Phytophthora Root Rot

By Berlin Nelson, Professor, Dept. Plant Pathology

Phytophthora root rot is a major disease of soybean, especially in areas where soybeans have been cultivated for many years. The disease is caused by the fungus Phytophthora sojae. Yield losses can be substantial; entire fields have been destroyed. The disease is common in the Red River Valley, expecially in the southern portion. The pathogen survives in soil as spores called oospores which are produced in infected plants. When there is high soil moisture the spores germinate and infect the roots. Infection and disease development can occur at any stage of plant development, but are most commonly observed and damaging at the seedling or young plant stage. Disease is most common in heavy, compacted clay soils and fields subject to flooding. Flooding rains, especially near planting, favor disease development. Reduced tillage, especially no-till, is reported to increase damage. The pathogen does not naturally infect other crops grown in this region. Only three Lupinis spp. and soybeans are natural hosts.

RACES AND VIRULENCE PHENOTYPE

There are more than 40 races of P. sojae. A race is a specific form of the pathogen that attacks certain resistance genes in the soybean. Races of the fungus are identified by the resistance genes they can defeat (defeat means cause disease on). In 2002 a race survey was conducted in North Dakota. The results are shown in the pie chart below. The most prevalent races in this area are races 3 and 4 (about 80% of the isolates). Races 1, 5, 8, 21, 25, 28, 41, 43, and 44 are also found, but at low frequency. Three new races were also identified. In the early 1990s, races 9, 14 and 34 were found, but they did not appear in the 2002 survey. Studies with single zoospores of isolates from North Dakota have revealed there is great genetic diversity within the population of P. sojae. There are strains of the fungus that can defeat most of the major resistance genes (14 Rps genes) for control of P. sojae. The bar chart below indicates the perecentage of isolates collected in 2002 that can defeat each of 7 resistance genes. The Rps genes 1a, 1c, 1k and 6 are the genes commonly found in cultivars adapted to North Dakota. The genes 1k and 6, for example, are still valuable genes becasue they protect plants from most races. But there are races that will attack and kill plants with those genes. As soybeans continue to be a widely planted crop there will be selection pressure for the development of new strains of the pathogen and we will continue to see more evidence of plants with the 1k and 6 genes becoming diseased.

The term race is now being supplemented by a new term, virulence phenotype, to designate the virulence of the fungus stain on the various host resistance genes. Most races are identified based on the differential reaction on 8 resistance genes (Rps genes 1a, 1b, 1c, 1d, 1k, 3a, 6 and 7). Because there appear to be so many races and the numbering system has no biological meaning, designating the strains by the genes in the host that they defeat has practical use. For example, race three is now designated with the virulence phenotype of 1a, 7. This means that of those 8 resistance genes, race three defeats the resistance genes 1a and 7. Race 4 has a virulence phenotype of 1a, 1c, and 7. So a cultivar with Rps 1c or 1a would not be resistant to race 4, because race 4 defeats those genes. Races 3 and 4, however, can be controlled with a cultivar with Rps 6 or Rps 1k because those races can not defeat those resistance genes.

SYMPTOMS
The symptoms are seed rot and pre- and post-emergence damping-off and wilting of plants. These are common in flooded soils and are often misidentified as water damage. On older plants, leaves may become yellow and plants will wilt with wilted leaves remaining on the plant. The lateral and tap roots are rotted and destroyed. A dark brown discoloration that can turn into a girdling lesion often appears on the lower portion of the stem. Disease is usually patchy in the field, often occurring in low or flooded areas. Symptoms can appear at any time during the year when wet soil conditions occur.

Management. Planting resistant cultivars is the best method to control Phytophthora root rot. Choose a resistant cultivar that contains a gene for control of races 4 and 3, since those are the most prevalent races. The genes Rps lk and Rps 6 will control races 3 and 4. Rps 1c will not control race four. The gene Rps la, which is found in some cultivars, will not control races 3 and 4. If you are using a cultivar resistant to races 3 and 4, and you observe Phytophthora root rot, it indicates that another race is appearing in the field.

Some cultivars are reported to have tolerance to Phytophthora root rot. These cultivars may not be as susceptible under low to moderate disease pressure, but can be severely damaged under high disease pressure. Crop rotation is not an effective method to reduce disease because the oospores are very long lived in soil. Metalaxyl and mefenoxam seed treatments will protect seedlings but not older plants. See the section on seed treatments