Tips For Management of Diseases
By Sam Markell & Robert Harveson*
Diseases are a fact of life with virtually any crop, including sunflower. It has been estimated that for every one crop grown, you have 100 different diseases. Fortunately, only a handful are present at any given time or are challenging enough to the crop that we need to actively manage them.
However, disease management is a moving target, and the most destructive diseases one year may not be the same diseases the following year. For example, different diseases cause problems in wet cycles than in dry cycles. (How much Sclerotinia head rot occurs in drought years?) . . . Pathogen race changes occur (think rust or downy mildew). . . . And a myriad of other factors, ranging from plant stress to host resistance, influences the diseases we have to manage.
Effective disease management can make a big difference in final sunflower yield and quality. Below are some key “general” points that will help you manage disease in sunflower, and in most of the other crops on your farm.
1. Integrated Pest Management (IPM) — Pathologists drop this acronym frequently, but for good reason. The simple concept is that you want to use multiple strategies to manage disease, not just one. Although pathologists tend to talk about miscellaneous (and sometime nebulous) potential negative consequences when growers rely on one management technique (how often have you heard us say “fungicide resistance development” or “race change”), the concept for a single grower is much simpler. IPM reduces your chances of a management failure occurring. If the resistance doesn’t work, you might be covered because you rotated well, are prepared to put down a fungicide, etc.
2. Know the Enemy — There is an increasing amount of information available to help in disease identification and management. The National Sunflower Association does an excellent job of providing good pictures and key points about diseases that you are likely to see. Pathologists realize that differentiating diseases can be difficult, and as a result, questions and/or submissions to county agents, research extension centers and diagnostic labs are encouraged.
Similarly, the NSA and the universities will try to get critical information out as diseases are occurring. Whether through the NSA website, the NDSU Crop & Pest Report or radio spots, it is important to stay informed.
3. Find the Enemy — Scouting is one of the most important parts of disease management. If you are uncertain what disease(s) you have, it is very difficult to make informed decisions about specific management tools.
For example, if you find out your ’flowers have downy mildew, then the next time you can go back in that field you might want to plant a downy mildew-resistant (DMR) hybrid or use a different seed treatment.
Similarly, foliar fungicides are most effective at the early stages of an epidemic. If you get behind on rust, for example, you can be “toast.” Furthermore, foliar fungicide applications would not be effective in managing downy mildew; thus, correctly diagnosing the problem is critical .
4. Rotation — Sunflower growers typically are very good at rotations. Four-year rotations are generally recommended. Almost all pathogens will be hurt by this length of rotation. They don’t disappear, but there’s a reason why growers don’t plant ’flowers on ’flowers. Also, be aware of Sclerotinia- susceptible crops in the rotation. Dry beans, canola and potatoes are all quite susceptible to Sclerotinia and are not the best choices for inclusion in sunflower rotations.
5. Resistance — For a variety of reasons, selection of a hybrid is perhaps the most important decision you make. Although pathogen changes are frequent with several diseases, there are some growers who get hit often with the same disease. Maybe it’s micro-climate, maybe it’s bad luck; but, if you anticipate getting rust, plant a resistant hybrid. If you had lots of downy mildew in the field you are going back into you, try to get a DMR hybrid.
Be aware that resistance does not guarantee control. None of the resistant varieties confer “immunity.” As an example, a new race of the downy mildew pathogen was identified, which calls into question the DMR status of hybrids if you have the new race.
6. Fungicide Seed Treatments — Most, if not all, sunflower seed comes treated. There is good reason for this. Downy mildew is the most important pathogen that you try to manage with seed treatments, but other root rot pathogens lurk. There is a limited amount of information concerning the pathogen spectrum found underground on sunflower; but in most crops, seed treatments will help protect the germinating seed during the most important part of the plant’s development. Establishment of a healthy stand is critical for a healthy crop.
7. Foliar Fungicides — On most crops, multiple diseases can be managed with foliar fungicides. On sunflower, the most important disease to manage is rust. Recent research has shown that rust pressure can reduce yield and test weight dramatically. In general, the time to pull the trigger is when rust severity on the upper leaves approaches 1%. However, this tended to correlate pretty well with the R5 growth stage (bloom), and an R5 application when you have rust often resulted in less yield loss and better disease control. The exception occurs if you see lots of rust in the vegetative stages, which likely mandates multiple (and early) applications.
Research on the management of other diseases with fungicides, most notably Sclerotinia head rot and Phomopsis stem canker, is under way. In both cases, disease reduction with fungicides has been observed in preliminary data. Due to the lack of information (and availability of fungicides), we are not yet ready to make concrete recommendations. That being said, the situation changes fast, so stay tuned.
8. Stay Engaged and Adapt — The world changes quickly in agriculture. In the future, we would anticipate diseases and disease management will be different. We may have new diseases (or races) to worry about, new resistance in hybrids to combat them, and new fungicides and new recommendations to go with them. The more knowledge you have about disease management, the more likely you are to be able to manage diseases in a changing world.
* Sam Markell is extension plant pathologist with North Dakota State University. Robert Harveson is plant pathologist with the University of Nebraska, Scottsbluff.
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