By Jaime Cummings and Ken Wise, NYS IPM
With spring underway, our fall planted wheat, barley, rye and triticale crops have woken up and are in early developmental stages, and some spring planted barley, oats and rye are emerging. By now, you’ve already assessed plant stands for winterkill or other weather-related damage and weed pressure. But did you look for signs of diseases?
Fungicidal seed treatments protect our small grains crops from soilborne pathogens that cause damping-off, and foliar fungicides may be warranted for many of our common early-season foliar diseases. Early epidemics from these pathogens may spread throughout the canopy as the season progresses, given favorable weather conditions, resulting in potential yield reductions. An integrated approach for managing these diseases involves crop rotation, residue management, planting pathogen-free seed of resistant varieties, proper fertility and canopy management, and foliar fungicide applications where necessary. Susceptible cultivars benefit most from fungicide applications for reduction of diseases. While scouting, keep an eye out for some of our most common early season diseases:
Powdery Mildew can be commonly found lower in the canopy of all small grain crops, and is easily identifiable by its white, fuzzy fungal growth on both the upper and lower leaf surfaces (Fig. 1). This pathogen overwinters on straw residues which provide inoculum in the spring to blow into growing wheat stands. Cultivars vary in their levels of resistance to powdery mildew, and resistance is the most cost-effective method of defense. Should you find high levels of powdery mildew in your field, you may consider a triazole foliar fungicide application at flag leaf emergence.
Septoria and Stagonospora Leaf Blotches are two other very common foliar diseases you may encounter in NY wheat fields. Though caused by two different pathogens, the leaf spot symptoms can be somewhat similar and difficult to differentiate for scouts (Fig. 2). Both fungal pathogens overwinter on crop residues, and initial infection often occurs on seedlings emerging in the fall, but may also occur in the spring. Many commercially available cultivars are available with varying levels of resistance to these pathogens. But if you notice high levels of leaf blotches in your field, you may consider a foliar application of a strobilurin or triazole fungicide at flag leaf emergence to protect yield.
Leaf Rust, or brown rust, can occasionally be found early in the season, though it often appears later as the spores migrate on winds from the south. Leaf rust is easily identified as bright orange pustules on the upper leaf surfaces throughout the canopy of all small grain crops (Fig. 3). Rust fungi are obligate pathogens, which require a living host, and therefore must either over-winter on alternate hosts, or arrive on wind currents from the south. Planting resistant cultivars is the most cost-effective management tool, and a flag leaf application of a strobilurin or triazole fungicide also offers effective control of leaf rust on susceptible cultivars.
Scald is another fungal leaf spot that easily identified on winter malting barley by its distinctive lesions that can be found on any above-ground plant tissue (Fig. 4). The fungal pathogen can over-winter on residues or can be seed-transmitted. On susceptible cultivars and under favorable environmental conditions, scald can spread quickly throughout the canopy and field, resulting in significant yield loss. Pay attention to scald resistance ratings when selecting malting barley cultivars to plant, as they vary drastically in susceptibility. Susceptible cultivars will benefit from a triazole fungicide application at flag leaf, or even earlier to protect yields.
When deciding whether or not to spray a fungicide, there are many points to consider. Not least of all, cost. Use the following information as a guide from the Cornell Integrated Guide for Field Crop Management for making fungicide decisions in small grains:
Activity Worksheet: Economical Analysis of Thresholds in 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. ____ Yes ___ No
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. ____ Yes ___ No
Have foliar diseases been observed during head emergence?
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. Fungicide applications made after early June may control some diseases but are unlikely to produce significant yield benefits. ___ Yes ___ No
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. What is your short to medium term weather conditions?
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. Check in the Cornell University Guide for Integrated Crop Management.
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. At a grain price of $4, producers will need to see approximately a 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 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 scab disease (fungus that infects heads at or following flowering), viral diseases (wheat spindle streak mosaic and yellow dwarf), soilborne diseases, or several other environmental factors.
Relative Profit = (Grain Yield Increase x Grain Price) – (Cost of Fungicide + Application Costs)