Nebraska Cooperative Extension NF02-518

Management of Phytophthora Diseases of Soybeans

[Previous Category] [Catalog] [Order Info]

Phytophthora diseases of soybean, caused by Phytophthora sojae, are present wherever soybeans are grown in Nebraska. The pathogen survives primarily as "resting" spores in the soil or in association with infested crop debris. Symptoms associated with Phytophthora sojae infections include seed rots, pre- and post-emergence damping off of seedlings and stem rot of plants at various growth stages. Disease development is most rapid at soil temperatures above 60°F with high soil moisture. This disease is most common in low-lying areas of a field, in poorly drained or compacted soils, and in soils with high clay content. It also may occur on well-drained hillsides during growing seasons with excessive moisture availability. Currently 55 races of the pathogen have been identified. In 2000, Phytophthora sojae race surveys for soybean were initiated with funding from the Nebraska Soybean Board. As a result of ongoing survey efforts, races 1, 3, 4, 25, and 28 have been recovered from soybean fields in Nebraska. Previous surveys were conducted in eastern Nebraska in 1980 and 1981 and resulted in recovery of races 1, 3, 9, 14, 18, and 23.

Once P. sojae is established in a field, it cannot be eradicated. The disease must be "managed" with the use of seed treatment fungicides and resistant varieties. Knowledge of the races present in the state and how varieties with different resistance genes have performed in a grower's field is critical to variety selection for maximum profitability.

Seed treatment fungicides can be used for management of early season seed rot and damping off caused by Phytophthora sojae. The compounds labeled for control are metalaxyl and mefenoxam. Mefenoxam is one of the chemical compounds which has been isolated from metalaxyl. A list of products containing these com- pounds and their recommended rate of application is provided in Table I. In general, all varieties of soybean grown in problem fields should be treated since conditions favoring Phytophthora sojae also favor Pythium spp., which commonly cause seedling problems in Nebraska.

Table I. Seed treatment fungicides1 labeled for Phythophthora sojae control on soybean.2
Product Name	Common Name	Application rate for Phytophthora control	Manufacturer
Allegiance LS	Metalaxyl	1.2-2.4 fl. oz./100 lbs. seed	Gustafson
Allegiance FL	Metalaxyl	0.75-1.5 fl. oz./100 lbs. seed	Gustafson
Allegiance Dry	Metalaxyl	1.5-2.0 oz/100 lbs. seed	Trace Chemicals LLC
Apron FL	Metalaxyl	0.75-1.5 fl. oz./100 lbs seed	Gustafson
Apron Flowable	Mefenoxam	0.82-1.64 fl. oz./100 lbs. seed	Wilber-Ellis
Apron Maxx RTA3	Mefenoxam and Fludioxonil	5.0 fl. oz./100 lbs. seed	Syngenta
Apron Maxx RTA + Moly3	Mefenoxam and Fludioxonil	5.0 fl. oz./100 lbs. seed	Syngenta
Apron TL	Metalaxyl	2.0-4.0 fl. oz./100 lbs. seed	Wilber-Ellis
Apron XL LS	Metalaxyl	0.16-0.64 fl. oz./100 lbs. seed	Syngenta
System 3	Pentachloronitrobenzene (PCNB), Metalaxyl, and Bacillus subtilits GBO3	4-6 oz/bu	Helena
Warden RTA	Mefenoxam and Fludioxonil	5.0 fl. oz./100 lbs. seed	Agriliance LLC
1Product list is intended for information purposes only. No criticism is intended for products not listed nor endorsement for products listed. Always read and follow label directions when applying any pesticide. 2Application rates on the high end of the labeled amount are generally necessary for adequate Phytophthora sojae control. 3In fields with a history of high phytophthora pressure, the label recommends using Apron Maxx RTA or Apron Maxx RTA + Moly at the recommended rate of 5.0 fl. oz./100 lbs. seed in combination with Apron XL LS at a rate of 0.16-0.48 fl. oz./100 lbs. seed.

Use of resistant varieties is the most effective means of managing phytophthora diseases in soybean. Genetic resistance in the host is expressed in terms of Rps (Resistance to Phytophthora sojae) genes. The genes are denoted as Rps 1a, 1b, 1c, 1d, 1k, 3, 6, and 7. The pathogen exists in races that interact with these genes. A race is identified by its interaction with the eight known Rps genes. In a resistant reaction, the plant survives when infection occurs; susceptible varieties are killed when infection occurs (Table II).

Table II. A list of the races of Phytophthora sojae identified in Nebraska over the last 20 years and the Rps gene reaction to these races. Race Rps Resistance Genes 1-a 1-b 1-c 1-d 1-k 3 6 7 1 R R R R R R R s 3 s R R R R R R s 4 s R s R R R R s 9 s R R R R R s s 14 R R s R R R R s 18 R R s R R R R R 23 s s R R R R s s 25 s s s R s R R s 28 s s R R s R R s s = susceptible or plant killed, R = resistant or plant not killed

Soybean varieties are marketed on the basis of their genetic make-up in relation to the predominate Phytophthora sojae races in a given area. The predominant races in Nebraska are 1 and 3. The most widely available resistance genes in the Midwest are 1-c and 1-k, commonly referred to as c or k in seed company literature. Gene 1-c protects against races 1, 3, 9, 23, and 28, where gene 1-k protects against races 1, 3, 4, 9, 14, 18, and 23. Gene 3 is the only gene that protects against all known races that occur in Nebraska. Currently 40 percent of the maturity group 2 and 63 percent of the group 3 soybean varieties marketed in Nebraska contain some resistance to Phytophthora sojae. Resistance within these maturity groups is listed below.

Maturity Group Percent Nebraska Marketed Varieties With Rps Gene Resistance 1-a 1-b 1-c 1-d 1-k 3 6 7 1k/1a Group 2 5 0 20 0 72 1.5 0 0 1.5 Group 3 9 0 34 0 54 3 0 0 0

Growers should consult local seed company representatives to request varieties with different Rps genes than marketed in a specific area.

Electronic version issued May 2002
pubs@unl.edu

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Elbert C. Dickey, Dean and Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.

University of Nebraska Cooperative Extension educational programs abide with the non-discrimination policies of the University of Nebraska-Lincoln and the United States Department of Agriculture.