ISSUE 9 July 10, 2008
FINAL STAGES FOR FUNGICIDE APPLICATION TO WHEAT AND BARLEY
The wheat and barley crops advanced considerably over the warm 4th of July weekend, and many wheat crops flowered over that weekend. For some producers, fungicide decisions have already been made, and attention will be turned to the later planted fields.
For those later planted fields, applications of fungicides to control head scab at flowering stage also provide excellent control of leaf rust. And the products available for head scab control also give excellent control of leaf rust and leaf spot diseases.
Example of leaf rust control: In 2003 at the Carrington Research Extension Center, Blaine Schatz, superintendent of the Center, applied Folicur fungicide (4 fl oz/acre) to four varieties of spring wheat, at two growth stages: Feekes 10 (boot stage), and early flowering (Feekes 10.51).
On varieties 2375 (S), Reeder (MS), and Alsen (MR), the application at flowering gave much better control of leaf rust than applications at boot (from 10 to 45% better control). On the most rust resistant cultivar of the four tested (Briggs), the flag leaf and flowering application gave equal control of leaf rust. In ND on spring wheats, the development of leaf rust and Septoria fungal leaf spot generally gets going later in the season than in states to our south. Application at fungicides at flowering often provides the 21 days of protection needed to get the crop through grain fill.
PLETHORA OF TEBUCONAZOLES
The federal registration of Folicur (tebuconazole) in May opened the door for other federal registrations as well, because the patent on Bayer Crop Science’s tebuconazole had expired. Jim Gray, Pesticide, Feed, and Fertilizer Team Leader at the ND Dept. of Agriculture, has quickly acted upon requests by various companies for supplemental labeling allowing the use of other tebuconazoles on wheat and barley. As of July 3, 2008, Mr. Gray announced that supplemental labels for seven tebuconazoles are now posted on the ND pesticide registration database (http://www.kellysolution.com/nd/pesticideindext.htm)
The names of the tebuconazoles and their manufacturer are:
Emboss (Winfield Solutions)
Embrace (Winfield Solutions/Agrilliance)
The use pattern of all the products is the same, but an applicator MUST have the supplemental label of the appropriate product in hand, when application is made.
NDSU IPM SCOUTS FIND MORE WHEAT LEAF RUST
Wheat: Detection of leaf rust picked up during the 4th of July week, with 11.3% of the 168 wheat fields scouted showing some level of leaf rust. These detections were all in counties in the southeast quadrant of the state. The average severity of leaf rust in infected winter wheat fields was 9.2%, while average severity in positive spring wheat fields was 1.9%.
Tan spot remained the most common wheat disease observed, in 60% of the fields, with the northeast region showing the least amount of disease. Loose smut was increasingly observed, in 15% of the surveyed fields, and the average percent incidence of tillers with symptoms was 5.8%, but as high as 20% in one field. Loose smut is controlled with systemic seed treatments.
Loose smut in wheat
Other wheat diseases observed included Septoria leaf blotch, bacterial leaf blight, black chaff, and some powdery mildew, all generally infrequently, although Septoria levels are increasing now that the crops are headed out.
Barley: The NDSU field scouts looked at 20 barley fields last week. Fungal leaf spots, including net blotch and those caused by Septoria species, were generally the only diseases observed in barley.
PRE-HARVEST INTERVALS FOR SMALL GRAIN FUNGICIDES cont.
Last week’s NDSU Crop and Pest Report contained a table indicating the pre-harvest interval required for common small grain foliar fungicides. This table related to pre-harvest intervals for wheat or barley used for grain. Different limitations may apply if the crop is used for forage or silage, grazed or used for bedding. Individual labels must be consulted for these additional restrictions.
Inadvertently, last week’s table on pre-harvest intervals did not include the mancozeb products such as Dithane, Manzate, and Penncozeb, nor were the copper containing products, such as Champ, Cuprofix and Kocide, included. The pre-harvest interval for mancozeb products is 26 days, and the copper product labels say "the first application may be made at early heading, and may be followed with another application 10 days later".
NDSU Extension Plant Pathologist
FUNGICIDES, FUNGICIDE RESISTANCE, AND LOW-RATE RECOMMENDATIONS
I have had several questions about applying multiple applications of reduced rate fungicides. The questions have been raised in part to the high value of commodities and the high value of inputs resulting in a lower breakeven point, and due to success with early-season low-rate applications on wheat. I understand why the questions are being asked, but to explain this well, I need to discuss a few things about fungicides first.
Fungicide applications have the potential to increase your profit margin. However, fungicide applications are not without risk. If disease pressure is low, you may not increase your yield enough to breakeven, which is why we encourage you to scout for disease and/or use one of the forecasting models (if available) to help make spray decisions. However, the greater concern over the long-term is related to the development of fungicide resistance.
