For many airblast operators, the spring of 2019 has been very difficult. The frequency and duration of rain events has left limited opportunity for orchard sprays. Even then, the periods between rains are transitions between warm and moist conditions and cold fronts, which makes wind gusty and changeable. These same periods leave wet alleys prone to rutting and compaction, and conditions that favour spraying may also favour pollinator activity.
In response, applicators get frustrated. Some may be tempted to spray in sub-optimal conditions and risk drift thinking even a little coverage is better than none. But the adage that “there is no wasted fungicide spray” does not apply here. Some may disagree, but spraying in wet and high-wind situations:
- greatly reduces coverage and subsequently, crop protection.
- may result in repeated sub-lethal doses that can encourage resistance.
- greatly increases the degree of surface run-off and off-target drift, risking environmental, commercial and residential contamination.
The argument itself may be moot because the decision to spray is not strictly a consideration of economics, productivity, and risk tolerance. When environmental restrictions exist on a pesticide label they are inviolate. That is, they are not suggestions but legal requirements. Statements might include:
- Not spraying when rain is forecast within 12 hours following application. This is, in part, to prevent water-soluble products from moving in surface or channel run-off.
- Not spraying in calm conditions (generally <3 km/h, as measured at the top or outside of the orchard). This is to prevent airborne spray from moving in unpredictable directions during a thermal inversion, or downhill with stratified air.
- Not spraying in gusting or windy conditions (generally >10 km/h, but there is no Canadian standard). This is to prevent airborne spray from moving with the wind. This is of particular import when there are sensitive downwind areas that can bring buffer zones into play
Technologies exist that extend the spray window, but they require long-term planning and may not be economical (or even completely proven). They are generally a combination of orchard architecture and sprayer design. Examples include:
- Tented orchards (more common in Australia) designed to exclude pests and insulate against hail, wind and inversions.
- Shrouded vertical booms (e.g. Lipco) designed for trellised orchards.
- Solid-set emitters (more common in Europe and still experimental in parts of the northern US) that reduce drift and can spray large areas quickly.
- Vertical towers with downward-oriented fans (e.g. Curtec Proptec or Sardi sprayers) that rely on the orchard itself to filter lateral/downward-directed spray.
Assuming the pesticide label does not prohibit application, there are adjustments that can improve coverage and reduce drift in sub-optimal conditions, but only marginally. These are compromises that sacrifice time, money, effort and/or the level of crop protection. Further, they are only intended for sprayers with towers (i.e. not axial sprayers):
- Do not spray the last two or even three downwind rows.
- Convert to air induction nozzles.
- Be certain to turn off any nozzles spraying over the orchard.
- Reduce fan speed to only reach the middle of the canopy on the upwind side.
- Turn off the boom on the downwind side of the sprayer and adjust airspeed and nozzle rates for upwind alternate row spraying only.
The best advice is unpopular: Park the sprayer until conditions improve. Like hail, there are environmental factors that are out of the farmer’s control. They are inconvenient and highly frustrating, but do not be tempted to takes risks on what might ultimately result in poor coverage and accusations of pesticide drift.