A.L. Black and Armand Bauer Soil Scientists, USDA-ARS,

Northern Great Plains Research Center

Mandan, ND 58554

Crop residues present on the soil surface are a deterrent to accelerated erosion and simultaneously conserve precipitation through reduction in water evaporation rate (compared to a bare surface) and through their water-trapping capability. Some reluctance to adopt or continue usage of tillage systems which leave residues on the surface ostensibly is being expressed by producers because of concern for plant disease infestation being accentuated by the presence of residues.

The purpose of this paper is to provide some soil and crop management guidelines that help reduce the risk and level of infestation of plant diseases of wheat. These management tips are based on many years of research experience and observations by the authors.

Three conditions must be present for a plant disease to occur. There must be a susceptible host, a pathogen. and the proper environment for the disease to flourish. If only one of these is absent or diminished. infestation will not occur or will be diminished. Many producers believe that nothing can be done to diminish diseases and that their wheat crop is always susceptible. that the pathogens are usually present or will blow in from their neighbor's land, and that they cannot do anything about the weather or the plant environment. There are many soil and crop management practices and cultural practices which influence the three factors of the disease-triangle and subsequently alter the incidence and magnitude of wheat disease infestations.

1. CONTROL VOLUNTEER WHEAT

Volunteer cereal grains, particularly wheat plants. harbor mites which transmit virus diseases and plant pathogens that serve as a source of inoculum for the succeeding crop. if not controlled. Most notably. disease which is transferred to a succeeding wheat crop is wheat-streak mosaic. A cardinal rule is to eliminate all volunteer wheat plants from the soil surface at least two weeks prior to planting a winter or spring wheat crop.

2. PLANTING DATE, RATE, AND ROW SPACING

For winter wheat, planting should be scheduled as early as possible in the fall after the mean soil temperature at the 1 to 2-inch depth is below 55oF. Planting in soils above 55oF increase the chance of infection of soil-borne pathogenic root rot diseases.

Because of the more rapid development of foliar diseases late in the season, late plantings of winter wheat (later than about Sept. 20) or spring wheat (after about May 20) increase the risk of diseases. Planting a spring wheat crop May 10 that does not germinate and emerge until June is the same as a late planting date. Usually the environment becomes more optimum for disease buildup in midsummer with warm, moist weather as opposed to somewhat drier, cooler weather in the late spring-early summer period. In our conservation tillage-cropping systems study, Dr. Krupinsky often wishes I would plant the wheat crops later, that is, in a more untimely manner so as to provide an environment more inducive to a higher pathogenic activity to enhance his study of diseases.

The planting rate and row spacing used also can be important because they influence the environment in which a given plant grows. The amount of plant stress caused by insufficient water, plant competition for nutrients etc. all alterthe environment. Planting rates of 300 to 500 (2) kernels/m(2) (1.5 to 2.0 bu/ac or more) and narrow rows (3 to 6 inches) increase plant population density, canopy cover, and humidity within the canopy for longer periods of time than in lower populations and wider row spacings. In other words, these practices change the plant environment to favor the growth of pathogenic organisms if not controlled by fungicide applications. Plant populations and potential yield goals can be set too high in relation to the amount of water available for the crop to sustain good growth rates through the flowering and grain-filling period. If water or nutrient stresses develop, the wheat plants become weakened and more disease-susceptible, so that diseases can have a more devastating effect. The combined effect of water and nutrient stress and foliar diseases on grain yield and quality are probably greater than the effect of any one of these alone.

A producer needs to plant early enough in the spring to avoid having the wheat plants in the 4.0 to 5.5 leaf stage of development (Haun) when average mean air temperatures are apt to be 78oF or. higher. Number of spikelets/spike are reduced from about 19 to 14 when mean maximum air temperatures during this development period are 78oF compared to 65oF. This decrease in spikelet number represents a 25 to 40% potential yield reduction that cannot be redeemed. It would be relatively easy to erroneously point to plant diseases as the cause of low kernel counts/spike if it were not known what change a given environment can impose on the wheat plant.

3. CROP ROTATIONS

The longer the time span between susceptible crops, the greater the chance of reducing the amount of pathogenic inoculum in the soil or in the crop residues. Continuous wheat culture poses the highest risk of increasing pathogenic activity and inoculum in a given field compared to other crop rotations. Even when a wheat-fallow cropping system is used, the disease risks are higher compared to an extended 3-year or 4-year rotation that includes an oilseed crop, spring barley, or oats. Inserting an oilseed crop every third or fourth year breaks weed and disease cycles. Of all the management practices available to a farmer to control plant diseases, the use of crop rotations of diverse crops is still the most dependable, effective, and economical way to reduce or eliminate the effect of wheat diseases on quality and yield.

In the higher rainfall areas (16 to 20 inches, or more) where diseases are often the yield limiting factor when other inputs are optimized, 3- or 4-year rotations of wheat, oilseed crops and cereal grains like spring barley or oats will provide the highest degree of disease control. In the lower rainfall areas (18 inches or less) where disease are often not the yield limiting factor and water usually is, rotations and residue management practices can be selected or developed more easily to minimize erosion and optimize production without increasing the risk of more plant disease.

