''Phytophthora sojae'' is an
oomycete and a soil-borne
plant pathogen that causes stem and
root rot
Root rot is a condition in which anoxic conditions in the soil or potting media around the roots of a plant cause them to rot. This occurs due to excessive standing water around the roots.-Hydroponics Root Rot: What is It, How To Treat It, How ...
of
soybean
The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean. Soy is a staple crop, the world's most grown legume, and an important animal feed.
Soy is a key source o ...
. This is a prevalent disease in most soybean growing regions, and a major cause of crop loss.
In wet conditions the pathogen produces
zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion in aqueous or moist environments. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Certain zoospores are ...
s that move in water and are attracted to soybean roots. Zoospores can attach to roots, germinate, and infect the plant tissues. Diseased roots develop lesions that may spread up the stem and eventually kill the entire plant. ''Phytophthora sojae'' also produces
oospores that can remain dormant in the soil over the winter, or longer, and germinate when conditions are favourable. Oospores may also be spread by animals or machinery.
''Phytophthora sojae'' is a
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...
organism with a
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
size of 95
Mbp (Millions of base pairs).
The natural chemical
farinomalein (a metabolite from
entomopathogenic fungus ''
Paecilomyces farinosus''
[Sastia P. Putri, Hiroshi Kinoshita, Fumio Ihara, Yasuhiro Igarashi and Takuya Nihira. ''Farinomalein, a Maleimide-Bearing Compound from the Entomopathogenic Fungus ''Paecilomyces farinosus. J. Nat. Prod., 2009, 72 (8), pp 1544-1546 ]) has shown potent and selective inhibition (0.15-5 μg/disk) against eight isolates of plant pathogenic ''Phytophthora sojae''.
[Sastia Prama Putri, Hiroshi Kinoshita, Masayasu Kato and Takuya Nihira. ''Antimicrobial and antioomycete activities of the novel antibiotic farinomalein''. Poster Presentation 2P-2124, Annual Conference, The Society for Bioscience and Bioengineering, Japan, 28 October 2010.] These results suggest that farinomalein might be useful as a candidate
pesticide
Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...
for the treatment of ''Phytophthora'' stem rot in soybean.
''Phytophthora sojae'' is so similar to ''
Phytophthora megasperma'' that they're often mistaken for each other. In the early years of research, ''Phytophthora sojae'' and ''
Phytophthora medicaginis'' were respectively known as ''Phytophthora megasperma''
f. sp. ''glycines'' and ''Phytophthora megasperma'' f. sp. ''medicaginis''.
Recent discoveries about their molecular structure, however, proved that they were indeed unambiguous
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
.
Hosts and symptoms
''Phytophthora sojae'' infects soybean plants ''(Glycine max)'' and many members of the genus ''
Lupinus
''Lupinus'', commonly known as lupin, lupine, or regionally bluebonnet, is a genus of plants in the legume family Fabaceae. The genus includes over 199 species, with centre of diversity, centres of diversity in North America, North and South A ...
''. They have the ability to infect soybeans at any point during its development process, including during seed development.
They cause seed decay and
pre- and
postemergence damping off
Damping off (or damping-off) is a horticultural disease or condition, caused by several different pathogens that kill or weaken seeds or seedlings before or after they germinate. It is most prevalent in wet and cool conditions.
Symptoms
There ar ...
when the soil is flooded after planting. Seedling roots may appear to have light brown soft rot as soon as the seedlings begin to sprout from the soil.
It also causes root and stem rot and the severity of the infection depends on how susceptible or tolerant the plant is to pathogens. In a highly-tolerant soybean plant, the root rot will simply cause the plant to be stunted and slightly
chlorotic instead of killing the plant. In contrast, infection of a low-tolerant soybean plant will most likely lead to the death of the plant. Infection initiates in the roots and then progresses several
nodes up the stem, turning the root and the stem brown and the leaves yellow.
