Plant Detective

23 Jun 2016

By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) -- A grower finds patches of wilting and dead soybeans in a field planted into a dense stand of rye cover crops. Is it a residual herbicide gone rogue or a bad case of Phytophthora root rot?

Another grower reports the appearance of frogeye leaf spot on the leaves of very young soybean plants. Is it an early attack of the disease or chemical burns from drifting gramoxone herbicide?

Disease symptoms can mirror crop damage from chemical injury, and vice versa. University of Tennessee plant pathologist Heather Kelly spends part of her time each summer playing detective for growers.

She recommends relying on context clues -- the pattern of the damage, the variety in use, and the appearance of new growth -- as well as a good understanding of the suspected disease and herbicides at use in the area.

CONTEXT CLUES

"Wait for the plant's newest growth to come out, and if doesn't display the same symptoms, it is most likely chemical injury," Kelly said.

That said, keep in mind the time of year. If farmers in the area are still in spraying mode, that new growth could still show symptoms, even if it is herbicide burn.

Next scrutinize the variety you planted and any seed treatments you added, Kelly said.

Is the continuous corn you just planted into no-till residue susceptible to gray leaf spot? If so, those tan lesions could well be the disease. Did you decline any fungicide seed treatments? The chances of a Pythium seedling disease, Fusarium, Phytophthera or Rhizoctonia root rot will be much higher.

(Also keep in mind that Bayer's ILeVO seed treatment targeting sudden death syndrome can produce a "halo effect" of yellow or brown edging on the cotyledons -- but it's harmless, Kelly said.)

Finally, note the pattern of the damage in the field. Does it follow the path of a sprayer? Is it concentrated along your neighbor's border and does it fade as you move into the field? Those patterns would suggest foliar herbicide drift.

Patches of damage concentrated in wet, low-lying regions of a field -- like the wilting soybeans in the Tennessee River bottom fields -- can suggest diseases that favor these conditions such as root rots, Kelly said.

ZOOM IN ON THE DISEASE AND CHEMICAL IN QUESTION

Doing a little research on the suspected disease's symptoms and timing can clear up the case as well, Kelly said.

Those young soybean plants with suspected frogeye leaf spot lesions? They're more likely gramoxone drift damage; foliar diseases like frogeye leaf spot don't usually develop until the reproductive stages of soybean growth, Kelly said.

A close look at the lesions can also help -- a chemical burn will be bleached out in the center with a dark ring and tidy circular shape. Frogeye leaf spot lesions tend to be grayish in the center and more angular in shape.

Pre-emergence herbicide damage can mimic seedling diseases, but some of their symptoms are unique to the pathogens, Kelly noted in a university blog. Look for "rotten, mushy seedlings with poorly developed roots, water-soaked lesions on the hypocotyl, cotyledons, or stem, and stunted or wilted seedlings," as signs of a seedling disease rather than chemical injury, she wrote.

Knowing which herbicides you used on your field or which ones are at use in the surrounding area can help, because different chemicals injure susceptible plants in different ways.

For example, growth regulators like 2,4-D and dicamba are more likely to result in cupped soybean leaves than leaf lesions. Yet an herbicide like Fomesafen could very well show bronzing or speckling on a soybean leaf, and a chemical like atrazine can produce interveinal chlorosis, not unlike sudden death syndrome.

See Purdue University's detailed and illustrated guide to the types of chemical injury associated with different herbicides here: http://bit.ly/….

See Kelly's blog here: http://bit.ly/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/CZ)