Making wise seed decisions for farm

Tony Nye - OSU Extension

It is amazing we are closing out yet another year. Where does time fly when you’re having fun?

As we enter another new year, be sure to reflect on the past enough to learn from, set goals for the coming year and plan for success knowing there may be a few “hiccups” along the way.

One of the first things you might be looking at is making your seed decisions for 2020. Now I realize some of you may have already done so, but for those that haven’t, I ask you” “Have you done your homework?”

Anne Dorrance, Ohio State University plant pathologist, recommends to first focus on the disease and insect scores. Every company uses a different scale based on 1 to 10 – but for some companies 1 is best and for others, 10 is best – so first read the fine print.

Next step: What key diseases and insect pests do we need to focus on?

Let’s start this week looking at soybeans. Dorrance suggests for soybeans, the four diseases that impact Ohio farmers the most are: Phytophthora (on poorly drained soils); Frogeye leaf spot (continuous soybean fields from central Ohio-south); Sclerotinia (fields with poor air drainage – Northeast and Eastern regions); and SCN (more than 50% of fields now have detectable populations of SCN – with more than 7% in severely high populations).

Phytophthora root and stem rot. This pathogen, Phytophthora sojae, can be found in most fields in Ohio but causes disease when those fields are saturated with rains for 24 hours or more.

Under these conditions highly susceptible varieties can have 100% yield loss. During 2019, we detected Phytophthora stem rot over a broader geographic region due to the amount of rain. In the seed catalogues, there are two ratings for resistance, 1) a listing of a Rps gene and 2) a quantitative resistance score on a 1 to 9 scale.

The Rps genes were the first line of defense and have been used since the 1960s (Rps1a was the first). Based on recent check-off funded research, we can confirm that most fields in Ohio have populations of P. sojae, where these genes are no longer 100% effective. They might work in one spot in the field, but not 2 feet away.

The next line of soybean defense is the quantitative resistance, which is many, many genes working together to limit the growth of the pathogen. This quantitative resistance has been called many things in the seed catalogues: partial resistance, field resistance, and tolerance.

Our best varieties have scores of 3 on a scale of 1 to 10 where 1 is very high resistance (really an effective Rps gene) and 10 is dead. Focus on the best score rating for that seed company.

Frogeye leaf spot. This has now become a recurring problem for soybeans in southern up to Central Ohio.

High levels of inoculum (lots of leaf spots) in the fall that can overwinter in Ohio, so this is especially important for those fields that are continuous soybean.

The first thing is if you had Frogeye at the end of the season in 2019, please do not plant the same variety back in that field. Any frogeye in fields in 2019 (conditions were not as favorable as previous years) means it’s time to choose something with better resistance scores.

A resistant cultivar will not develop frogeye, so no yield hit and no added input costs for fungicides if conditions are favorable for disease to develop.

Sclerotinia stem rot. The infections for this disease occur during flowering under conditions of cool temperatures (70s F) and high humidity. High plant population and poor air drainage can also favor this disease.

Resistance to this pathogen, Sclerotinia sclerotiorum, is also quantitative (many genes) and some are associated with limiting pathogen growth but also with longer internodes to help with disease escape. The structure, sclerotia, looks like a mouse or rat dropping, and can survive for long periods of time if they are buried.

This is one soybean disease where no-till can favor the degradation of the sclerotia.

Soybean Cyst Nematode. This nematode continues to expand in the number of fields it can be detected in (> 50%) in Ohio. More importantly, we are also identifying fields with super high numbers of SCN (7% of those sampled during 2018 & 2019)!

It is very important to continue to purchase varieties with SCN resistance. This is a success story. Planting soybean varieties with resistance has kept this nematode at very low levels for over 20 years.

Now, as you would expect, similar to Phytophthora, where the same resistance has been deployed for 20 years, we do have a number of fields in Ohio where the SCN populations are adapting to PI 88788 or Peking or both sources of resistance.

Interestingly, not like the Rps genes of Phytophthora where they work or they don’t, SCN adapts slowly by increasing the number of successful feeding sites on the roots of resistant plants. So early in the process, we don’t see the decline in SCN numbers when soil tests are collected and yields begin to drop.

Later in the process, increases in SCN occur overall in the field and yield loss is similar to that of a susceptible variety.

Tony Nye is the state coordinator for the Ohio State University Extension Small Farm Program and has been an OSU Extension Educator for agriculture and natural resources for over 30 years, currently serving Clinton County and the Miami Valley EERA.

Tony Nye

OSU Extension