By necessity, commercial cropping concentrates plants of the same species into a small geographical location. As a result, this makes it difficult to eliminate pests and diseases once they have gained a foothold. In perennial crops such as fruit trees, nut trees, and vines, optimal plans for combatting pests and diseases must begin with preventative measures put in place before the growing season.
Deciduous reduce potential infection sites by shedding their leaves, oftentimes dropping the inoculum with the onset of defoliation, or the dropping fruits and nuts. However, both pests and disease-causing organisms have developed mechanisms whereby survival is secured, despite these preventative measures.
The temperature drops occurring in the winter literally slow the activity and growth rates of disease and pest organisms. In many cases, these organisms remain in a state of dormancy during the cold season, reemerging with the onset of warm spring weather and the resurgence of new growth. However, this dormancy does not involve total physiological inactivity. They are in fact very sensitive to the effects of applied spray materials. This is why dormant sprays represent a key starting point in the foundation of sound, preventative measures for pest and disease control.
In the past, experts have underestimated the value of dormant sprays.
There are a considerable number of pests and pathogens targeted by dormant sprays—from twig borers and grape mealybugs to brown rot and shot hole. Many such pests have in the past been considered by experts as organisms beyond control during the dormant season. For example, many standard recommendations for almonds have indicated that dormant sprays are unnecessary and yield no benefit.
However, much of the information compiled to date has relied upon standard water and oil emulsions applied with an air-blast sprayer. Further, with respect to the previous almond example, many of the tests designed to delineate the worth of dormant sprays were conducted during drought years in which disease pressures were relatively minimal.
It should be noted that any effect dormant spray program must address three key issues:
- Complete coverage is necessary for any dormant spray to work effectively.
- Pest or disease pressures must exist at elevated levels to manifest differences when dormant sprays are tested for their worth.
- The economic benefit to the grower should not be borne of chance (in which a grower gambles on the benefits or relative neutrality of sprays), but rather be considered as consistent insurance.
This is especially important, considering the cost-price squeeze experienced by all growers, and thus, the drive to avoid mistakes at all times. For many growers, a single pitfall, such as suffering an outbreak of shot hole from missed sprays, during a season when crop prices are high, can mean the difference between continued farming and filing for Chapter 11.
Dormant sprays represent a preventative approach to plant health, stopping disease from ever taking hold.
The thought processes underlying the need for dormant spraying coincide with the principles of prevention. That is, the best pest and disease control resides in setting insurance plans in motion to:
- Reduce inoculum levels of pathogens or infesting pest populations.
- Reduce levels of pests and pathogens near vulnerable crops, and then in the surrounding area.
- Maintain a preventative program that minimizes the opportunity for build-up of disease inoculum and pest populations.
- Avoid waiting for signs and symptoms of disease or infestation before treatments are implemented.
Preventative care in agriculture follows a similar philosophy found in human health care. Spending money today on conscientious efforts to ward off disease through exercise and balanced nutrition can mean improved quality of life for an individual. Similarly, a proactive nutrition plan, coupled with preventative sprays and maintenance, ensures crops are better prepared to ward off disease while providing consistent yields.
The effectiveness of dormant sprays is often undermined by surfactants used, many of which provide incomplete coverage.
With the benefits of dormant, protective sprays so well-defined, why and from where does the question of its worth arise? To answer this question, we must first begin by describing the mechanics of the art and science of orchard and vineyard spraying in general. Spray procedures fall into three main categories:
|Grouping of Spray Practice||Essential Characteristic of the Spraying|
|1. Protection of in-season tissues against pests||Deposits a film of protectant onto the surface of growing tissues|
|2. Plant nutrient, foliar sprays||Expose absorbing tissues to extended droplet longevity of appropriate nutrient concentrations|
|3. Dormant protective sprays||Thoroughly covers target tissues with protectants|
The first group relies primarily upon the depositing of a protectant or combination of protectants onto the surfaces of target tissues. In rare cases, a systemic material may be used, which goes beyond surface deposition. However, in this scenario the grower can achieve effective control with volumes of spray as low as that utilized with concentrate or semi-concentrate spraying.
