Plant pathology is a discipline that studies diseases in plants. Disease agents can be very diverse, from phytoplasmas, virus, bacterias, fungi, nematodes to insects. Fungal plant diseases affect all types of plants and crops, causing abundant economic and ecologic loses worldwide.

 

Fungi are one of the most hyper diverse groups of organisms and play a fundamental contribution in all ecosystems – the estimated number of fungal species can be higher than 1.5 million species, being more than 8,000 plant pathogens (1,2). They participate in the carbon and nutrient cycles; decompose organic matter, and their roles range from mutualists, endophytes, symbionts, saprotrophs, and parasites and pathogens of plants, animals and humans. They are also able to produce valuable substances, like secondary metabolites and antibiotics, being penicillin the most famous one, discovered by Alexander Fleming in 1942, which is derived from a fungus called Penicillium.

Oomycetes, also known as water molds, are microorganisms situated in a different kingdom, named Chromalveolata. They are very similar morphologically to fungi and have been historically studied in mycology, but however, they are closely related to brown algae. This group of microorganisms host some of the most damaging plant pathogens to date, affecting forests, ornamental plants and agricultural crops (3).

 

One of the most tragic and remarkable plant diseases is potato blight caused by an oomycete, Phytophthora infestans. It was the causal agent of the Irish Potato Famine (or Great Famine) in mid 19th century, producing disease, starvation and emigration, reducing the Irish population more than 25%. Anton de Bary, a German mycologist who is considered the father of the actual plant pathology discipline, named P. infestans as the cause of the potato blight two decades after it (4).

(1) Anonymous. 2017. Stop neglecting fungi. Nature Microbiology 2, doi: 10.1038/nmicrobiol.2017.120
(2) Hawksworth, D.L. 2001. The magnitude of fungal diversity: the 1.5 million species estimated revisited. Mycology Research 12, 1422-1432.
(3) Erwin, D.C., Ribeiro, O.K. 1996. Phytophthora diseases worldwide. St. Paul, Minesota, USA. The American Phytopathological Society. 562 pp.
(4) Lamour, K. 2013. Phytophthora. A global perspective. Knowville, USA. CABI Plant Protection Series 2. 244 pp.

Relevant fungal plant pathogens

Phytophthora ramorum

Phytophthora ramorum is the pathogen causing ramorum blight, sudden oak death (SOD) in North America, and sudden larch death in Europe. It has caused massive deaths of coastal tanoak and oaks (Lithocarpus densiflorus, Quercus agrifolia, Q. kelloggii, Q. parvula var. shrevei), japanese larch (Larix kaempferi), as well as woody ornamentals like rhododendron plants, camellias or viburnums. This disease has also a high impact in the nursery industry. 

It is believed the pathogen is introduced in both territories and strong quarantine regulations are being applied to contain its spread. P. ramorum is already present in Europe, in both nurseries and forest ecosystems, being recently reported in France affecting larch trees, and confirming its spread.

Currently, this pathogen has four different clonal lineages, NA1 and NA2 present in North America, and EU1 and EU2 in Europe, named after the continent they were reported for the first time. However, nowadays EU1 lineage has been found in USA.

Fusarium circinatum

The Ascomycete Fusarium circinatum is the causal agent of the pitch canker disease. It affects different pine trees (over 60 pine species), and Douglas-fir, Larix and Picea species, causing an extensive mortality. Its presence in Europe is limited to Spain, Portugal, and it has been already eradicated in Italy and France. 

The main symptom is the presence of a characteristic white resin exudate in the trunk of the infected tree and yellowing of pine needles. It also infects seeds, causing damping off and root disease in seedlings. 

This pathogen is considered native from Mexico, Haiti and southeastern USA, and it has spread to different continents, like South America, Africa, Asia and Europe. 

In Europe it is considered and listed as a regulated quarantine species, and regulations were placed in 2007 to avoid its introduction in EU territory and spread (Commission Decission 2007/433/EC). 

Phyllosticta citricarpa

Phyllosticta citricarpa is an important fungal pathogen of citrus trees (Citrus spp.): orange, lemon, grapefruit, lime and mandarin trees. However, other plant species are able to host this pathogen, like almond trees or eucalyptus, among others.

This ascomycete causes the CBS disease (Citrus Black Spot), and produces enormous losses in citric crops, depreciating the value of fruits and making them not suitable for the fruit market. The main symptom is the appearance of hard spot lesions in the outer peel of the fruit, and subsequently, early fall from the tree.

It is absent in the EU, but is present in Asia, USA, Central and South America, Oceania and Africa, being Tunisia the last report of this species. In Europe, it has been reported in citrus leaf litter in Italy, Portugal and Malta, but up date, it has never been found affecting citrus trees. P. citricarpa is also listed in A1 EPPO list, as welll as in EU quarantine list.