Integrated Crop Management

Aphanomyces in Iowa

Aphanomyces euteiches is a fungus that causes seedling blight and root rot of alfalfa and certain vegetable crops. It can cause extensive seedling death and reduce yields in established plants. Aphanomyces, a well documented pathogen in Wisconsin and a few other states, also occurs in Iowa. Because it is not widely recognized, it probably is not properly diagnosed in many cases. In 1994, ISU plant pathologists and extension field specialists collected soil and plant samples throughout the state and used a baiting technique to determine the presence of A. euteiches. A modification of the same technique can be used to detect Phytophthora medicaginis, the cause of Phytophthora root rot. A total of 118 samples were collected from 44 counties. Some samples were soil only, some were roots only, some were both. Sites suspected of having root disease problems or poor drainage were selected primarily, so it was not a random sample. Areas with more alfalfa production were sampled intensively. Table 1 shows the results from soil and root samples by crop district. A. euteiches and P. medicaginis were detected in 31 and 26 of the 44 sampled counties, respectively. Recovery of both pathogens was greater from soil than roots. A. euteiches was recovered from 75 percent of soil samples and 16.9 percent of root samples. P. medicaginis was recovered from 35.6 percent of soil samples and 7.7 percent of root samples.

Of the 51 soil and root samples, on only one occasion was a pathogen (Phytophthora) recovered from the roots but not the soil. In the 14 samples that were roots only, A. euteiches was recovered from only one sample, and P. medicaginis was recovered from two samples. Therefore the plant samples did not contribute greatly to our knowledge of A. euteiches distribution in Iowa. Less frequent recovery from roots is common with this technique; the fungus is easier to detect from soil. However, the fungus often may be present in the soil but not infecting the particular plant samples we collect.

P. medicaginis is somewhat more difficult to recover using the seedling bait technique than is A. euteiches. Therefore the relative frequency of recovery of the two fungi may be slightly biased in favor of A. euteiches. Table 1 indicates that Aphanomyces euteiches is a very common inhabitant of Iowa soils where alfalfa is grown. In fact, it appears to be more common than the well-known Phytophthora medicaginis . Both fungi can often be found in the same sample; most of the soil samples positive for Phytophthora were also positive for Aphanomyces. These results do not allow us to estimate the amount of damage being caused by either fungus. Under wet conditions and/or heavy soils, both pathogens can cause serious problems in stand establishment and reduce yields. The most effective way to manage both diseases is by using resistant varieties and improving drainage. Apron seed treatment protects against Phytophthora but not Aphanomyces. Proper moisture conditions in the seed bed reduces seedling losses to either fungus. Rotation with clover, peas, or Phaseolus beans increases the Aphanomyces population, but corn and soybeans are not hosts for either fungus. Since both fungi can survive a long time in the soil, rotation does not substantially reduce their populations. It is possible that seeding with a nurse crop such as oats reduces Aphanomyces severity, but I am not aware of any direct evidence for this.

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