On the other hand, foliar sprays require a different approach. This is predicated on the principles of foliar nutrient technology, which have correlated the effectiveness of foliar nutrition with increasing longevity or retention of the spray droplet. Thus, such a principle goes beyond mere deposition of a material onto the surface of target tissues. The issue in this case is one of extended droplet longevity and appropriate nutrient concentration.
Such requirements tend to steer away from concentrate sprays, which host a higher probability for evaporative loss. Semi-concentrate to semi-dilute volumes can be used in this case. However, they must be applied in the early morning or late evening hours, when evaporative loss is minimized.
The last grouping, dormant spraying, is by necessity aimed at controlling pests or pathogens which inhabit host tissues that are hidden and difficult to reach. A few such examples include:
- Shot hole spores are oftentimes lodged within the bud scales.
- Anthracnose fruiting bodies are not only recessed into host tissues, but characteristically exude a difficult-to-penetrate wall of viscous gelatinous materials.
- Yeast stages of leaf curl coat the inner leaves of active buds.
- The twig borer overwinters in a minuscule tunnel, called a ‘hibernaculum.’
- The grape mealybug and the grapeleaf skeletonizer overwinter beneath the bark of vines.
- The omnivorous leafroller overwinters as larvae in, among other places, mummies replete with water-repellent fungal mats.
- Scale adults have an impermeable, wax-like shell that is tightly appressed to the host tissues, protecting the delicate insect underneath.
The difficulties of reaching and effectively coating target pests and pathogens with dormant sprays are evident. The standard practice to date has relied upon the coating and creeping qualities of spray oils. Oils have gained acceptance in part because of their smothering effects on insects and mites, and the increased longevity of spray deposits in wet weather.
Our research has shown that organo-silicones are by far the most effective surfactants for dormant sprays.
However, in extensive tests conducted with various surfactants at two standard concentrations (1% and 2%), examining the spreading qualities on a polished glass surface, we found that oils were the least effective. That is, oils, despite their coating and creeping qualities, contributed least to the spreading of the spray droplet. The surfactants that performed with exceptional efficiency were organo-silicones. These organo-silicones (OS) were tested under various challenging regimes:
|Laboratory Challenge Regime||Outcome|
|Ability to wet a spore-laden water-repellant spherical fruit||Upon placing the tip of the fruit into the OS solution, the liquid crept up the sides of the fruit, wetting the entire sphere.|
|Ability to move down a simulated hibernaculum||A thin insect needle was used to create 1.5” hibernacula into compressed Styrofoam blocks. Dyes mixed with oil, standard surfactant, and OS were placed atop the holes. Only the dye solution with OS penetrated the simulated hibernacula.|
|Solutions of OS were tested for ability to creep beneath the wax shell of various scale insect adults||All solutions of OS were observed to creep beneath the wax shell, wetting the wax shell and the delicate adult insects protected by the shell.|
The merits of replacing oils with organo-silicone surfactants were then put to the test under challenging field conditions:
|Field Challenge Regime||Outcome|
|Heavily infected nectarine orchard with various diseases, including leaf curl, shot hole, brown rot and scab. The grower had been unable to spray due to wet field conditions that spring.||A late fall spray with fungicide, OS, and nutrients was applied at 250 GPA. The next spring, the field was clean of disease to an extraordinary extent.|
|Heavily infested plum orchard with San Jose Scale. Grower had an history of severe infestations over the last 5 years.||A late fall spray with fungicide, OS, insecticide, and nutrients was applied at 250 GPA. The next spring, there was exemplary control of SJ Scale, PTB and disease.|
|Almond orchard with extensive infections of scab and shot hole, and severe infestations of PTB.||A late fall spray with fungicide, OS, insecticide, and nutrients was applied at 250 GPA. Excellent control attained of both diseases and PTB.|
Both laboratory and field studies indicated that OS surfactants had a viable role in the dormant spray regime. Because of their superior spreading capabilities, the timing of the OSdormant spray program was selectively applied during periods of high moisture on the plant tissues (i.e. heavy dew). This prevented quenching and absorption of the spray solution into the bark, minimizing the spreadability of the spray solution. The results were quite remarkable.
Using organo-silicone surfactants reduces the quantity of chemicals necessary, a key advantage in an era when regulatory agencies take a dim view of pesticide usage in agriculture.
