Soybean Disease Outlook for August

By Anne Dorrance

We have soybeans in all different growth stages but the majority of the crop looks great but there are a few highlights based on some scouting and reports from last week.

1. Frogeye leaf spot – the fungicides do seem to be holding this at bay for those fields where it got an early start.  It is now showing up in northern Ohio, but too little too late to do any damage.  The caveat for this will be to monitor these fields to identify the susceptible varieties – and then avoid those varieties in those fields for 2019.

2. Sclerotinia stem rot – white mold is just beginning.  At two of my field research plots there were a few plants with 1” long lesions and white mold.

In approximately 2 weeks, it will be time to begin to scout in fields with a history of this disease.  Soybean varieties that develop a high incidence of this disease (>20%) are considered susceptible and should not be planted back in these fields.  It takes this amount of disease to begin to measure yield losses.

3. Late season Phytophthora stem rot. On varieties that have low levels of field tolerance or partial resistance, stem rot is popping up in fields that have received some of these heavy down pours over the last few weeks.  Make notes of these varieties and remove them from the line up for next year.

4. There are reports of some soybean plants with early symptoms of Sudden death syndrome.  Several of these late season diseases can have similar symptoms so the first step is to check the pith.  If it is white that fits with sudden death syndrome.  If it is brown-chocolate brown it is brown stem rot and if it is a reddish brown then it is Diaporthe (southern). Also look at the roots, the fungus that causes sudden death syndrome is also a very good root rotter, but the fungi that cause brown stem rot and Diaporthe are wimps in this category.

The best means to manage all of these diseases is first with variety selection.  As the sales have started for the 2019 season, take a look at those ratings first and for any other issue that you are finding affecting soybeans this year.  It’s a great year to weed out the wimps.

Source: osu.edu