Disease pathogens and soil-inhabiting insects in cereals

Date:04-27-2012

Unpredictable weather and machinery breakdowns are just a few of the many noticeable challenges growers regularly encounter. Some obstacles affecting productivity, like soilborne diseases and insects, are not as obvious as they lurk below the ground’s surface. In cereal crops, Fusarium, Pythium and Rhizoctonia attack seeds and roots, stealing valuable nutrients and resources needed to produce high-quality crops.

Possessing the potential to act synergistically with soilborne diseases, nematodes, and soil-inhabiting insects, like wireworms, also feed on and damage plant roots. This interaction leaves crops susceptible to more disease, other environmental stressors and ultimately, reduced yields. To help ensure vigorous crop growth, it is critical to understand the challenges impacting root health and stand establishment and determine viable, protective solutions.

Over the last 11 years, Tim Paulitz, Ph.D.,research plant pathologist, USDA-ARS in Pullman, WA, and his research associate, Kurt Schroeder, Ph.D. at Washington State University, have been closely studying soilborne pathogens and root diseases in cereal crops. Because soilborne diseases like Rhizoctonia and Pythium grow rapidly, they can quickly infect germinating seeds and attack young seedlings. These fungal pathogens have the ability to prevent seedlings from ever breaking ground. Paulitz says in most crops, one of the major symptoms of these diseases is damping-off – the seed simply rots and never germinates. If crops do in fact get the chance to emerge, they will likely be stunted and patchy as the diseases continuously nibble away at the root tissue throughout the season.

Luckily, Fusarium, Pythium and Rhizoctonia have different life cycles and thrive under differing weather conditions, so their opportunity to increase levels of destruction in unison is fairly limited. Nevertheless, each pathogen has the ability to cause severe damage individually.

• “Fusarium infects the roots and eventually gets into the crown, or the bottom part of the stem near the soil. It manifests its symptoms later on in the growing season. This disease seems to be predisposed by drought stress.

• “Pythium infects germinating seeds and young seedlings very quickly and nibbles away at root tips. The disease is more prevalent in areas that have higher precipitation – more than 18-20 inches of precipitation annually and is favored by wet, poorly drained soils.

• “Rhizoctonia, similar to Pythium, infects roots and young seedlings early in the growing season. The disease is favored by wet, cool conditions, although the strains that cause bare patch symptoms are more common in soils that have more of a sandy or silty texture and receive less than 15 inches of precipitation a year," Paulitz and Schroeder explained.

Unfortunately, it’s not just soilborne pathogens that can harmfully impact root health – certain soil-infesting insects can cause gradual yet severe damage and economic loss, too. Wireworms are among the most difficult insects to detect, and as with soilborne pathogens, symptoms are tough to observe until bare patches and thin stands appear in fields.

Differing greatly in habitat, size, feeding patterns and susceptibility to insecticides, wireworms are small pests that typically feed on plant crowns, developing roots, germinating seeds, and specifically, wheat kernels’ endosperms, leaving behind only the seed coat. As they move upward in the soil, they can even attack wheat as soon as the crop is seeded. Populations of this pest can be found as high as 1.2 million wireworms per acre, and depending on the species, they can concentrate in low, poorly drained fields or in high, drier areas. Typically, wireworm damage is most severe where wheat was seeded after fallow or after a number of years of grass growth.

Bob Vernon, Ph.D., with the Canada-based Pacific Agri-Food Research Centre, is a global leader in wireworm research who has been conducting wireworm surveys since 2004.

"Wireworms are attracted by carbon dioxide," Vernon explained. “So as wheat germinates, wireworms will cue in on that carbon dioxide trail and move in to feed. What we’ve found in the lab is that large wireworms can easily consume two or more seeds, and they also eat roots and other growing parts later on. So once they’ve polished off one seed, they’ll move to the seed next to it. So you can end up losing the entire crop if the population is high enough."

In addition to wireworm- and soilborne disease-induced damage, studies have also shown wheat nematodes can interact synergistically with Rhizoctonia to further impede crop development. In fact, certain cropping systems, such as no-till, can accelerate that interaction to cause higher levels of crop destruction. In the Pacific Northwest, Paulitz and Schroeder observed wireworm and nematode symptoms similar to Rhizoctonia bare patch.

"Wireworms feed on the crown of the crop, attacking seeds and root systems, while lesion nematodes feed on the crop roots. The nematodes can even get inside the roots. We can hypothesize that nematodes provide more wounds for the fungus to get in," Schroeder said.

"Soilborne diseases, nematodes and insects like wireworms are driven by cropping practices, and therefore there is a lot of concern with direct-seeding, especially with the increasing wireworm problems lately," Paulitz explained.

To make matters worse, crops experiencing weather-related stress, such as drought or excess moisture, are even more vulnerable to damage and reduced yields. “Crops under weather-related stress react to attacks in various ways. Much of it relates to the speed of growth of the host. If the host is growing more slowly under cool conditions, there is more chance of damage by the fungus," Paulitz noted.

Roots are unique in that it can be difficult to monitor their health until undesirable symptoms appear above ground. While solutions to unfavorable interactions between soilborne pathogens, root-feeding pests and uncontrollable weather stressors are gradual and limited, they can be effective and profitable in the long run. One viable solution is to plant winter wheat within seven to 10 days of the first freeze – planting any earlier will put the crop at risk of early-season wireworm feeding. Because wireworm damage occurs primarily in the spring, during its cool, moist weather, careful consideration of seed treatment insecticides is important before planting spring wheat. Ultimately, the best way to confirm whether wireworms pose a threat is to scout before planting.

Growers increasingly understand the value in protecting their seed and profit potential through use of a top-performing seed treatment. A combination of seed treatment fungicide and insecticide, such as Dividend Extreme® seed treatment fungicide and Cruiser® seed treatment insecticide, can help cereal growers effectively protect crops against Rhizoctonia and Pythium, as well as other root rots and nibblers like wireworms. This pair safeguards crops as it stimulates plant growth and increases crop vigor, even under stressful conditions.

New Product in the Pipeline

Additionally, a new, experimental seed treatment fungicide from Syngenta is expected to be registered in 2012. Its formulation contains a new mode of action, which will offer an unmatched level of disease protection for stronger, more powerful roots and greater profit potential for growers. When it comes to battling the tough-to-detect soilborne diseases and insects, awareness of their presence and knowledge of field history are keys. Implementing proactive, protective measures, such as seed treatment applications, can also help alleviate hindrances to root health and crop productivity.