A survey by the journal, Molecular Plant Pathology, had 495 responses from international fungal pathologists on what they thought the most scientifically and economically important fungal plant pathogens were. Several of the ‘top 10’ fungi from these results are those that infect cereal crops, which isn’t surprising as cereals such as wheat and rice are some of the most highly produced crops worldwide.
1. Magnaporthe oryzae (rice blast)
High economic importance as it is highly destructive to rice, which is a staple food for over half the world’s population. The disease can cause losses of 10-30% of the grain yield. The main method of control is to use resistant cultivars of rice. However, there is a constant race to produce new cultivars as the fungus usually overcomes resistance within a few years. There are some fungicides used that prevent the fungus from developing a specialised infection cell that would penetrate the host.
2. Botrytis cinerea (grey mould)
Known to infect more than 200 plant species. Causes cell death of the host. This species is a problem all the way along the crop production chain as it can infect seedlings but also not become apparent until the product has been sold. This has been the case in the reduction of shelf life of roses, grapes and soft fruit such as raspberries. Often these economic losses aren’t taken into consideration because the product has already been sold, but customers may be put off from buying these products in the future.
3. Puccinia spp. (wheat rust)
There are three wheat rust diseases: stem (black) rust caused by P. graminis f.sp. tritici, stripe (yellow) rust caused by P. striiformis f.sp. tritici, and leaf (brown) rust caused by P. triticina. The emergence of a particularly virulent strain of stem rust, Ug99, in East Africa has raised worries of food security. Since its discovery in 1999, it has spread via wind movements to South Africa and parts of the Middle East. Its ability to overcome so many different resistant cultivars and cause yield loss of over 70% is a serious threat to global wheat production.
4. Fusarium graminearum (head blight)
This fungus can infect all cereal species, and mainly leads to a reduction in grain quality rather than yield. It causes the production of mycotoxins within the grain, the levels of which are strictly controlled as they are hazardous to human and animal health if consumed. The most effective ways to control the disease are ploughing and rotating cereal crops.
5. Fusarium oxysporum (Fusarium wilt)
There are many F. oxysporum formae speciales that infect specific crops including banana (f.sp. cubense), tomato (f.sp. lycopersici) and melon (f.sp. melonis). These often cause wilt, vascular discoloration and plant dieback.
6. Blumeria graminis (powdery mildew)
Affects grasses such as wheat and barley, reducing grain yield by taking nutrients from the host plant. This pathogenic species persists as it evolves resistance to fungicides and becomes immune to the varietal resistance of cereals.
7. Mycosphaerella graminicola (Septoria tritici blotch disease)
Infects wheat by penetrating the stomata, producing necrotic lesions on the leaves. This fungal disease particularly affects wheat grown in temperate regions. The fungus has a particularly high genetic variation and is therefore able to rapidly evolve in response to new fungicides or resistant wheat cultivars. The availability of data on this species since the mid 19th century has enabled long-term studies into the evolution and population dynamics of plant pathogens.
8. Colletotrichum spp.
Causes anthracnose (a disease that produces dark lesions) on parts of the plant that are aboveground. This genus isn’t specific to a particular crop – almost all crops are susceptible to at least one Colletotrichum species. The fact that this disease often only manifests post-harvest means that stored fruit can be badly affected, with losses of up to 100%.
9. Ustilago maydis (corn smut)
This symptoms of this species are very noticeable but this pathogen hasn’t had a large scale, devastating effect as most of the other ‘top 10’ fungi. Many of the experts questioned for this ‘top 10’ survey nominated U. maydis for its scientific importance as a model organism. This species can easily be cultured, and its infection cycle of maize completes in two weeks, making it ideal for laboratory studies. It also has a fully mapped genome that is relatively straightforward to manipulate.
10. Melampsora lini (flax rust)
Another model system, the study of which has contributed to knowledge of pathogenicity genes that can be targeted when breeding crops for pathogen resistance. Further understanding of the species at a molecular level has led to advancements in plant immunity research.
This is by no means a comprehensive list of fungal pathogens that are important to plant science and agriculture. Limiting the number of “top” fungi has meant that species such as Phakopsora pachyrhizi (Asian soybean rust) and Rhizoctonia solani (which causes rice sheath blight and also affects many other crop species) are not included despite causing significant damage in parts of the world. The concept of a pathogen’s importance, particularly in the economic sense, can often differ between regions, and change over time. Which pathogens do you think should be in the ‘top 10’? Leave a comment below with your suggestions!
Source: DEAN, R., VAN KAN, J. A. L., PRETORIUS, Z. A., HAMMOND-KOSACK, K. E., DI PIETRO, A., SPANU, P. D., RUDD, J. J., DICKMAN, M., KAHMANN, R., ELLIS, J. and FOSTER, G. D. (2012), The Top 10 fungal pathogens in molecular plant pathology. Molecular Plant Pathology, 13: 414–430. doi: 10.1111/j.1364-3703.2011.00783.x