21st Century Guidebook to Fungi, SECOND EDITION, by David Moore, Geoffrey D. Robson and Anthony P. J. Trinci

Table of Contents

1. Chapter 13: Ecosystem mycology: saprotrophs, and mutualisms between plants and fungi
2. Chapter 14: Fungi as pathogens of plants
   1. Fungal diseases and loss of world agricultural production
   2. A few examples of headline crop diseases
   3. The Rice Blast fungus Magnaporthe grisea (Ascomycota)
   4. Armillaria (Basidiomycota)
   5. Pathogens that produce haustoria (Ascomycota and Basidiomycota)
   6. Cercospora (Ascomycota)
   7. Ophiostoma (Ceratocystis) novo-ulmi (Dutch Elm disease or DED) (Ascomycota)
   8. Black stem rust (Puccinia graminis f. sp. tritici) threatens global wheat harvest
   9. Plant disease basics: the disease triangle
   10. Necrotrophic and biotrophic pathogens of plants
   11. The effects of pathogens on their hosts
   12. How pathogens attack plants
   13. Host penetration through stomatal openings
   14. Direct penetration of the host cell wall
   15. Enzymatic penetration of the host
   16. Pre-formed and induced defence mechanisms in plants
   17. Genetic variation in pathogens and their hosts: co-evolution of disease systems
   18. Chapter 14 References and further reading
   19. Buy a PDF of Chapter 14
3. Chapter 15: Fungi as symbionts and predators of animals

14.10 Necrotrophic and biotrophic pathogens of plants

Plant pathogens are often divided into biotrophs and necrotrophs, (and, more recently, hemibiotrophs) according to their lifestyles. The definitions of these terms are:

• biotrophs derive energy from living cells, they are found on or in living plants, can have very complex nutrient requirements and do not kill host plants rapidly;
• necrotrophs derive energy from killed cells; they invade and kill plant tissue rapidly and then live saprotrophically on the dead remains;
• hemibiotrophs have an initial period of biotrophy followed by necrotrophy.

This classification suggests a range of generalisations (Table 1) that together indicate clearly that the biotroph/necrotroph division is biologically meaningful. In particular, the genetic analysis of disease resistance with plants for which the full genomes are available, such as Arabidopsis thaliana, demonstrates that the division is based on how defence against fungal pathogens is controlled. Hane et al. (2020) suggest that this traditional classification (into biotrophs, hemibiotrophs, or necrotrophs) should be replaced by a seven-class scheme of trophic phenotypes based on genome-derived analysis of carbohydrate-active enzyme gene content. It is too early to know how widely this scheme will be applied.

Defence against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defence responses regulated by the salicylic acid-dependent pathway. Necrotrophic pathogens benefit from death of host cells, so they are not limited by this defence, but by responses activated by jasmonate acid and ethylene signalling pathways (see below). Such a ‘mode-of-defence’ division successfully distinguishes necrotrophs and biotrophs but it does limit the biotroph category to fungi that produce haustoria (Oliver & Ipcho, 2004; Glazebrook, 2005).

Table 2 gives some examples of necrotrophic and biotrophic pathogens, and includes some hemibiotrophic pathogens, like Phytophthora infestans, that exhibit characteristics of both biotrophs and necrotrophs.

Updated January, 2020