Plant pathologist reviews 2017's top crop diseases

Ray Mueller
Pest management guidelines outlined by UW Extension experts.

KIMBERLY – White mold in soybeans, stripe rust in winter wheat, and a mix of them in corn were the most prominent diseases in Wisconsin's major crops during 2017, plant pathologist Damon Smith's reported at the Extension Service's annual pest management update meetings.

Based on phone calls he received as early as June and until the end of the growing season in October, Smith said white mold stood out as the major disease in soybeans during the year, especially in both the western and northern areas. There were some outbreaks of sudden death syndrome in southern counties and some incidents of brown stem rot – for which the B genotype is now appearing more often, he added.

Damon Smith

Of the 33 soybean fields that were inspected twice during the growing season, a few had white mold infestations on 60 to 70 percent of the plants, Smith reported. To inform growers on how to deal with such threats, he noted that the Extension Service continues to document the effect of various fungicides in its research plots.

Management practices

Beyond tracking the performance of the differing products, Smith emphasized that one consistent finding is that the best application time is at the R1 to R3 growth stage (when plants are still flowering). Among the products, COBRA (a herbicide that can also control white mold) is the cheapest but its use also creates the likelihood of foliage damage that can affect plant growth, he pointed out.

In terms of return on investment (ROI), the best results occur when the disease pressure was high, Smith observed. For actual ROI from the product lineup, COBRA ranks at the top but only because of its low cost, he stated. For actual control, Aproach, Endura, and Proline often stand at or near the top.

Short of treatment applications for white mold, Smith advised considering planting in 30 rather than 15-inch rows, noting that trial plots had as much as 50 percent less infection in the wider rows. He also suggested checking the Extension Service's summaries of the 2017 research plot for the ratings of differences in white mold susceptibility between varieties and keeping informed about white mold on iPhone and Android apps.

Corn disease list

Among the 2017 corn diseases, Smith listed common rust, northern corn leaf blight, grey leaf spot, ear and stalk rots, Goss's wilt, and southern rust, which he noted is a relatively new one becoming more frequent in Wisconsin.

Among the 2017 corn diseases, southern rust relatively new one becoming more frequent in Wisconsin.

On corn intended for harvest as grain, scout for the presence of diseases before tasseling and expect to earn a return on the cost of a fungicide application only if the foliar disease pressure is already high at that time, Smith recommended. He explained that continuous corn (with pathogen carryover in plant residue), late planting, irrigation, and susceptible hybrids increase the odds of diseases.

At current corn prices, national data suggests a 20 to 40 percent chance of an economic return for a treatment at the V6 growth stage to as high as a 50 to 60 percent chance at the tasseling stage provided that the disease pressure was high and the per acre cost of an application is between $20 and $30, Smith indicated. An application at both stages lowers the probability of a return to between a 15 to 45 percent chance, he added.

For corn going into silage, Smith has far less concern about plant disease effects on yields. What could be a problem is the development of mycotoxins, stemming from organisms that infected the silks, he explained.

If a decision is made to apply a treatment, do not apply a fungicide with strobilium ingredients during the silking stage because this could stimulate mycotoxin production, Smith cautioned. Because the presence of molds on ears does not necessarily equal the presence of a mycotoxin and because the lack of molds does not guarantee the absence of mycotoxins, he advises testing to answer the doubt.

Stripe rust on wheat

In a change from some recent years, stripe rust replaced fusarium head blight as the top disease affecting winter wheat in 2017, Smith reported. He noted that head blight and septoria leaf spot had a minimal presence for the season.

Stripe rust in a “striped pattern” on winter wheat leaves.

Smith attributed the stripe rust outbreak to the lack of temperatures low enough to kill the pathogen spores during the previous winter. He noted that spores were active by early as March 29 in the Extension Service plot near Sharon in southern Wisconsin.

As a starting point, growers should choose wheat varieties with good resistance to stripe rust, Smith stated. In terms of fungicide treatment, he reported that the best controls are being found with the Headline and Prosaro products applied at the Feekes 8 growth stage.

Applications of those fungicides at that stage (flag leaf) boosted per acre yields by 3 to 4 bushels on a very susceptible wheat variety (Pro Seed 420) but a wheat price of $4 per bushel did not cover the added costs, Smith pointed out.

Despite its minimal presence in 2017, Smith urged growers not to forget about fusarium head blight. He advised planting wheat following an alfalfa stand or soybean crop rather than after corn and applying an additional fungicide if there's lots of wet weather when wheat is heading.