What is the Difference Between Pythium and Phytophthora

The main difference between Pythium and Phytophthora is that Pythium primarily attacks monocotyledonous herbaceous plants and some species of Pythium attack mammals, fish, and red algae whereas Phytophthora especially attacks dicotyledonous woody trees, shrubs, and herbaceous plants. Furthermore, the rotting of roots, slow growth, stunting, and chlorotic foliage are the symptoms of Pythium disease while the rotting of roots and stem, stunting, discoloration and wilting are the symptoms of Phytophthora disease. 

Pythium and Phytophthora are two genera of plant-damaging oomycetes. The fungi that belong to the class Oomycota are also known as water molds. 

Key Areas Covered 

1. What is Pythium
     – Definition, Characteristics, Importance
2. What is Phytophthora
     – Definition, Characteristics, Importance
3. What are the Similarities Between Pythium and Phytophthora
     – Outline of Common Features
4. What is the Difference Between Pythium and Phytophthora
     – Comparison of Key Differences 

Key Terms 

Parasitic Fungi, Phytophthora, Pythium, Soil-Borne, Sporangia, Zoospores

What is Pythium 

Pythium is a genus of destructive, parasitic fungi mainly responsible for rotting roots in plants. It belongs to the family Pythiaceae of order Pythiales. Also, it contains around 355 described species. Especially, this genus infects monocotyledonous herbaceous plants. Therefore, it causes serious crop damages to both cereal crops and turf grass. Furthermore, it leads to the soft rot of fruit, rot of roots and stems, and pre- and post-emergence of seeds and seedlings by infecting mainly juvenile or succulent tissues. Pythium infections are common in tropical to temperate regions. On the other hand, it causes pythiosis in mammals including humans, fish, and marine algae. Some Pythium species produce substances including polyunsaturated fatty acids, arachidonic acid, and eicosapentaenoic acid, biotin, folic acid, pantothenic acid, riboflavin, thiamine, and vitamin C, etc. Other mycoparasitic species of Pythium can be used to control plant diseases caused by other fungi.

Figure 1: Pythium

The sporangia of Pythium lack the apical thickening as seen in Phytophthora. Also, these sporangia are produced in water and they are non-caducous. Moreover, the discharge of the protoplasm of the sporangium through an existing tube, forming a vesicle at the end is responsible for the differentiation of zoospores. The differentiated zoospores are released upon the rupture of the vesicle.  

What is Phytophthora 

Phytophthora is another genus of destructive, parasitic fungi responsible for rotting roots. Most importantly, this genus has been newly classified under the family Peronosporaceae of order Peronosporales. The genus also contains 313 described species. However, Phytophthora is a soil-borne pathogen that can infect dicotyledonous woody and herbaceous plants. As it infects both natural and cultivated plants, this genus causes a serious loss in both natural forest ecosystems and agriculture respectively. It may cause root rot, basal stem rot, leaf spot or blight as well as fruit rot.

Figure 2: Phytophthora

Furthermore, the right-angled branches of Phytophthora are the main distinguishing feature that helps to identify the aseptate fungi. Also, hyphal swellings can often be identified in Moreover. Furthermore, there are three types of sporangia in Phytophthora based on the characteristics of the apex. They are (I) conspicuously papillate with hemispherical apical thickening over 3.5µm deep (II) inconspicuously papillate (semi-papillate) with shallow apical thickening under 3.5µm deep, and (III) non-papillate, without noticeable apical thickening. Therefore, the differentiation of zoospores in Phytophthora occurs inside the sporangium. In addition, after the proper maturation, these zoospores released in an evanescent vesicle at the sporangial apex.  

Similarities Between Pythium and Phytophthora 

• Pythium and Phytophthora are two genera of fungi that belong to the class Oomycota. 
• Generally, these two genera are fungal-like-organisms or psuedofungi classified under the Kingdom Chromista or Kingdom Straminipila, distinct from Kingdom Fungi.  
• Although their taxonomic classification is highly controversial, their production of globose oogonia in the sexual reproduction has placed them in the phylum Oomycota.  
• Moreover, they were known as Pythiaceous fungi before and have been classified in the same family and order, Pythiaceae and Peronosporales respectively.  
• Also, they have very similar morphology with coenocytic, hyaline, and freely branching mycelia. 
• Furthermore, their mycelia are diploid and their cell walls are made up of cellulose. 
• During their sexual reproduction, they produce oospores; during their asexual reproduction, they produce zoospores.  
• Besides, their zoospores are heterokont and contain two laterally inserted flagella; the anterior flagellum being tinsel-like and the posterior flagellum being smooth and whiplash, inserted at the same point.  
• Additionlly, their oogonia contain a single oospore.  
• However, they have a definitive differentiation in zoospore differentiation and discharge. 
• Along with the other two genera, Fusarium and Thielaviopsis, they are the most common soil-borne pathogens that infect bedding plants. 
• They are mainly responsible for root rots.  
• In addition, they produce common symptoms such as chlorosis, stunting and wilting in plants upon infection. 
• Both genera can be grown in the V8 medium. 