4. NITROGEN MANAGEMENT (SOIL-N PLUS FERTILIZER-N)

Soil nitrogen, measured principally as the amount of nitrate-nitrogen (N0(3)-N), at any one time in the soil profile, is influenced primarily by the amount of soil organic matter present, N-fertilization level for previous crop, and the crop rotation employed. In a crop-fallow system soil water and soil N0(3)-N) accumulate during the fallow period to subsequently become available to the succeeding crop. Most fallowed soils contain 80 to 120 lb./acre of N0(3)-N) in the soil profile at spring wheat planting time. Therefore, it is relatively easy to over-fertilize with N on fallow. When this is done it can stimulate vegetative growth and total leaf area, and delay maturity and increase potential for foliar diseases.

In a continuous cropping system, like spring wheat-winter wheat-sunflowers, the NO -N levels in the soil profile in the spring are (3) low (20 to 40 lb. NO -N/acre). In these systems it is relatively easy to under-fertilize with N. Inadequate supply of N0(3)-N) weakens the plants resistance/tolerance to foliar diseases and more plant disease occurs at low than at more optimum N-levels. (See Joe Krupinsky's paper.) Nitrogen fertilizer management must be based on soil test information and the available water supply (stored soil water plus growing season precipitation) used together to determine a realistic yield goal. Other soil nutrients, particularly phosphorus and potassium need to be present in appropriate proportions to provide a balanced supply of nutrients. Unbalanced plant nutrition, like excessive N and low soil P, can prolong maturity of the crop and thus increase the exposure time to ravages of disease.

5. WHEAT VARIETIES

One of the most important aspects of variety selection is its resistance to a wide complex of soil-borne and foliar diseases. The degree of resistance or tolerance to foliar diseases of a given wheat variety is often amplified as production inputs and yield levels increase. Those varieties which exhibit little increase in leaf damage from foliar diseases as N-levels and crop yields increase should be the ones selected as opposed to those varieties that show greater leaf damage as production input increase. The importance of continually selecting the most disease resistant varieties of cereal grain crops in any crop rotation cannot be over-emphasized as an additional means of reducing the amount and source of inoculum for subsequent crops.

Although implied under crop rotations, source of stubble from previous crop is a factor in disease incidence and intensity. When the planted crop and stubble source are the same species, the least desirable management scheme is to plant a disease susceptible variety. The potential for a buildup of diseases is accelerated compared to situations where more disease-resistant crop varieties were used in the rotation.

6. CROP RESIDUE MANAGEMENT

The quantity and orientation of previous crop stubble such as; standing, leaning, flat on the soil surface, or incorporated In the soil has an effect on the amount of disease inoculum present to potentially infest succeeding crops. In a spring wheat-winter wheat-fallow rotation, winter wheat planted no-till into 15-inch stubble had more foliar diseases than when seeded into 2-inch short stubble (unpublished data, Krupinsky and Bauer). However, highest average yields (4-year average) of three different winter wheat varieties were obtained with no-till plantings into 8 to 10 inch standing stubble.

Spring wheat or winter wheat planted into conventional-, minimum-, or no-till cropping systems In our studies have various amounts of crop residue present, varying from mostly incorporated (conventional till), to leaning and flat (minimum-till), to mostly standing (no-till). The amount of foliar disease present in 1987 was similar in no-till and conventional till in the spring wheat-fallow rotation. But in the spring wheat-winter wheat-sunflower rotation, more foliar diseases were present for both spring wheat and winter wheat using conventional tillage rather than no-till. (See Joe Krupinsky's paper in Proceedings of this Conference). Also, variety selection and N-fertilizer management were also simultaneously affecting the amount of foliar diseases present. These interactions are complex in themselves, but when combined with environmental parameters, the possibility of separating cause and affect factors becomes even more complex. For Instance, if no-till cropping systems conserve more soil water and reduce evaporation losses more while the crop is growing than a conventional-till system, then the crop may not be under as much water stress and therefore less susceptible to foliar diseases. In 1987, the combined effect of water plus N stress in the conventional-till spring wheat plots following sunflower, increased foliar diseases significantly compared to the no-till systemwith less visible signs of water and N stress.

CONCLUSION

The importance of conservation tillage systems (minimum- or no-till) in maintaining crop residue on the soil surface to provide a protective cover against erosion and to conserve more precipitation in the soil should not be underestimated. Before producers condemn or abandon conservation tillage-crop production systems, they should consider the possibility that one or more of the first five management factors discussed in this presentation may very well be more important in developing superior soil and crop management technologies to avoid or reduce wheat disease effects on wheat quality and yield than the crop residue management factor Itself. More succinctly, be sure to consider all cause and effect possibilities pertaining to incidence and intensity of foliar disease when developing your management strategies.