[Jee, H., W. Kim, and W. Cho. 1998. Occurrence of Phytophthora root rot on soybean (Glycine max) and identification of the causal fungus. Crop Protection 40:16-22.] As the pathogen progresses, the entire plant transforms into an orange-brown color. The wilted leaves bend towards the plant and remain attached as it succumbs to death.
[Erwin, D.C. and O.K. Ribeiro. 1996. Phytophthora Diseases Worldwide. APS Press, St. Paul MN.]
Foliar blight is also a symptom of ''Phytophthora sojae'', especially when the plant has recently experienced heavy rain. The soybean plant has an age-related resistance in which the older leaves are not susceptible to foliar blight.
Soybean fields infected with ''Phytophthora sojae'' can be easily spotted by looking for stunted soybean plants or looking for empty patches where the soybean seed had been planted.
Microscopic identification of an oospore that measure around 40 micrometers in diameter from a soybean plant sample is a definite sign of ''Phytophthora sojae''. Oospores, in general, measure around 20-45 micrometers in diameter and have very thick cellulose cell walls for overwintering.
[Agrios, George N. 2005. Plant Pathology. 5th ed. Elsevier Academic Press, Burlington, MA.]
Disease cycle
''Phytophthora sojae'' overwinters in plant debris and soil as oospores. Oospores are made after the male
gamete
A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
,
antheridium, and female gamete,
oogonium
An oogonium (: oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes.
In the mammalian fetus
Oogonia are formed in large ...
, undergo fertilization and then sexual recombination (
meiosis
Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
). They possess thick
cell wall
A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...
s with
cellulose
Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...
that enables them to survive harsh conditions in the soil without
germinating for several years. They begin to germinate once the environmental condition is favorable during spring (see
§ Environment) and produce
sporangia
A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...
. They can either germinate directly or indirectly. In direct germination, sporangia directly penetrate the host cells at the plant's root tips (if it's within reach). Indirect germination involves sporangia releasing
zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion in aqueous or moist environments. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Certain zoospores are ...
s (if the root is at a farther distance from the sporangia) which
encyst on the host plant cells and germinate. Zoospores are
biflagellate asexual motile spores. They are dispersed by water flow in the soil and are able to inoculate the roots of plants or seeds. Chemicals such as
daidzeins and
genistein are released at the tip of the plant roots which attract the liberated zoospores.
Once zoospores have made contact with the host root, they encyst on the surface, break down the plant cell wall with
proteolytic enzymes and begin to germinate.
Their
hypha
A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium.
Structure
A hypha consists of one o ...
e will begin to grow through the intercellular space of the plant cells. After establishing its haustoria for nutrients, more oospores will begin to form in the cortical cells of the root. The host plant will begin to exhibit secondary symptoms such as stem canker, wilting, and chlorosis as ''Phytophthora sojae'' continue to reproduce. This continuous reproduction renders the plant dead at the end of the season. The oospores are then left to overwinter in the dead plant's debris and the soil. The cycle is repeated once again in the spring when environmental conditions are favorable (see
§ Environment). The disease is mostly localized where zoospores initially infected the host plant.
''Phytophthora sojae'' is considered to be a monocyclic pathogen and has one effective infection in its cycle. This is because the oospores don't germinate together at the same time; rather they each have their own distinct favorable condition in which they'll initiate their germination.
Environment
''Phytophthora sojae'' favor fields that are poorly drained or highly-susceptible to flooding. Solving it only by creating optimal drainage does not restrict the pathogen because the field may be subject to continuous heavy rain which induces flooding.
[Schmitthenner, A.F. 1988. Phytophthora rot of soybean. Pages 71-80 in: Soybean Diseases of the North Central Region, T.D. Wyllie and D.H. Scott, eds. APS Press, St. Paul, MN.] Similar to other ''Phytophthora'', warm soil, intermittent rain (including the rain splashes that results from rain), and windy weather are favorable conditions for development and the spread of the disease respectively. The optimum temperature for its disease development is above .
