Controlling stylet-borne plant virus transmitted by aphids is one of the greatest challenges facing vegetable growers today. Whereas many tens of thousands of fungal spores or bacteria can reside on healthy plant tissue without causing injury to the plant, it only takes one virus particle to infect a healthy plant. This single virus particle can be trans- mitted in less than 20 seconds during the initial probes from a winged aphid.

Attempts to control these diseases by killing winged aphids with potent aphicides have proven futile. Aphids transmit viruses in periods of time much less than required for an insecticide to work. Once the virus enters the plant, the plant is infected for the rest of its life. Since there is no chemical cure for a virus infected plant, prevention programs play a key role in suppressing spread of the disease.

The effectiveness of prevention programs is largely dependent on grower understanding and input. The greater the level of understanding and input by the grower, the greater the likelihood a virus prevention program will succeed. With no ‘magic bullet’ to fall back on, prevention programs often need to be implemented long before the first plants germinate.

Prevention Programs Are Comprehensive And Include:

• planting resistant varieties
• field sanitation
• monitoring winged aphid populations
• early diagnosis
• vector control
• and, in some cases application of Stylet-Oil

Early Diagnosis: Virus spread can be categorized as either primary spread or secondary spread. Both types of spread are the result of winged aphids transmitting virus from infected plants to healthy plants.

Primary spread is characterized as the first virus to show up in a crop. This type of spread is the result of winged aphids transmitting virus to healthy plants from infected plants (usually from a virus source outside the crop). Solitary infected plants within a crop indicate the location of primary spread.

Secondary spread, on the other hand, is the result of winged aphids probing into primary source plants within the crop, then transmitting the disease plant-to-plant within the crop. Infected plants adjacent to one another usually indicate secondary spread.

Generally, primary spread amounts to no more than 2% of the total infection in a crop. This is sufficient inoculum for winged aphids to acquire and infect the remaining 98% of the crop. Limiting the amount of primary spread [the earliest infection] in a crop becomes a major goal for a grower because it can have a profound effect on limiting secondary spread (and disease epidemics) later in the season.

Unfortunately, virus infected plants do not express symptoms for 8 to 14 days after inoculation. This leaves a window of time when transmission has occurred but growers are unaware the disease is even in the crop. Virus multiples in an infected plant so that after 4 to 5 days enough is present for aphids to acquire it. Because of this time lag and the inability of growers to recognize early virus symptoms until it is too late, we recommend growers monitor winged aphid populations and / or spray Stylet-Oil before virus symptoms appear in the crop.

Monitoring Winged Aphid Populations: Transient aphids usually transmit the first virus into a crop. These flights often occur at very low population levels leaving the grower unaware that disease transmission may be taking place. For this reason we recommend that winged aphid populations be monitored using yellow pan traps. Yellow-painted 9-inch metal pie or cake pans mounted 3-feet above ground level and filled with a solution of 50% water + 50% antifreeze (yellow color) should be adequate. (Yellow antifreeze will both attract winged aphids and retard water evaporation from the pan.)

When 5 or 6 winged aphids are trapped in a 24-hour period, application of Stylet-Oil should begin. Increase spray frequency to twice per week during peak aphid flight periods (15 or more aphids trapped in a 24-hour period). Spraying insecticides to control these transient aphids is futile and results in a loss of time, chemical and money.

Vector Control: Aphid colonies should not be allowed to develop in the crop. Winged adults are certain to develop from these colonies, fly plant-to-plant and aggravate virus spread. Aphid colonies should be controlled with label rates of insecticides such as methomyl (Lannate), acephate (Orthene); dimethoate (Cygon); or, imidacloprid (Admire).

Abandoned fields or crops no longer being harvested which contain high aphid populations should be sprayed with an insecticide prior to mowing to prevent aphid movement from infected plants.

Succession Plantings: Whenever possible, plant succession plantings into the prevailing wind to retard aphid movement into the younger crop. Young crops can be overwhelmed by virus infection coming out of nearby infected plantings. These young crops can be adequately protected using Stylet-Oil pro- vided oil sprays are maintained until the time of destruction and the older plantings are destroyed after harvest in a timely manner. Abandoned fields serve as a source of virus inoculum and nurseries for aphids.

Field Sanitation: The effectiveness of Stylet-Oil applications is influenced by the amount of virus disease pressure in or adjacent to commercial plantings as well as the level of aphid activity in or adjacent to commercial plantings. Virus infected weed host plants frequently represent the primary source of virus inoculum early in the season. These are weeds which have grown along ditch banks and in spray middles during the early part of the growing season. Thus growers should make every effort to eliminate weed hosts prior to and concurrent with oil applications.