As with the many pesticides that are either banned by regulatory agencies or voluntarily removed from the market by chemical companies due to extensive restrictions and trends, oils will soon follow suit. This is one of the reasons why we explored viable alternatives.
Our studies indicate that organo-silicones are superior. Spray patterns are uniform, coating multiple areas of the target tissue surface, whereas oils tend to clump unevenly, leaving pockets of unprotected tissue. They are even able to reach beneath bud scales, providing superior control of diseases like shot hole and leaf curl. OS treatments are also able to penetrate sunken fruiting bodies, such as the cupshaped body of anthracnose, and the vase-shaped fruiting body of Phomopsis and Cytospora. OS sprays are also able to thoroughly wet and coat the spherical overwintering fruiting body of powdery mildew, and the otherwise velvet, water-repellent fungal mats of scab and brown rot.
One example we examined closely was the grape mealybug. The habit of this pest is to overwinter beneath the bark of cordons (the ‘arms’ of grapevine trunks), as well as up and down various parts of the trunk. Knowing this habit, growers were resigned to targeting their sprays at the trunk and cordons with high volume sprays. Some had gone to the point of stripping the bark with the use of power washing equipment.
Our investigation of the biology of the grape mealybug indicated that a spring migration would bring nearly 100% of the overwintering population towards the upper canopy. On spur-pruned varieties, for example, the mealybug could be found in high numbers between the bark and wood of spurs, oftentimes reaching as far down as 1.5 to 2 inches from the tips of the spurs. Others could be found beneath the bark scales created around the buds.
Lab tests with cut spurs indicated that the OS/insecticide treatments eradicated all mealybugs residing beneath the bark of various varieties. Because treatment required the thorough wetting of the upper canopy and spurs, we used a specially constructed boom equipped with flood-jet nozzles to deliver the spray without air blasts. The reason for this system was that air-blast spraying was oftentimes observed to throw past the target before sufficient wetting could occur. To put the system to the acid test, we used a low rate of Diazinon (2 pounds per acre), but with a volume of 150 GPA. The result was nothing short of phenomenal eradication. We did not see grape mealybugs for the rest of the season.
Another advantage of the OS dormant spray is absence of the bark darkening often observed with oil sprays, which can cause extensive sunburning of the bark. The annual losses to growers from sunburning can be quite dramatic, providing additional motivation to consider an OS dormant spray program.
Further, given the desire of regulatory agencies for decreased pesticide usage, the use of OS surfactants allows for a more efficient performance of applied materials, producing better results with less pesticide. It is a much-needed step, helping to keep our necessary arsenal of chemicals on the market, slowing or negating entirely across-the-board banning.
Caution must be exercised when using OS sprays during the growing season, as most have a degree of phytotoxicity, related not only to its superior spreading capabilities but also to a slight capacity for cutting the cuticular wax coating of plant tissues. However, this characteristic can be mitigated by adjustments in rates.
Which is more important for dormant sprays—the longevity of coverage, or the completeness of coverage?
The traditional approach to sprays, particularly dormant sprays, has been to retain the material on the surface of tissues for as long as possible. However, experience has shown us that targeting sticking quality or longevity of spray droplet invariably sacrifices spreading capacity. Microscopic examinations of sprayed tissue reveal many pockets of unprotected tissues when sticking ability is prioritized. This in part explains why disease and pest outbreaks recur or are never truly eliminated.
Sacrificing complete coverage for longevity and incomplete coverage is akin to utilizing a long-lasting antiseptic on parts of a large wound. While the antiseptic may last for some time, the incomplete coverage of the wound allows infection to take hold and negate the benefits of medicinal longevity. In short, you still get an infection. The better approach is to completely disinfect the wound with a material that is short-lived.
This is the rationale behind the use of OS surfactants. For one, studies with various diseases indicate that the most common source of inoculum for disease outbreaks of epidemic proportions resides within the diseased tree or vine itself. That is, the pathogen source which most effectively results in disease is from the parent plant. This is another reason for the immensely effective performance of fungicides when combined with OS surfactants. The technology is one that PCAs and growers should examine closely, and seriously consider instituting on a regular basis.