Pythium ultimum is aplant pathogen. It causesdamping off androot rot diseases of hundreds of diverse planthosts includingmaize,soybean,potato,wheat,fir, and many ornamental species.[1][2]P. ultimum belongs to theperonosporalean lineage ofoomycetes,[3] along with other important plant pathogens such asPhytophthora spp. and many genera ofdowny mildews.P. ultimum is a frequent inhabitant of fields, freshwater ponds, and decomposing vegetation in most areas of the world. Contributing to the widespread distribution and persistence ofP. ultimum is its ability to growsaprotrophically in soil and plant residue. This trait is also exhibited by mostPythium spp. but not by the relatedPhytophthora spp., which can only colonize living plant hosts.
P. ultimum is a species complex that includesP. u. var.ultimum andP. u. var.sporangiiferum.[4] One major distinguishing feature between these two genetically distinct organisms is the production ofzoospores (swimming spores) -- which are produced only rarely byP. u. var.ultimum. Asexual reproduction of bothP. u. var.sporangiiferum andP. u. var.ultimum results in the formation of sporangia that develop at the tips of hyphae.[5] Wind, water or other disruptions to the soil can disperse the pathogen by causing the sporangia to detach. In the case ofP. u. var.sporangiiferum, the free sporangia release zoospores in response to outside stimuli. These zoospores can then "swim" to susceptible root tissues. This infection process is referred to as "indirect germination". Contrary toP. u. var.sporangiiferum, the free sporangia ofP. u. var.ultimum do not release zoospores, instead, they undergo a process called "direct germination", during which the sporangia themselves form invasive hyphae that serve as the primary inoculum source. Generation of these infectious hyphae is initiated once the free sporangia have made contact with susceptible plant tissues. Once attached, they form appressoria; specialized infection structures that can generate enough turgor pressure to punch through the plant cuticle.[6] From there, both variants engage in necrotrophy, a process by which pathogenic organisms kill host cells in order to access and incorporate their contents to meet their nutritional needs.[7]P. u. var.ultimum in particular, is known to release a cascade of unique effector proteins to break down and degrade various cellular components of plant tissues.[8] Both species makeoospores, which are thick-walled structures produced by sexual recombination that can serve as survival structures during times of stress.[9] Both varieties are self-fertile (homothallic), which means that a single strain can mate with itself.[10]One important ecological difference between the different types of spores produced by these organisms, is that sporangia and zoospores are short-lived, while the thick-walled oospores can persist for years within soil, surviving even winter freezes.[11] Common signs of aPythium infection include stunting of the plants, brown coloration of root-tips, and wilting of the plant during the warm part of the day.[12] Management of disease is challenging but focuses onsanitation,fungicides, andbiological control. Fungicides includemefenoxam,thiadiazole,etridiazole,propamocarb,dimethomorph, andphosphonates. Biological control agents include the bacteriaBacillus subtilis,Enterobacter cloacae,Streptomyces griseoviridis, and the fungiCandida oleophila,Gliocladium catenulatum,Trichoderma harzianum, andT. virens.[13][14]
Effective resistance in the plant host is generally not available. Sanitation is very important since the pathogen can be easily introduced into pasteurized soil or even soil-free potting mixes on dirty tools or pots. Especially in greenhouses, fungus gnats may also help move the pathogen from place to place. A recent study of greenhouses in Michigan revealed that the same pathogen populations were responsible for the root rot of all greenhouse ornamental plants over a two-year period. These results stress the importance of sanitation and encourage greenhouse growers to improve their scouting of all incoming plant material to prevent additional root rot.[15]
Pythium ultimum isdivided into varietiesultimum andsporangiiferum, the genomes of both of which have been sequenced.[16][17] Analysis of the genomes suggest that the two species encode 15,290 and 14,086 proteins, respectively.
While fungicides and proper sanitation measures remain important means ofPythium control and management, natural means of suppression via the formation of disease suppressive environments, is becoming better understood, and could pave the way for more sustainable practices in commercial production of crops susceptible toP. ultimum.
Disease suppressive environments are defined as environments in which environmental conditions are favorable, a susceptible host is present along with a virulent pathogen, but disease levels remain low.[20] The soil microbiome of plants is known to influence soil-borne diseases incidence and severity by either indirectly combatting disease by fortifying plant defenses or through direct microbe-microbe interactions, thus helping to create a disease suppressive environment.[21][22][23] BothP. u. var.sporangiiferum andP. u. var.ultimum are not immune to the impacts of microbiome-based disease suppression. Many studies have correlated increased suppression ofP. ultimum with an increase in total abundance of microorganisms present in growth media microbiomes.[24][25] For example, increased microbial activity and biomass has been correlated with a decrease inP. ultimum induced damping-off of cucumber.[26][27] The method of microbe-mediated suppression ofPythium remains uncertain, however, these and other studies suggest that competition in the rhizosphere for carbon and nitrogen resources could play a role.[28] Another theory suggests that at high enough concentrations, bacteria can quickly coat and protect susceptible plant tissues, leaving no room forPythium adherence and subsequent infection.[29]
^Schroeder, Kurtis L.; Martin, Frank N.; De Cock, Arthur W. A. M.; Lévesque, C. André; Spies, Christoffel F. J.; Okubara, Patricia A.; Paulitz, Timothy C. (2013). "Molecular Detection and Quantification of Pythium Species: Evolving Taxonomy, New Tools, and Challenges".Plant Disease.97 (1):4–20.Bibcode:2013PlDis..97....4S.doi:10.1094/PDIS-03-12-0243-FE.PMID30722255.