Difference Between Pythium and Phytophthora 

Definition 

Pythium refers to a genus of destructive root-parasitic fungi having filamentous sporangia, smooth-walled spherical oogonia, and stalked antheridia and including forms that cause damping-off while Phytophthora refers to a genus of destructive parasitic fungi having conidia that usually act as sporangia, especially under moist cool conditions and sporangiophores that are simple or branched. Thus, this is the fundamental difference between Pythium and Phytophthora.

Classification Based on the Recent Molecular Phylogenetic Studies 

Moreover, Pythium is placed in the family Pythiaceae of order Pythiales while Phytophthora is placed in the family Peronosporaceae of order Peronosporales.  

Types of Infection 

Furthermore, Pythium primarily attacks monocotyledonous herbaceous plants and some species of Pythium attack mammals, fish, and red algae whereas Phytophthora especially attacks dicotyledonous woody trees, shrubs, and herbaceous plants. Thus, this is the main difference between Pythium and Phytophthora.

Symptoms 

Also, an important difference between Pythium and Phytophthora is their symptoms. The rotting of roots, slow growth, stunting, and chlorotic foliage are the symptoms of Pythium disease while the rotting of roots and stem, stunting discoloration and wilting are the symptoms of Phytophthora disease. 

Width of Hypha 

Width of hypha is another difference between Pythium and Phytophthora. Pythium has narrow hyphae, 4-6 μm in diameter, while Phytophthora has wider hyphae, 5-7 μm in diameter. 

Growth Rate 

Additionally, Pythium is fast growing while Phytophthora is slow growing. 

Type of Growth on Agar  

Besides, Pythium produces more flexuous or meandering hyphae while Phytophthora produces approximately right-angle branching hyphae. Hence, this is also a difference between Pythium and Phytophthora.

Differential Media (Ex: PARPH-V8) 

One other difference between Pythium and Phytophthora is that the hymexazol present in the PARPH-V8 medium inhibits the growth of Pythium species while PARPH-V8 medium allows the growth of most of the Phytophthora species.  

Zoospore Differentiation  

In Pythium species, the protoplast of a sporangium is transferred usually through an exit tube to a thin vesicle outside the sporangium while, in Phytophthora species, the zoospores are differentiated within the sporangium proper. 

Zoospore Discharge 

Zoospores in Pythium are differentiated and released upon the rupture of the vesicle while the zoospores in Phytophthora are released in an evanescent vesicle at the sporangial apex after maturation.  

Conclusion 

Pythium is a genus of destructive, root-parasitic fungi in plants. It belongs to the family Pythiaceae of order Pythiales. Also, it has narrow, more branched hyphae and shows fast growth. In addition, the protoplast of a sporangium of Pythium is transferred usually through an exit tube to a thin vesicle outside the sporangium where zoospores are differentiated and released upon the rupture of the vesicle. On the other hand, Phytophthora is another genus of destructive, root parasitic fungi that belong to the family Peronosporaceae of order Peronosporales. It contains wider hyphae and shows slow growth. Moreover, the zoospores of Phytophthora are differentiated within the sporangium proper and when mature, released in an evanescent vesicle at the sporangial apex. Therefore, the main difference between Pythium and Phytophthora is zoospore differentiation and discharge. 

References:

1. Ho HH. The taxonomy and biology of Phytophthora and Pythium. J Bacteriol Mycol Open Access. 2018;6(1):40‒45. DOI: 10.15406/jbmoa.2018.06.00174

Image Courtesy:

1. “Pythium (257 23)” By Doc. RNDr. Josef Reischig, CSc. – Author’s archive (CC BY-SA 3.0) via Commons Wikimedia  
2. “Phytophthora parasitica sporangia and zoospores” By Supattra Intavimolsri Department of Agriculture, Thailand (CC BY 3.0 au) via Commons Wikimedia