The Need For Quality Spray Coverage:
Because Stylet-Oil is not a systemic chemical, the performance of the oil is directly related to the quality of the spray coverage. In the case of virus control, spray application to the upper leaf surface is essential. This is because winged aphids land on the upper leaf surface then immediately probe to determine if the plant is a host or non-host. If the plant is a host the aphid walks to the under leaf surface where it begins to feed; if the plant is a non-host it flies off in search of a host.

The upper leaf surface is the location where virus is either inoculated into a healthy leaf or acquired from an infected leaf. By the time the aphid has crawled to the under leaf surface, virus transmission has occurred. Typically, transmission occurs in less than one minute (for stylet-borne Poty viruses) during these short host-detection probes to the upper leaf surface.

To Enhance Thorough Coverage:
Thorough coverage of the upper leaf surface is necessary for optimum performance. To enhance thorough spray coverage:

• maintain a constant sprayer speed
• check nozzles for wear before and during the season. Replace worn nozzles when they eject 10% more gallonage than new nozzles
• spray every row, never alternate rows
• do not spray when wind speed exceeds 10 mph or apply the oil to wet foliage
• adjust nozzle distance to within 10 to 15-inches of the plant canopy
• vary the volume of dilute spray according to the size /density of the spray target
• spray to the point of runoff
• use hydraulic boom sprayer at 400 psi spray pressure with the recommended nozzle

Method of Application
Type of Sprayer Used to Apply Stylet-Oil*:

• high pressure, hydraulic boom sprayer*

* Do not use low volume sprayers (such as mist blowers or low volume electrostatic sprayers) or air assist sprayers designed to create turbulence to enhance under leaf coverage. The oil needs to be applied to the upper leaf surface where virus transmission occurs.

Spray Nozzles: We recommend the Albuz ceramic hollow cone ATR lilac color or the HCA green color nozzles for virus control. These nozzles produce a median droplet size of 70 microns @ 400 psi spray pressure. This combination of nozzle and spray pressure will produce oil droplets two-tenths of a micron in diameter which are  small enough to lodge between the epidermal cells of a leaf ― the exact location where aphids probe and transmit virus. Also, with such tiny oil droplets, the oil can be safely sprayed at twice per week intervals on fast growing crops without worry of plant injury. NOTE: Re-ducing the size of the spray droplet by one-half results in 8 times more droplets to impinge on the target.

Tractor Speed: Maintain a tractor speed of 3.5 to 4 mph (6 to 7 kph).

Agitation: Use constant agitation in the spray tank. Use either mechanical or bypass agitation to insure the oil remains an emulsion in the tank. Never leave a spray tank of water + oil overnight to be sprayed out the next day.

Application Rate and Spray Frequency
Concentration: Mix Stylet-Oil at the rate of 3 qts per 100 gals of water (0.75% spray concentration) in the spray tank.

Gallonage: From 25 to 150 gallons of dilute spray emulsion per acre depending on plant size. Growers should adjust gallonage according to the size / density of the spray target to achieve thorough coverage.

Gallonage Should be Adjusted by:

• adding or removing nozzles
• changing the nozzle tip orifice size

We do not recommend adjusting spray pressure to change gallonage because this will change the spray droplet size which could compromise spray coverage.

Spray Frequency: If virus disease pressure is expected before germination, plants should be sprayed at the time of germination.

On Rapidly Growing Crops Such as Melon, Squash or Cucumber:

• make the first application at 50% germination and a second application the following day
• twice weekly applications must be made to compensate for new plant growth regardless of the size of the winged aphid population
• after fruit set in these crops, weekly oil applications should be adequate

On Slower Growing Crops Such as Tomato or Pepper:

• weekly applications
• under heavy aphid flight pressure (15 or more aphids trapped in a 24-hour period) twice weekly oil applications are recommended

Post Harvest Considerations:

• destroy old plantings to prevent development of aphid ‘nurseries’ and to reduce disease inoculum
• continue to spray old plantings until their destruction if young succession crops are nearby

Re-entry Interval ― The re-entry interval (REI) for Stylet-Oil is 4-hours.
Residue Tolerance ― Stylet-Oil has no residue tolerance.
Pre Harvest Interval ― There is no pre harvest interval (PHI).