Determining when wheat diseases limit input effectiveness

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When a farmer starts noticing he has disease in a wheat field, it doesn’t matter how much water or nitrogen is applied, it can be a waste, said Charlie Rush, Ph.D., Texas A&M AgriLife Research plant pathologist in Amarillo.

“It’s a waste because the root system can’t take it up,” Rush said. “Anything you put on severely diseased wheat, you are wasting your time and resources and energy – it just doesn’t pay off for the current crop.”

But that doesn’t mean site-specific management isn’t possible, especially on plants infected later in the season which are still capable of producing good grain and forage yields, he said.

Rush is leading a research team to study mite-vectored virus diseases, which are the main pathogenic constraints to economically sustainable to profitable wheat production, and transmitted by the wheat curl mite. The project is funded by the Ogallala Initiative and also by a federal grant from the U.S. Department of Agriculture-Agriculture and Food Research Initiative.

Wheat plants infected with mite-vectored diseases, such as wheat streak and triticum mosaic, not only have reduced grain and forage yields, but also greatly reduced root weight and water-use efficiency, he said. Their progressive nature makes it difficult for producers to know when additional crop inputs, such as fertilization and multiple spring irrigations, are economically feasible.

Rush said the study’s goal is to help producers develop some site-specific practices to make those critical economic decisions.

Wheat streak mosaic, one of the mite-vectored diseases, develops in gradients across the field, starting on one edge and spreading across the field. Almost every plant eventually becomes infected.

Because this is known, Rush and his plant pathology crew are studying the hot spots and making transects across the field, starting where the disease started and going into the center of the field where there is no disease yet. The study is being conducted at the AgriLife Research station near Bushland and in farmers’ fields.

Members of the team detect and quantify plant diseases using a variety of remote sensing devices. Most recently, a hand-held hyperspectral radiometer was used to quantify severity gradients in the field. By recording hyperspectral readings of disease symptom severity over time and associating each reading with GPS coordinates, it will be possible to determine how a specific reflectance reading, at a particular location and point in time, relates to grain and forage yields.

Knowing when specific crop inputs were made, in relation to disease severity at the time, will allow economic cost/benefit analysis and development of an economic threshold for irrigation of diseased wheat, Rush said.

“We take radiometer readings and also visual readings on how severe the disease is at that particular point,” he said. “We are making those ratings each week at the same spot over the entire season. At the end of the season, our statistical guys will be able to say ‘if you had a particular level of disease at a particular time of the season, then it is not going to pay off for you to put any more inputs into that wheat.’

“Or, if you have a disease level that is below a threshold, then we would say based on what we measured in the past, it would be economical for you to irrigate or apply that top-dress of nitrogen,” Rush said.

This year the study has been somewhat limited by the freeze damage and hail, but the wheat that is growing out of the damage is showing the same trends as the wheat prior to the freeze, he said.

“We are going to continue to do the study for three more years,” Rush said. “At the end, we believe we will be able to provide an economic threshold that will tell you when it is worth it to continue to irrigate or put nitrogen on if you have disease in a field."

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