and table 1).

• Biotrophs derive nutrients and energy from living cells. They enter intact plant cells and create feeding organs in the epidermis to derive nutrition from their host. The host is gradually infected through leaves, stems and fruits and loses its photosynthetic capacity and will become less productive. The host does remain alive during a long period of infection
• Hemibiotrophs have dual life-styles and start feeding as a biotroph and then switch to necrotrophy. Initially, they invade living cells with minimum damage to the plant tissues, while the fungus obtains nutrients from the host. Due to a shift in environmental conditions they will start with their necrotrophic lifestyleand start killing the host cells.
• Necrotrophs actively attack living organisms and then feed on the dead ordecaying plant cells. They benefit from wound openings and once they enter the plant, the host plant rapidly declines and systemically infected plant parts can’t be cured. Saprotrophs share similarities with necrotrophs, except for that they only live on dead organic material and these are not able to actively infest a plant.

New research: CAZ focuses on carbon degrading enzymes

2024 research put forward a new classification based on genome-derived analysis of carbohydrate-active enzyme (CAZyme) gene content called CAZyme-Assisted Training And Sorting of -trophy (CATAStrophy). See figure 2.

• Saprotrophs: In this new classification, the saprotrophs, or so-called ‘pioneer’ fungi can be present on fruit stems or flower buds and include Penicillium, Mucor or Rhizopus species. These are not considered as plant pathogens as they are not able to infest healthy plant tissue, but their presence causes cosmetic damage.
• Monomertrophs: Monomertrophs are characterized by low CAZyme and secondary metabolite gene contents, producing fewer pathogen and damage- associated molecular patterns, (PAMP & DAMP) and few secondary metabolites. 1) The biotroph fungi are reallocated and grouped as monomertrophs and feed via specific structures, haustoria, which disrupt the host cell membranes and permit the adsorption of nutrients directly from the host cytoplasm (Mildew,Rust, Pythium, Phytophthora). 2) Within this group, the waterborne fungi, Pythium and Phytophthora are not considered true fungi, as they have cell walls made primarily of cellulose and β-glucans instead of chitin. They produce swimming spores, by which they can spread rapidly by rain, irrigation and splash water.‍
• Mesotrophs: The group of mesotrophs can be divided into fungal species that invade host tissues intracellular or extracellular. Extracellular Mesotrophs fungi don't produce virulent toxins and do not form appressoria before penetration of plant cells (e.g. Venturia, Mycosphaerella). They have a relatively long latent phase before they switch their lifestyle strategy and speed up the infection process. The intracellular Mesotrophs do produce appressoria prior to infection (Colletotrichum graminicola) and have a shorter latent growing phase than the extracellular Mesotrophs.‍
• Vasculartroph and Polymertroph: Fungal pathogens with higher carbohydrate-active enzyme production are classified as vasculartroph or polymertroph with a broad or narrow host range. These pathogens secrete a battery of toxins and cell-wall degrading enzymes, which explains their ability to infest a broad range of plant species.

The 6 classes of fungal pathogens that PAW is targeting

Keeping the specific mode of action of PAW in mind, the best efficacy can be expected when the fungal spores remain at the surface of the plant for a period of their lifecycle and produce low amounts of secondary plant metabolites.

These fungal species are classified as monomertrophs or extracellular mesotrophs and include the following pathogen categories.

• Monomertrophs: Mold, Rust, Mildew, Spots
• Extracellular mesotrophs: Venturia, Mycosphaerella

The saprotrophic fungi can be also targeted, when they are present on the outside of stems or leaves, before they enter wounded plant tissue.

Extensive and independent trial evidence is available for the efficacy of Plasma Activated Water against Powdery Mildew in crops like Cucumber, Strawberry, Gerbera and Cut Roses. Multi-year evidence is available for Venturia in Apple and initial research has been conducted for Mycosphaerella in Cucumber.

The combination of expected efficacy of Plasma Activated Water combined with market relevance, has led to the following pathogens currently being in scope: Powdery Mildew, Downy Mildew, Venturia, Cladosporium, Phytophthora Infestans, Rust.

All pathogens mentioned above are part of our research program, and aim to collect trial evidence for the crops and regions where these pathogens lead to significant yield loss.

References

• 1. Judelson, H.S. and A. M. V. Ah-Fong (2019). Update on Plant-Oomycete Interactions, Exchanges at the Plant-Oomycete Interface That Influence Disease. Plant Physiology 179: 1198–1211.
• 2. Hane, J.K., J. Paxman, D.A.B. Jones, R.P. Oliver and P. de Wit (2020). “CATAStrophy,” a Genome-Informed Trophic Classification of Filamentous Plant Pathogens – How Many Different Types of Filamentous Plant Pathogens Are There? Frontiers Microbiology 10:3088.