To describe how fungicide resistance develops, we will use a human example. Turn on the T.V. and you will see people of all shapes and sizes, but we are all humans. This is just natural variation in the human population. Humans are also variable in ways that we can’t see. Imagine that you have a stress on the human population, like the flu. We all know people that don’t ever seem to get sick when everyone around them does. They are just a little better at fighting off illness.
Pathogen populations have a lot of natural variation too. Instead of the flu however, we apply stress to the pathogen by applying fungicides. Most of the time, the disease is significantly reduced after an application, however, there may be an individual pathogen in that population that is not affected by that fungicide, just like your neighbor that never gets sick. The pathogen that is able to survive in the presence of the fungicide is called - resistant. If all the other individuals are sick or killed, resistant individuals will multiple and spread very easily because they have no competition from other pathogens. If the fungicide is applied repeatedly, the resistant individual’s offspring can become predominant in the population, and the fungicide is no longer effective.
There are some factors that influence the development of fungicide resistance. First, some pathogens are more variable than others, and have a proven track record for developing fungicide resistance. Second, some fungicides are more likely to lose effectiveness than others (this is because of the biochemical mode-of-action they have on the pathogens). Third, the frequency and rate that the pathogen is exposed to the fungicide will influence the development of fungicide resistance.
We can’t control the variability of the pathogens, or the inherent risk of the fungicide itself, but we can control the exposure. Anytime the pathogen is exposed to a fungicide, there is a chance that a resistant member of the population will survive and become predominant. This is most true when the fungicide is applied at a reduced-rate. If the fungicide is effectively ‘weak’, there is a greater chance to select for some individuals that have a lower level of resistance (a greater likelihood that a more people will be able to fight off a weak strain of the flu, while they may not be able to fight off a stronger strain). Over time, the odds of fungicide resistance developing may increase with reduced rates. Consequently, we do not recommend using a below-label rate.
The one low-rate exception to this is a recommendation that NDSU has made on wheat. The recommendation is for a low-rate fungicide application to control tan spot of wheat, but only when a susceptible cultivar is planted on wheat ground. NDSU has accumulated years of data that show this reduced-rate is able to effectively kill the pathogen and control diseases when applied early. Additionally, there is good evidence to suggest that the tan spot fungus is unlikely or very slow to develop fungicide-resistance. Only in recent years has the Septoria fungus, another similar leaf spot pathogen, showed some level of resistance to continued strobilurin fungicide use in Europe.
In most broadleaf crops however, we don’t have nearly the amount of fungicide data that we do on wheat. We simply don’t know if reduced rates will actually control the diseases in many of the broadleaf crops. Furthermore, we know that some pathogens on broadleaf crops are prone to developing fungicide resistance quickly. Fungicide resistance has been observed in pathogens of potato, sugarbeet, and chickpea in North Dakota. Many related pathogens cause disease on other broadleaf crops in the state, making fungicide resistance a real concern for us.
Consequently, we recommend that you use a fungicide application only when you need it to control disease, and use the recommended rate.
Ext. Plant Pathologist
Ext. Plant Pathologist
TAKE NOTE OF DOWNY MILDEW OF SUNFLOWER
Downy mildew of sunflower has recently been observed in North Dakota. Although control of the disease at this point in the season is not possible, the observation of downy mildew is important for future years.
Downy mildew generally causes a systemic infection, which results in a severely stunted plant and death in the seedling stage. Although it can also cause a secondary infection, which occurs later and causes limited damage to plants. When plants are systemically-infected with downy mildew they will often be stunted and their growth will be distorted.
The leaves may appear ‘crinkly’ with yellowing on the top sides of the leaves (particularly along the leaf veins) with a white cottony growth on the underside. Secondary infections appear as small chlorotic (yellowed) spots on the leaves and do not result in a systemic infection or stunting.
Sunflower with a systemic downy mildew
infection next to a healthy sunflower.
Note: plant stunting with deformed and yellowing leaves.
Yield losses to downy mildew are generally low because systemically infected plants often die, and adjacent sunflowers are able to compensate by producing larger heads. In general, yield loss is not usually observed if less than 5% of the plants are infected. However, when infected plants are in clusters or in rows the compensatory ability of sunflowers is reduced, and yield loss in that part of the field may be significant.
Secondary infection usually causes no yield loss.
Once downy mildew occurs there are no tools to control it, thus, prevention of downy mildew is the best management strategy. The pathogen produces spores that may remain in the soil for many years, and when the environment is favorable, may germinate and cause infection for up to five years after downy mildew was first observed in the field. For this reason, it is important to take note of any downy mildew occurring in your fields. Several years of rotation, downy mildew resistant varieties, and seed treatments are recommended when returning to a field with a history of downy mildew.
Extension Plant Pathologist