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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 4 - Disease Control, No. 12, Summer 1987


Cephalosporium Stripe Control Strategies

Roger Veseth

For successful winter wheat production under conservation tillage, growers must develop an effective management strategy to control the complex of soilborne diseases in their area. One of several important soilborne diseases of winter wheat in the Northwest is Cephalosporium stripe. Growers need a basic understanding of the disease cycle and pathogen survival in order to determine how their management options affect the potential for crop loss.

STEEP researcher Tim Murray, Washington State University plant pathologist at Pullman, recently published Extension Bulletin 1434, "Cephalosporium Stripe Disease of Cereals. " This publication provides a useful guide for identifying, understanding and managing the disease. This article highlights some of the key points in the bulletin and current research efforts.

Life Cycle

Cephalosporium stripe is a soilborne fungal disease of winter wheat, winter barley and other winter cereals and grasses. Under cool moist conditions in the fall, the fungus on infected residue produces millions of spores (conidia) which act as primary inoculum for infecting young wheat plants, Infection occurs mainly through root injury during the winter and early spring. Soil freezing and frost-heaving cause the most common root injuries through which infection takes place. This is one of the reasons the disease level often fluctuates sharply from year to year, depending on the winter and early spring weather.

Once the fungus is inside the plant, it moves upward, colonizing and eventually plugging the water-conducting tissue (xylem) of the stems and leaves. Infected tillers die prematurely and set little or no seed. If seed is produced, it usually is shriveled and light in test weight, much of which can be lost through the combine at harvest.

Yield losses can be high. In years of severe infestation, as in 1984, yield reductions up to 75 percent or more were reported in the region. Murray estimates that for every 1 percent of the tillers with the stripes in the flag leaf, there is about 0.8 percent yield loss. In conducive years, infection rates of 40 to 60 percent are common. This potentially means a 32 to 48 percent yield loss, He adds that, in years unfavorable to the disease, up to 20 percent of the tillers can commonly be infected and not have a noticeable effect on yield.

Survival of the fungus from year to year depends on the presence of the infected crop residue on the soil surface or shallowly buried in the soil, Research has shown that once the residue decomposes, the spores can only survive independently in the soil a few months under the best conditions. Murray stresses that the fungus survives only in the plant residue which it colonized when the plant was alive. The fungus does not colonize the residue of other crops. In other words, residue from spring cereals, legumes or other non-host crops, remaining on the soil surface under conservation tillage systems does not become infected with the fungus and carry the disease from one year to the next.

Cephalosporium stripe is most common in 2-year rotations in the annual cropping areas receiving 18 inches or more of annual precipitation. It can occur in the 14- to 18-inch intermediate precipitation areas in the winter wheat-fallow rotation or other 2-year rotations when the fall and winter weather is conducive. The disease rarely occurs in the winter wheat-fallow areas below 14 inches annual precipitation, but again the incidence is dependent on weather conditions.

Symptoms

Cephalosporium stripe symptoms become visible in the spring after the wheat resumes growth. The most obvious symptoms are long yellow stripes that extend down from the tip of the leaf blade to the sheath, then down the sheath to the node. Typically, two to three yellow stripes appear on each leaf. Extension Bulletin 1434 contains color pictures and descriptions of other diagnostic characteristics for identifying the disease.

Management and Environmental Factors

The severity of Cephalosporium stripe depends on many management and environmental factors including: amount of inoculum present, seeding date, varietal susceptibility, weather conditions and soil pH. Murray points out that although the factors individually influence the amount of disease, they also interact with each other, resulting in more or less disease. The entire production operation must be considered to understand how the factors influence the disease potential.

Inoculum Level

Inoculum density (number of spores in the soil) depends on the length of crop rotation, tillage and residue management and fall weather conditions. The exact relationship between the amount of disease and amount of inoculum is unknown, but more inoculum increases the chance of the disease.

Rotation Murray stresses that crop rotation is the single most effective management option for controlling Cephalosporium stripe. Longer rotations, which allow time for residue decomposition, reduces the amount of inoculum in the soil. The pathogen dies when the infected host residue decomposes.

Even with conventional tillage or residue removal, a 2-year rotation can still result in high levels of Cephalosporium stripe if environmental conditions promote spore production and root injury. Deep burial or removal of infested residue is rarely complete and sufficient inoculum may be produced to severely infect the subsequent wheat crop. Very high levels of Cephalosporium stripe have been reported on early-seeded winter wheat on conventional "black" fallow in a wheat-fallow rotation.

A 3-year rotation, which includes 2 years out of winter wheat or winter barley, is most effective in reducing the inoculum level. Examples of rotation options include nonhost crops such as spring wheat, spring barley, peas, lentils, or rapeseed, and fallow. Many winter annual and perennial grassy weeds are also host for the fungus and must be controlled for the rotation to be effective.

Tillage The impact of tillage systems on Cephalosporium stripe disease potential is complicated by several interacting factors including: surface residue influence on soil freezing and frost heaving; crop rotation; tillage rotation; seeding date and inoculum level. Under the higher surface residue levels of conservation tillage systems, soils freeze less often and to a shallower depth than under conventional tillage systems. Consequently there is less soil heaving, which reduces root injury and decreases the potential infection sites for the pathogen.

On the other hand, the fungus produces spores more profusely when infested residue remains on or near the soil surface, resulting in more inoculum in the soil. In contrast to infested crop residue, which tend to promote the disease, non-infested and non-host crop residue remaining after winter wheat is seeded under minimum tillage or no-till systems limits Cephalosporium stripe development. This is a result of the insulating effect of the residue reducing the potential for root injury.

