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Chapter 4 - Disease Control, No. 4, August-September 1985

No-till or Minimum Tillage Seeding of Winter Wheat Can Reduce Disease Problems

Roger Veseth

The most severe soil erosion in the Pacific Northwest is from winter wheat seeded on a free, conventionally tilled seedbed with no surface residue. Maintaining at least a portion of the previous crop's residue on the surface after winter wheat seeding can effectively reduce soil loss over the fall and winter. No-till and minimum tillage seeding of winter wheat into the previous spring grain, legume, rapeseed or other non-cereal crop residue has advantages other than erosion control. These include fewer tillage operations, lower labor and machinery expenses and increased overwinter storage of soil water, making possible a higher yield potential. In addition is an advantage that many growers may not be aware of. . . a potential reduction in two major winter wheat diseases.

Some misconceptions exist today about the impact of no-till and minimum tillage seeding of winter wheat on disease levels. In the past, no-till and minimum tillage seeding have been associated with higher disease levels, usually because of short crop rotations and improper residue management. However, seeding winter wheat under these conservation tillage systems can actually reduce the level of some important diseases compared to a fine, conventionally tilled seedbed with little or no surface residue. This is the case with two important winter wheat diseases in this region — Cephalosporium stripe and strawbreaker foot rot.

Cephalosporium Stripe

One of the diseases that can be reduced by no-till or minimum tillage seeding of winter wheat into spring grain or non-cereal residue is Cephalosporium stripe. STEEP researcher Tim Murray, Washington State University plant pathologist, explains that the potential reduction in this fungal disease with no-till and minimum tillage seeding is related to reduced root injury, Overwinter injury to winter wheat roots from frost heaving and freeze-thaw cycles is one of the main ways the Cephalosporium stripe fungus enters the plant.

Crop residue on the soil surface acts as an insulating blanket on the soil over the winter. STEEP research has demonstrated that surface crop residue can be effective in reducing frost depth and the number of soil freezing cycles through the winter compared to little or no surface residue on a fine seedbed under conventional tillage. Consequently, root injury is reduced and potential Cephalosporium stripe infection is less where non-host crop residue remains on the surface after winter wheat seeding. The impacts of different combinations of seedbed residue levels and seeding dates on Cephalosporium stripe is currently being researched.

Murray stresses that crop rotation is one of the most important management decisions in Cephalosporium stripe control. Two years out of the susceptible winter wheat or winter barley crops allows time for the residue to decompose and reduce the fungal inoculum level. The fungus survives in the winter wheat or winter barley residue and many perennial and winter annual grasses. It does not survive on spring grains or non-cereal crops. Once the infected residue decomposes, the fungus has no substrate on which to survive and can only live in the soil as spores for a few months.

Besides crop rotation and no-till or minimum tillage seeding, other management practices can also help assure a reduction in Cephalosporium stripe levels. These include delayed seeding date, selection of more resistant varieties and grass weed control.

Reducing the size of the overwintering wheat root system by delayed seeding also reduces the degree of root injury and the potential sites for Cephalosporium stripe infection. However, late seeding of winter wheat can reduce yield potential, particularly if seeding is too late to permit plants to develop a crown and at least three leaves before overwintering. The potential for increased Pythium root rot damage to late seeded winter wheat also exists because the fungus is active in the cold, moist environment of late fall when the wheat root system is small and growing slowly, Small winter wheat plants provide little protection from soil erosion as well.

Although no current varieties are highly resistant to Cephalosporium stripe, growing moderately resistant varieties will reduce the level of plant infection and yield losses, and subsequently reduce the inoculum available for carryover to the next winter wheat crop. This is particularly important where Cephalosporium stripe has been a problem in the past. Stephens, Hyslop and McDermid are all highly susceptible varieties. Lewjain is moderately resistant, with Nugaines, Luke, Hill 81 and Daws following in order of decreasing resistance.

Many winter annual grass weeds such as downy brome are also susceptible to Cephalosporium stripe. Control of these weeds in the winter crops and throughout the crop rotation is critical for reducing inoculum levels.

Strawbreaker Foot Rot

Another major winter wheat disease that is reduced by no-till and minimum tillage seeding is strawbreaker (Pseudocercosporella) foot rot. One of the main mechanisms leading to infection is "raindrop splash" of spores from the soil onto the plant. The fungus enters the plant through older, senescing leaf sheaths on the lower stem near the soil line. Young leaf sheath tissue is generally resistant to the fungus.

Fall and early winter plant infections contribute to the majority of the yield loss from strawbreaker foot rot. Spring infections generally do not significantly affect yield but do contribute to the inoculum carryover. Later-seeded winter wheat provides less plant surface area for infection and also fewer senescing leaf sheaths. Consequently, delayed seeding has been used as a foot rot control practice, as well as for reduction of Cephalosporium stripe infection, But again, the potential for yield reductions and increased Pythium root rot damage with late-seeded winter wheat must also be considered.

Crop residue remaining on the surface after winter wheat is seeded under no-till or minimum tillage also reduces strawbreaker foot rot levels. STEEP researchers Maurice Wiese, University of Idaho plant pathologist, and Tim Herrman, research assistant, evaluated the impacts of different tillage systems on strawbreaker foot rot in winter wheat in 1981, 1982 and 1983. In an example of their results from 1982 (Fig. 1), strawbreaker foot rot infection decreased with increasing surface residue levels both with and without Benlate fungicide applications. Currently, three fungicides are registered for strawbreaker foot rot: Benlate, Mertect and Topsin M.

Two reasons are proposed for the reduction in foot rot with reduced tillage and no-till. First, higher surface residue levels would reduce the amount of raindrop splash of inoculum from the soil to the plant compared to residue free conventional tillage. Second, higher levels of microbial activity near the soil surface associated with residue decomposition may compete with the foot rot fungus and inhibit its activity. Regardless of the reason, maintaining higher surface residue levels with no-till and reduced tillage planting can be an additional management tool for strawbreaker foot rot.

Fig. 1. Strawbreaker foot rot incidence by tillage and benomyl (Benlate) fungicide treatment, May 1982. Different letters indicate significant differences. Disease levels in benomyl and no-benomyl treatments were significantly different within all tillage treatments (University of Idaho Cooperative Extension Service Current Information Series 737).

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.


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