Leaf Injury Tells The Story In Almonds
Many different experts over the last several decades have preached the virtues of proper ground speeds with the use of air-blast spray rigs in orchards. The key issue discussed revolves around the fact that in order for the spray mist to reach all areas, the volume of air it is carried in must displace the volume of air in the target area for coverage to occur. Just because a grower owns the largest dual-fan, turbo-diesel airblast sprayer that costs more than $100,000 doesn’t mean that there are not limits to the speed of travel for best coverage throughout the canopy. From a distance it may look impressive that the sprayer blows two rows over in each direction with spray, or looking from a hilltop and seeing the mist several feet above the tops of the trees. However, seeing the refractive halo of a spray mist through backlighting from the sun or headlights, and good coverage, are two different things.
Recent research funded by the Almond Board of California has re-confirmed what has been known for years regarding ground speed. USDA entomologist Joel Siegel has shown a reduction in spray coverage by as much as 32% when ground speed is increased from 2 miles per hour (mph) to just 2.5 mph. University of California-Davis agricultural engineer Ken Giles also found that navel orangeworm survival increased threefold when ground speed was increased from 1.8 mph to 2.4 mph.
I’m sure we all have seen at one time or another, an example where a grower or his employee accidentally put the herbicide glyphosate in a spray tank and sprayed the trees with it. I saw this on a number of occasions when I was with the University of California, and still see it today when diagnosing problems in orchards. The interesting thing is the injury to the tree is an excellent indication of just how poor spray coverage is, especially with a highly phytotoxic material like glyphosate. A look at the two photos on this page shows two different responses. In photo 1, you can see how little effect the glyphosate has on the half side of the tree that wasn’t sprayed, even though the opposite side has major limb death. In photo 2, you can see what little damage occurred in the upper canopy, even though both sides of the tree were sprayed. Definitely something to think about for those who spray every other row or travel at high ground speeds.
A similar phenomenon can be seen by those who still use the fall zinc sulfate sprays to defoliate trees. As great as coverage appears to be, the trees commonly look like photo 2 where the bottoms show major defoliation, while the top is hardly affected. As anecdotal as this may sound, this can be observed in multiple cases regarding glyphosate or zinc usage.
I believe that many of the cases of poor insect or disease control that are blaming product performance or insect resistance can be attributed to excess ground speed and poor coverage. I know there are those who will assert that they have traveled at 4 to 5 mph for years and have never had problems with insects or disease. While that may be true for them, there are other cases where poor control can be directly linked to “every other row” programs or excessive speed. The leaves can tell the story.