New “Spot-Form” Of Net Blotch Confirmed In MonDak Barley

Agricultural Research Service (ARS) scientists in Sidney, have recently confirmed the arrival of a new, troublesome form of net blotch in barley in the MonDak region. Sidney ARS Plant Pathologist Robert Lartey identified the new form recently at the Northern Plains Agricultural Research Laboratory from samples collected near Sidney and at Nesson Valley in Williston. Dr. Lartey used a new DNA technique which he developed that can accurately identify the presence of the disease in just two days versus several weeks for the standard method.

The arrival of “spot-form” net blotch in the region caught many by surprise. It’s been present in the Red River Valley, Fargo, ND, only since 2010, and while it has been reported in parts of western Montana as early as the 1990s (at Fairfield), it wasn’t expected to move into this area for another three or four years. However, when North Dakota State University scientists from Fargo attending the Williston Extension and Research Center Field Day in July, reported seeing symptoms of suspected spot-form in Dickinson, local scientists worried that it could also be in northeastern Montana and northwestern North Dakota and began collecting samples from MonDak area barley fields for testing. Their concern was further justified this past fall when a number of MonDak producers harvesting their barley reported seeing clouds of what looked like black smoke coming from the rear of their combines. The “smoke” was made up billions of dried black spores, released in plentiful supply by the new spot-form version of the disease.

“Spot-form” net blotch, like “net-form” net blotch, is caused by the Pyrenophora teres fungus, although by genetically different forms: P. teres f. maculate versus P teres f. teres. Like its counterpart, “spot-form” net blotch is a foliar disease of barley that can survive over winter on barley residue, on volunteer barley plants (and some grasses) and on seeds from the previous season, all lying in wait to infect the next year’s crop. Spores are produced and spread by wind and rain during the growing season, accounting for the bulk of the spread of the disease within the crop. Infected seeds are the more common source for geographic spread over longer distances, according to Lartey, and may have been the original source for our new local infections. In addition, over the past two years, conditions have been ideal for the spread of a number of fungal diseases, including net blotch. Cool, wet springs are favored by the fungi, Lartey said, aiding expansion in dryland barley, as well as irrigated.

The two forms of net blotch display slightly different symptoms. Net-form net blotch is evident from brown spots on the barley leaves that elongate into a net-like pattern and then join together into dark brown strips as the disease progresses. In contrast, spot-form net blotch exhibits elongated brown spots on its leaves surrounded by yellow zones throughout the disease’s progression.

Both forms of net blotch lower barley grain yield and brewing quality by reducing the carbohydrate content of kernels. The spot form of the disease can cause grain and yield losses from 10 to 40% to as high as 100% for some growers depending on how much of the leaf area of the plant’s upper two leaves is destroyed. Local producers finding evidence of the new spot-form of the disease in their barley fields this fall estimated their losses at between 50% and 75%. Spraying fungicides at the right time and avoiding continuous barley rotations are probably the best ways to minimize effects of this disease, in both its forms.

But while spot-form net blotch will respond to several of the same management options used for net-form net blotch, there is one big difference, Lartey said. Resistant varieties developed to help manage net-form net blotch are unlikely to be effective against spot-form. “Resistance to the two forms of net blotch is inherited separately,” he noted, “which means barley resistant to net-form is generally not adequately resistant to spot-form.” That’s due in part because good resistance against spot-form net blotch appears to require the presence of two or more genes to be effective, according to studies by the Australian Government Grain Research and Development Corporation. The multiple genes requirement significantly complicates development of resistant varieties and researchers in Australia have had only limited success to date in developing new varieties for their region. In part, the lack of effective resistant varieties may account for the rise of spot-form net blotch in Australia, as well as in other countries around the world, including South Africa, Canada and the United States.

Back in the MonDak, resistance in current varieties is limited to net-form net blotch and is largely ineffective against spot-form, although work on new spot-form resistant varieties is continuing. However, there are also other new alternative management strategies under study at the Sidney ARS lab. Lartey is currently studying three promising new fungal biocontrol agents for Cercospora leaf spot in sugarbeets, which have also been shown to inhibit net-form net blotch in barley. He plans to test these new agents against spot-form net blotch in upcoming lab and greenhouse trials. Ultimately, successful agents could provide the basis for a new “living” fungicide for MonDak growers to use in battling both forms of net blotch, as well as Cercospora.