Management
Host resistance is the primary method of control for ''Phytophthora sojae''. There are three types of resistance:
R gene mediated resistance, root resistance, and partial resistance. Currently there are 14 ''Rps'' genes, meaning 14 different single-resistance genes, which have been identified for R-gene mediated resistance and mapped in the soybean
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
. Effectively, the most damage that the oomycete can induce is a lesion. Root resistance is inherited and is generally expressed in the roots. In this case, the stem of a germinating seedling is most susceptible. Once the first leaves begin to emerge, the partial resistance of the plant is expressed. Colonization is reduced and lesions are smaller in comparison. This management prevents the zoospores from germinating in the root tip and therefore unable to produce hyphae, which it needs to survive.
''Phytophthora sojae'' can also be controlled using
fungicide
Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
s. For example,
Metalaxyl, a fungicide that is specifically used for oomycetes, is used for treating soybean seeds. It's used to prevent seed decay and pre-emergence damping off. This fungicide has been observed to be more effective on highly-tolerant soybean plants. Metalaxyl is most effective when applied to the soil as it allows the plant to take it up through the roots and elongate the control period in comparison to a seed application.
[Anderson, T. R., and Buzzell, R. I. 1982. Efficacy of metalaxyl in controlling ''Phytophthora'' root and stalk rot of soybean cultivars differing in field tolerance. Plant Dis. 66:1144-1145] Metalaxyl prevents the spores of ''Phytophthora sojae'' from entering the soybean plant tissues. As with all fungicides, Metalaxyl is effective for prevention only and should be applied before the disease has established itself inside the tissues of the soybean plant.
Replanting must be done once severe pre-emergence damping off is observed.
Improving field drainage and soil
tillage
Tillage is the agriculture, agricultural preparation of soil by mechanical wikt:agitation#Noun, agitation of various types, such as digging, stirring, and overturning. Examples of manual labour, human-powered tilling methods using hand tools inc ...
are cultural practices that can help minimize the effect of ''Phytophthora sojae''. Improving soil tillage can help eliminate oospores from the soil. Oospores are very sturdy and can remain stagnant in the soil for a long time and therefore
crop rotation
Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. This practice reduces the reliance of crops on one set of nutrients, pest and weed pressure, along with the pro ...
alone is not effective.
Proper field drainage prevents flooding and therefore inhibit zoospore movement towards the host.
Importance
''Phytophthora'' root and stem rot of soybean was first observed in the
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
in
Indiana
Indiana ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Michigan to the northwest, Michigan to the north and northeast, Ohio to the east, the Ohio River and Kentucky to the s ...
in 1948 and its causal agent, ''Phytophthora sojae'', first identified in 1958.
[Kaufmann, M.J., and J.W. Gerdemann 1958. Root and stem rot soybean caused by ''Phytophthora sojae'' n. sp. Phytopathology 48:201-208.] In the 1970s, soybean plants only had one single-resistance gene, meaning they were more susceptible to an infection.
Eventually plants with this gene were killed by new
races of ''Phytophthora sojae''. As a result, several states suffered significant yield losses particularly in the state of
Ohio
Ohio ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...
, which lost 300,000 acres of soybean plants in a year. Soon thereafter, a variety of new disease prevention methods were implemented and as a result this disease is currently one of the well-managed and well-known soybean diseases in the USA.
[Schmitthenner, A.F. 1985. Problems and progress in control of Phytophthora root rot of soybean. Plant Disease 69:362-368.]
Origin
Recently there's been evidence that soybean plants from
South Korea
South Korea, officially the Republic of Korea (ROK), is a country in East Asia. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...
and
China
China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
had a diversity of resistance that is much higher in these countries compared to other soybean-cultivating nations.
This indicates that soybean plants have been around in these areas longer and thus had more time to develop resistance against a variety of diseases including ''Phytophthora sojae''.
References
{{DEFAULTSORT:Phytophthora Sojae
sojae
Water mould plant pathogens and diseases
Soybean diseases