Last month I wrote an article about how to identify soybean seedling diseases caused by different fungi, including Phytophthora (see ICM, page 51). This article discusses seedling disease caused by Phytophthora in more detail, and it has updated information.

Damping-off of soybean.

Frequent rains the past few weeks have put Iowa soybean planting a week behind the five-year average, according to information from Iowa Crops and Weather. A wet spring last year delayed planting in southern Iowa and many fields had a poor stand, some caused by Phytophthora damping-off. This may happen again in some fields where Phytophthora has been a problem in previous years.

Unlike Pythium, Phytophthora is aggressive in warmer soil temperatures (20-25° C or 68-77° F). In Iowa, soil temperatures are warmer by the end of May and early June. Parts of the state also have soils saturated by excessive amounts of rain. Flooding, along with warm temperature within one week after planting, are favorable for development of Phytophthora damping-off. Soybeans that will be planted in June may have more Phytophthora problems than Pythium damping-off if we continue to receive excessive rain.

Soil type is another factor that influences Phytophthora disease. Poorly drained or compacted soils are favorable to Phytophthora root rot. Early this year we collected and analyzed soils from fields in southern Iowa that had damping-off in the spring of 1995. These fields had poorly drained soil and the amount of Phytophthora fungi was unusually high. We also received quite a few samples or reports from this region that resistant soybeans with the Rps1-k gene were defeated. Our tests of those samples showed that isolates either belonged to race 25 or were those that can defeat the Rps1-k gene.

Use of no-till further complicates the disease problem. Soil in no-till fields normally has higher moisture and more compaction, which worsen in poorly drained soils. Studies from Iowa State University showed that Phytophthora risk is higher for no-till fields than tilled fields.

Early detection of Phytophthora disease helps manage the disease risk in the long run. At seedling stage, Phytophthora causes damping-off (see ICM, page 51). The fungus can continue to attack the soybean through the reproductive stage if weather conditions are suitable. Plants infected by Phytophthora fungi yellow, followed by leaves that wilt but remain attached to the plant. Diseased plants are easy to pull and the tap root is discolored (dark brown). Often the entire root system is rotted. The brown discoloration extends from the root toward the upper portion of the stem.

If you see the disease in your resistant soybeans, find out the resistant gene in the variety you are using. Select other varieties with alternative resistant genes that are known to be effective to the major Phytophthora races in Iowa, races 1, 3, and 4.

Be alert if you find Phytophthora in varieties having the Rps1-k gene, which is effective to control races 1, 3, 4, and some other races--but not race 25. As I mentioned earlier, race 25, which can defeat the Rps1-k gene, is a relatively new race to Iowa soybean. It also has been frequently found in our disease-monitoring program.