Guidelines for Wheat Fungicide Decisions
The following criteria (summarized in Table 5.7.2) should be useful when considering a fungicide spray to winter wheat.
Does the crop have a reasonable yield potential?
Assess the crop in early May (stem elongation stages) for adequate stand (density of approximately 30 strong stems per foot of row for 7-inch rows on good soils) and plant vigor. If the stand is sparse or plants are not vigorous or show widespread virus symptoms, fungicide application should not be considered further.
Have foliar diseases been observed before flag (last) leaf emergence?
Assess upper three leaves for symptoms and signs of powdery mildew, leaf spots, or leaf rust in early to mid-May, before flag leaf emergence. If disease (any amount) is observed on approximately 50 percent of main tillers, averaged across the field, a spray should be considered now. This threshold is exceeded in less than 50 percent of location/year situations in New York, so there is a significant risk of making an unnecessary fungicide application.
Have foliar diseases been observed during head emergence until the initiation of flowering?
Assess upper two leaves for foliar diseases in late May to early June; if disease (any amount) is observed on approximately 50 percent of main tillers, a spray should be considered now. Only applications made at the initiation of flowering (i.e., yellow anther stage) on main tillers will have significant efficacy against Fusarium head blight and mycotoxin (deoxynivalenol) contamination. Only triazole products are recommended after heading because late strobilurin application has been associated with increased accumulation of deoxynivalenol in certain situations. The Penn State FHB Risk Assessment Tool (www.wheatscab. psu.edu/riskTool_2010.html) is an additional source of weather-based information to help you decide on the risk of Fusarium head blight development in your fields at the initiation of flowering.
Are climatic predictions conducive for further disease development?
Powdery mildew development is reduced dramatically once the average daily temperature rises above 70˚F; this disease often disappears by June. Severe leaf spot development is favored by extended periods of wet weather; it may be insignificant if dry weather persists in May and June. Listen for regional advisories on the threat from leaf rust; rust inoculum often builds up in areas to the south and west of New York and is deposited here by thunderstorms in June or July. In addition to disease observations, use long-range local weather forecasts in making your spray decision.
Have I selected fungicides appropriate for the disease spectrum and have I read the label carefully?
Be sure that the materials you spray will be effective against the range of diseases found in your field; e.g., some products effective against powdery mildew are ineffective against leaf spots or vice versa. Consult Table 5.7.1.
Is the spray decision consistent with my perception of risk?
A simple formula for evaluating the relative economics of a fungicide spray is: Relative Profit = (Grain Yield Increase x Grain Price) - (Cost of Fungicide + Application Costs). If ground spray rigs are used, the yield lost to wheel traffic should also be factored in. Each of these variables influences the relative economics of fungicide application as illustrated in Table 5.7.3. At a grain price of $4 and a fungicide plus application cost of $30, producers will need to see approximately a 7.5 bu./A yield increase to break even on the added costs of fungicide application. Because disease occurrence is erratic over years and locations, fungicide application cannot be expected to result in a 7.5-bushel or greater yield increase every year. Spray decisions should be tied closely to disease scouting information. When considering your economic risk, also be aware that foliar fungicides will not protect potential yield components that may be diminished by viral diseases, soilborne diseases, or several other environmental factors.
Sound Agronomic Practices
Management practices that optimize cereal yields in the absence of disease are basically the same ones used to limit the impact of diseases. The following are most critical:
Optimal planting date. Fall grains should be planted soon after the Hessian fly-free date. This strategy helps avoid peak populations of aphids carrying barley yellow dwarf virus and also minimizes fall infections of seedlings by soilborne diseases. The strategy for spring grains is to plant as early as soil temperatures and moisture allow. By doing so, plants will often be past the most vulnerable stages when they are exposed to viruliferous aphids and pathogenic fungi.
Rotational sequence. Planting a small grain, especially the same species, twice in succession should be avoided. Rotation helps reduce the buildup of disease inoculum in the soil and in crop debris. Planting small grains following corn that is badly affected with stalk rot is also inadvisable because the Fusarium-infected debris can provide inoculum for scab epidemics on small grains.
Balanced fertility. Although increased fertility, especially nitrogen, often increases the severity of diseases, the well-fertilized, vigorous stand has an increased yield potential even in the presence of disease.
Weed and insect control. Weed and insect management is a part of disease management because these pests serve as reservoirs and vectors, respectively, of plant pathogens. Also, weeds reduce airflow in the crop canopy and produce higher humidities conducive to disease development.