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Oomycete oospores © Richard Shattock

Oomycete oospores © Richard Shattock

In its latest issue, Molecular Plant Pathology has published a list of the top 10 oomycete pathogens based on scientific and economic importance, as voted for by 62 scientists. This is the latest of the journal’s “Top 10” articles which have previously covered plant pathogenic fungi, bacteria and viruses.

Oomycetes, also known as water molds, are often likened to fungi as they resemble filamentous fungi, and have similar feeding and reproduction methods. However, genetic analysis has shown that the two groups are actually phylogenetically very different and are now classified in separate kingdoms. Below you will find more information about the “Top 10” plant pathogenic oomycetes, with links to further information available on the Plantwise knowledge bank.  


1. Phytophthora infestans

Tomato affected by late blight © CABI

Tomato affected by late blight © CABI

Probably the most famous of all the oomycete pathogens is Phytophthora infestans, the causal organism of potato late blight, a disease that has caused thousands of hunger related deaths throughout history. Today, late blight remains an important threat to the yield of potato and other host crops such as tomato and pepper.

Leaves of potato plants infected with late blight initially show irregular water soaked grey lesions, which later become darker and more circular. Symptoms also appear on stems and tubers.

There are several disease management options for P. infestans, including the use of clean seeds at planting, proper plant spacing and application of fungicides such as mancozeb.

To find out more about Phytophthora infestans, click here.

= 2. Hyaloperonospora arabidopsidis

Hyaloperonospora spp. cause downy mildew on hundreds of plant species which can result in high crop losses. H. arabidopsidis is the causal organism of on Arabidopsis and has played a key part in the research into plants’ immune response to plant pathogens and the role that disease resistant mutants can play.

To find out more about Hyaloperonospora spp., click here.

Stem canker caused by P. ramorum on a tan oak © Matteo Garbelotto/U.C. Berkeley, USA

Stem canker caused by P. ramorum on a tan oak © Matteo Garbelotto/U.C. Berkeley, USA

= 2. Phytophthora ramorum

Coming joint 2nd in the poll is Phytophthora ramorum, the causal organism of diseases such as sudden oak death, sudden larch death and ramorum blight, and widely recognised as being the most damaging pathogen of oak trees around the world. Its wide host range, ability to survive in harsh climatic conditions and its propensity to reproduce asexually are just some of the attributes that make P. ramorum such a persistent and invasive pathogen.

To find out more about Phytophthora ramorum, click here.

4. Phytophthora sojae

The third Phytophthora species on the list, P. sojae, is responsible for root and stem rot of soybean, a disease which costs the soybean industry a substantial sum of money each year. Symptoms include decay, discoloration and damping off of the whole plant. The severity of the disease is dependent on the levels of resistance in the host. P. sojae was one of the first oomycetes to be genetically sequenced and was therefore key in finding out the molecular basis of oomycete virulence.

To find out more about Phytophthora sojae, click here.

Fruit rot of chilli © AgrEvo

Fruit rot of chilli © AgrEvo

5. Phytophthora capsici
Phytophthora capsici causes stem and fruit rot in a number of common crops such as pepper, tomato and cucurbits. It is one of the most genetically diverse oomycete species and is very difficult to control once it has infected a field. One method of overcoming infection is to rotate the crop by planting a non-host that cannot be infected by the pathogen.

To find out more about Phytophthora capsici, click here.

6. Plasmopara viticola

Coming 6th in the poll is Plasmopara viticola, which is responsible for downy mildew of grapes. This disease affects the leaves fruits and shoots of grape and left untreated, can cause losses of up to 75%. The control for this disease is usually the application of fungicides. P. viticola has been involved in numerous studies into fungicide resistance and the development of IPM techniques.

To find out more about Plasmopara viticola, click here.

Root rot of pineapple © Scot Nelson

Root rot of pineapple © Scot Nelson

7. Phytophthora cinnamomi

Phytophthora cinnamomi is a notorious plant pathogen, responsible for dieback in many hosts, including avocado, pineapple and economically important trees such as pines and firs. Its ability to move and survive in soil, water, dead plant material and wind, coupled with its wide host and destructive nature make this species a prime choice for ISSG’s 100 “World’s Worst” invaders.

To find out more about Phytophthora cinnamomi, click here.

=8. Phytophthora parasitica

The 6th and final Phytophthora species on the Top 10 is P. parasitica (=P. nicotianae), which is responsible for many diseases across a wide host range. Symptoms vary between hosts but the disease can affect any plant part and cause devastating losses to crop yields.  P. parasitica is able to survive in a range of conditions, making it difficult to control.  It is found throughout the world and is likely to benefit from the warming climate in future.

To find out more about Phytophthora parasitica, click here.

=8. Pythium ultimum

Pythium ultimum causes root rot and damping off on over 300 host species, including maize and wheat. It is ubiquitous and is able to survive in a range of conditions, making it very difficult to control. Common methods of control centre around field sanitation and fungicide application, but it has been the subject of research into biological control, using biocontrol agents such as as Trichoderma and Streptomyces.

To find out more about Pythium ultimum, click here.

Symptoms of white rust on turnip © J.P. Tewari

Symptoms of white rust on turnip © J.P. Tewari

10. Albugo candida

Albugo candida is the causal organism for white rust of crucifers and other host species. A plant infected with white rust has reduced photosynthetic ability, and therefore a lower yield, weaker stems and malformed inflorescence, resulting in a lower seed yield. This disease is associated with “green islands” – areas of seemingly healthy tissue on infected leaves, and can suppress plant immunity, increasing host susceptibility to diseases such as downy mildew.

To find out more about Albugo candida, click here.


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