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Phytophthora sojae

Prepared by Kestrel Lannon

Class Project for PP728 Soilborne Plant Pathogens

Fall 2010

 

Pathogen: Phytophthora sojae syn. Phytophthora megasperma f.sp. glycinea

Host and Range: Glycine max and Glycine soja; found in all soybean producing regions.

Introduction: Phytophthora sojae is a soilborne pathogen, which forms a parasitic relationship with soybean, Glycine max, causing severe economic damages. In the late 1970s, 10% of the entire soybean population in Ohio was destroyed by this pathogen (1). P. sojae causes 1-2 billion dollars in damages globally every year (2). If resistance is not incorporated into planted varieties, then losses would be many acres of soybeans each year. This pathogen causes a root and stem rot of soybean and can infect seeds, seedlings, and mature plants if conditions are favorable for disease. Symptoms usually develop one to two weeks after inoculation (1).

Symptoms: P. sojae can infect and rot seeds causing pre- and post-emergence damping-off as well as root and stem rot of soybean plants. Symptoms include necrosis of roots and hypocotyls, chlorosis of leaves, and the whole plant will turn reddish-orange to orange-brown in color. Sometimes a lesion will form on one side of the plant. In susceptible varieties, losses can reach up to 100%, with losses decreasing to 20-50% in low to moderately resistant cultivars. In highly resistant cultivars, roots are still colonized and symptoms are limited to root necrosis and reduced yields (1).

Stem Lesion.jpg

Diseased Field.jpg

Necrosis of a soybean stem. Courtesy of A. Dorrance (APSnet).

Soybean damping off in a low area of the field. Courtesy of A. Dorrance (APSnet).

 

Pathogen Biology: P. sojae is an Oomycete and is in the Kingdom Stramenopila. It is closely related to brown algae and Phytophthora spp. are often referred to as water molds. P. sojae is fungal like in growth because it produces hyphae as its vegetative structure. Hyphae are coenocytic, which means they lack cross-walls known as septations. False septa may form in older cultures (1). P. sojae is heterotrophic and saprophytic and can grow in culture on V8 medium, Lima Bean Agar, and minimal Nutrient Agar. P. sojae grows best on lima bean agar (LBA) between 25 and 28C. On LBA, mycelium grows close to the surface of the medium and has no color (1).P. sojae produces three asexual spores, the chlamydospore, sporangia and zoospore, and the sexual oospore. The sexual spore is formed by the fusion of the female oogonium and the male antheridium (2). Oospores have thick cell walls and are used for survival. P. sojae is homothallic (self-fertile), so oospores can form in 3-4 days in culture without mating between different mating types. Oospore walls are smooth and 40 m in diameter. Oospores will germinate and directly infect the host roots and stem, or a sporangium will form on the end of a hypha. Sporangia are lemon shaped and have a small papilla on their tip. They are formed on the end of long pedicels, are thin walled, and 40 m long by 28 m wide. The primary infectious propagule is a zoospore, which is formed in sporangia. Zoospores are motile spores with two flagella and are released by sporangia when the soil is saturated.

Medium.jpg

Oospore.jpg

P. sojae growing on water agar medium. Courtesy of D. Mills (APSnet)

P. sojae oospore. Provided courtesy of K. Broders (APSnet)

 

Ecology and Disease Cycle: Phytophthora sojae survives in the soil as oospores that are able to withstand extended periods of cold weather in winter. In fact, oospores remain dormant for years as spores produced in the same season must be conditioned by a cold soil period before germination will occur. In near-saturated soil conditions, pre-conditioned oospores will germinate to form mycelium that then produce sporangia. Sporangia mature quickly and when soil becomes saturated, zoospores are released to infect host roots. Zoospores are negatively geotactic and are attracted to root exudates such as daedzein and genestein. Once released, zoospores swim towards host roots. Once they come into contact with a root, zoospores encyst, losing their flagella and forming a cell wall. Then they germinate to form one germ tube and penetrate the root epidermis. Temperatures between 25 and 30C and moist or saturated soil favor disease development. Diseases caused by this pathogen are considered to be monocyclic, because the contributions of secondary inoculum are not known (1). Below is a depiction of the disease cycle. (Illustration from APSnet Education Center (1)).

Lifecycle_apsnet2.png

 

Management: P. sojae is a wide spread pathogen present in all soybean production areas around the world. It is primarily managed with host resistance, but chemical and cultural practices are also available for control. Disease is managed by single gene resistance and partial (quantitative) resistance. There are 14 different single gene resistance genes that are designated Rps genes. There are many pathotypes of P. sojae and they are determined by inoculating a series of differentials with an isolate. P. sojae has many races and because of their variable distribution, cultivar selection can be difficult for growers. Below is a brief list of races and their corresponding pathotype. (source APSnet (1)).

Race

Pathotype

rps

Rps1a

Rps1b

Rps1c

Rps1d

Rps1k

Rps3a

Rps6

Rps7

1

7

S

R

R

R

R

R

R

R

S

2

1b, 7

S

R

S

R

R

R

R

R

S

3

1a, 7

S

S

R

R

R

R

R

R

S

4

1a, 1c, 7

S

S

R

S

R

R

R

R

S

10

1b, 3a, 7

S

R

S

R

R

R

R

R

S

17

1b, 1d, 3a, 6, 7

S

R

S

R

S

R

S

S

S

25

1a, 1b, 1c, 1k, 7

S

S

S

S

R

S

R

R

S

30

1a, 1b, 1k, 3a, 6, 7

S

S

S

R

R

S

S

S

S

31

1b, 1c, 1d, 1k, 6, 7

S

R

S

S

S

S

R

S

S

35

1a, 1b, 1c, 1d, 1k

S

S

S

S

S

S

R

R

R

36

3a, 6

S

R

R

R

R

R

S

S

R

45

1a, 1b, 1c, 1k, 6, 7

S

S

S

S

R

S

R

S

S

51

1c, 6, 7

S

R

R

S

R

R

R

S

S

55

1d, 3a, 6, 7

S

R

R

R

S

R

S

S

S

 

Fungicides are only applied as seed treatments for prevention of seed decay and damping off. Metalaxyl and mefenoxam are two chemicals that are used as seed treatments. Disease is the most severe in low lying areas with poor drainage, so any practice that increases drainage would decrease the infection period of this pathogen. Rotation and tillage are not controls of P. sojae because oospores can remain viable in the soil for many years (1).

Links to other Sites:

Phytophthora Root and Stem Rot in Wisconsin

Plant Health Initiative

References:

1.       Dorrance, A.E., D. Mills, A.E. Robertson, M.A. Draper, L. Giesler, and A.Tenuta, 2007. Phytophthora root and stem rot of soybean. The Plant Health Instructor. DOI: 10.1094/PHI-I-2007-0830-07

2.       Tyler, B.M. 2007. Pathogen Profile Phytophthora sojae: root rot pathogen of soybean and model oomycete. Molecular Plant Pathology 8(1), 1-8.