From HLB (Huanglongbing) to MRLs (maximum residue levels), fruit growers face all kinds of problems when it comes to diseases. Thank goodness for R&D (research and development).
The various crop protection companies put a lot of horsepower into their R&D departments, and growers can reap the benefits. To find out more about what’s happening in the world of disease control, we posed five questions to two experts: Mike Cunnane, product manager, specialty fungicides and herbicides, DuPont Crop Protection; and Layne Wade, technical service manager for the Arysta LifeScience horticulture group. The two managers explore a wide range of issues.
1. What are the biggest challenges fruit growers are facing regarding disease control these days? Please be as specific as possible.
Cunnane: Understanding the various fungicidal modes of action, connected to the active ingredient/brand names, connected to the multiple new brand names that are pre-mixes of two different active ingredients, and lastly, connecting the FRAC (Fungicide Resistance Action Committee) code to these same products is a big challenge. The best reference guide I know of, to cut through clutter, is the FRAC code list from 2012 (www.frac.info). By simply identifying the active ingredients in fungicide products, a production manager or grower can identify both the FRAC code and the potential of that active ingredient to develop resistance. Generally, the most active and effective fungicides have the greatest risk of developing resistance, due to “strong” selection pressure against the pathogen. At the same time, a balance when using these products is to use a full rate to not leave semi-tolerant pathogens behind to reproduce. This “balance” is what makes rotation to different FRAC code category fungicides, at full rates, a strong strategy to protect fruits and vegetables while keeping all products effective, longer.
The other challenge growers face is exporting their crops to multiple countries which may or may not have MRLs (maximum residue levels) established on all the fungicides they’ve applied. In worst-case scenarios, exported crops can get delayed or rejected through inspection when active ingredient residues are detected and the country’s import group can’t determine if they are a health concern.
Wade: The biggest issues fruit growers currently face include disease control—pathogens developing resistance to commonly used materials, new varieties which show higher susceptibilities to various disease causing pathogens, and newly emerging horticultural practices which sometimes provide conducive environments for established pathogens, as well as encouraging new pathogens to become bigger problems than they were historically. Specific examples include powdery mildew of pome fruit developing resistance to strobilurin fungicides; the new Coral cherry variety, which has extreme susceptibility to bacterial blast caused by Pseudomonas syringae; and Alternaria leaf spot, which became a serious disease of almonds when standard plant spacing become “tighter” leading to increases in orchard humidity levels.
2. It seems like there are more diseases threatening fruit crops than ever these days. Is that actually true, that disease pressures are greater, or is the fruit growing community just becoming more aware?
Cunnane: I suspect that the fruit growing community is becoming more aware of the presence of diseases and how and when the conditions exist. The increasing annual value of exported fresh fruit and vegetable crops, driven by population growth, is making additional investments in planned fungicide applications a good business practice, versus waiting to “‘see” disease.
Wade: In some instances there are more disease issues appearing in fruit crops today. For example, citrus greening (or HLB), which is caused by a bacterium, has become a serious threat to citrus in California and Florida as the Asian citrus psyllid (which vectors the organism) has increased its range.
In other cases, it may seem like more diseases are occurring; but this impression is created by new varieties of fruit which have more susceptibility to common, long-established pathogens.
3. Have any diseases emerged in recent years that are particularly challenging for fruit growers?
Cunnane: I’ve only been a fungicide product manager a short time and none come to mind.
Wade: Citrus greening is one of the most concerning new diseases emerging in fruit crops, in large part because of the relationship it has with its vector as well as its characteristic of being a systemic (or internal) infecting organism. This makes it very difficult to get control agents to reach the pathogen. Other “bacterial diseases” to emerge are simply becoming bigger problems because of their preponderance to develop resistance to the few materials registered to control them. There are several bacterial isolates which have developed reduced sensitivity to copper exposure (i.e., Xanthomonas spp. in fruiting vegetables and Pseudomonas spp. in tree fruit crops).
4. A lot of researchers are concerned about resistance. Are there any diseases for which that’s a particularly acute problem? What is your company doing to address
Cunnane: Our company’s strategy on new fruit fungicide introductions is single-active materials that “live” in FRAC code categories not currently under resistance duress. Five reasons why a single active fungicide like DuPont Fontelis at full rates is a better resistance management approach than premixes of two different modes of action:
1. The best practice to prevent resistance is to reduce exposure (premixes increase exposures to both active ingredients).
2. A premix is only a valid resistance management tool if both active ingredients are fully active on the target disease.
3. A better approach to mixtures is actually combining single sites with multi-sites versus combining two single sites. (Thirty years of field experience is demonstrating this.)
4. Premixes frequently put on the wrong ratio and the wrong rate of one or both active ingredients versus what is needed in the orchard at the time of application.
5. You know better than anyone else what you need in your orchard. Giving you the ability to prescribe the best spray program for your orchard lets you do what you do best.
Wade: Most bacterial pathogens have tremendous ability to develop resistance to control materials largely because of the huge number of “generations” they produce in short amounts of time. In general, the agricultural industry has had very few efficacious bactericides available to control diseases caused by bacteria, which increases the exposure of isolates to the same material.
5. Has your company come out with any new products and/or label additions that would be of interest to fruit growers?
Cunnane: We’re anticipating the introduction of Fontelis fungicide into California this month (Editor’s Note: this news was made official in January), as the California Department of Pesticide Regulation 30-day public comment period is over. Fontelis will be registered on strawberries, apples, pears, cherries, peaches, plums, and tree nuts.
Wade: Arysta LifeScience North America has worked diligently over the last eight years to register Kasumin (kasugamycin), an agricultural antibiotic that will provide a new mode of action. We have extensively researched techniques to maintain efficacy of Kasumin and minimize the incidence of resistance for preparation of Kasumin’s registration and subsequent introduction into commercial use. Additionally, Arysta LifeScience North America has registered the fungicide Ph-D for use against diseases caused by various fungi. It has a unique mode of action, chitin inhibition, which prevents fungal cell wall development. This makes it an outstanding rotation and tank-mix partner with other fungicides.