In the Northwest, the most severe erosion problem occurs where winter wheat is seeded late into a residue-free, conventionally-tilled seedbed. Therefore, the most important tillage-residue management consideration for erosion control is at the time of winter wheat seeding and not after harvest of the winter wheat crop. A flexible tillage program can then be selected for optimum disease reduction and erosion control.

For example, consider a 3-year rotation of winter wheat spring grain-legume. A conventional or minimum tillage system could be used to seed the spring grain, following the winter wheat crop, to speed decomposition of the winter wheat residue if it was infested with Cephalosporium stripe. The third rotation crop (a legume) could be seeded with minimum tillage into the spring grain residue. Winter wheat could then be seeded with minimum tillage or no-till after the legume. This tillage-crop rotation sequence would reduce Cephalosporium inoculum production potential, and reduce root injury and infection potential in the winter wheat. This would also help control the critical, overwinter soil erosion in winter wheat. In the lower precipitation areas where take-all disease is not a problem, a second spring grain crop or fallow could be substituted for the legume.

Seeding date tends to reduce the differences in disease level associated with tillage systems. In a recent field study, under a winter wheat-fallow rotation near Palouse, WA, Murray found that winter wheat seeded under both conventional tillage and no-till had 10 percent of the tillers infected with Cephalosporium stripe when an October 1 seeding date was used. However, with a September 20 seeding date, the percent infected tillers increased to 25 percent under conventional tillage and 31 percent under no-till.

Fall Weather — Cool, wet fall weather provides good growing conditions for the wheat, but also stimulates spore production by the fungus. A 40 to 50F temperature range appears to be optimum. Fall weather, which is conducive to spore production, increases the potential for future plant infection. However, the most important weather factor determining infection level is weather during the winter and early spring when frost heaving and soil freezing results in root injury.

Seeding Date

Early seeding of winter wheat in a 2-year rotation favors Cephalosporium stripe. Any factor that promotes rapid fall growth will probably increase the potential for infection. Early seeding produces larger wheat plants than late seeding by the time winter weather slows growth. These larger plants have larger root systems which are more susceptible to overwinter injury.

In a 3-year rotation, however, an earlier (or' 'normal") seeding date would not significantly increase Cephalosporium stripe because of the decreased inoculum level. An earlier or "normal" seeding date, instead of late seeding, is also the most effective method of reducing damage from Pythium root rot.

Varietal Susceptibility

None of the winter wheat varieties in the Northwest are

highly resistant to Cephalosporium stripe but some are more resistant than others. Of the current, widely grown varieties, Stephens is the most susceptible. Lewjain, Daws, Luke and Hill 81 have intermediate susceptibility. Lewjain has generally produced the highest yields when the disease level was severe.

Weather Conditions

While fall weather determines the amount of disease inoculum produced, weather conditions during the fall, winter and early spring influence the infection potential of the plant. Like early seeding, weather which promotes vigorous growth of the crop in the fall also provides an extensive root system. The larger the root system is going into the winter, the greater number of potential infection sites there are if weather conditions promote root injury.

One of the most important factors determining crop infection level is winter and early spring weather. Murray points out that optimum weather for infection includes mild, open winters with cold periods which result in frozen soils. Similar conditions in the early spring lead to frost heaving and root breakage.

Soil pH Impacts

Experiments by Murray and other researchers has shown that increasing soil acidity (declining soil pH) increases the incidence and severity of Cephalosporium stripe in the absence of root injury from soil freezing. The reason for the disease increase with lower soil pH is unknown and is currently being researched. Several theories are being explored. Winter wheat may have a reduced resistance to infection at lower soil pH, either through lower root resistance to penetration by the pathogen or lower internal plant resistance. Increased infection may also be because of less competition from other soil microorganisms or that the pathogen grows better under low pH conditions.

Experiments have shown that soils having a pH less than 6.0 favor disease development as well as spore production and survival. Under the same weather conditions, seeding date and inoculum level, more disease developed when the soil pH was less than 6.0. Research results indicate that the fungus produces the most spores in the 3.9 to 5.5 pH range.

Research also indicates that the fungus survival in infested straw increases as soil pH declines. This is because of a wide-spectrum antibiotic produced by the fungus which inhibits other microorganisms in the soil. This decreased microbial activity slows decomposition of the residue colonized by the fungus. The antibiotic is produced more profusely and is more active under acidic conditions. In addition to surviving longer in the slowly decomposing residue, the spores also appear to survive longer in the soil as the soil pH declines,

Breeding winter wheat lines for resistance to Cephalosporium stripe has been difficult because of the yearly variations in disease severity in field plots. Murray currently has a greenhouse testing program underway to determine if increased soil acidity and resultant increases in disease can be used as a screening test for plant resistance. If the tests are successful, selection and development of resistant varieties could be accelerated.

Control Recommendations

Murray stresses that a 3-year rotation provides the most effective control of Cephalosporium stripe and allows the most flexibility in choosing a variety, tillage system and planting date. In addition a 3-year rotation may help control Pseudocercosporella (strawbreaker) foot rot. The earlier seeding date possible under a 3-year rotation also can control Pythium root rot by avoiding cool wet conditions favored by the disease at seeding.

If a field has a history of Cephalosporium stripe and a 3-year rotation is not possible, Murray recommends using a less susceptible variety and a slightly-delayed seeding date to help reduce the disease potential. Minimum or no-till seeding of winter wheat into non-host crop residue, regardless of rotation, may also reduce infection potential by reducing root injury.

     
 

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