Copper is also sprayed at this time for control of fire blight and to
aid in the suppression of apple scab pathogens, both being severe
diseases that can destroy a crop as well as the trees. We also have to
be aware of the spring critical temperatures as the buds progress in
development. Each spring I post the spring critical temperatures chart
from Utah State to help you determine at what stage your fruit trees may
be at as spring progresses.
Ever wonder how the
fruit trees know when it's time to come out of dormancy? Well, the trees
won't come out of dormancy until they have endured a certain amount of
time with temperatures between 32 and 45 degrees Fahrenheit. Once the
number of chill hours they need is achieved and temperatures warm in the
spring, the trees come out of dormancy and resume their normal growth.
The number of hours required at cooler temperatures is known as the
chill requirement or chill hours. As of this afternoon we had
accumulated approx. 751 chill hours from October 1 of last fall. Most
apple varieties require 400-1000 chill hours, so most of the trees in
our area have met their requirement and will come out of dormancy just
as soon as temperatures warm. Growth resumption can be predicted by
tracking what we call growth units. Growth units are the number of
degree hours above 41 F. For example, if the temperature averages 51 F
for and hour, then 10 degree units are accumulated. Bud break initiates
after approx 3710 F growth units accumulate, and progresses depending on
the temperature. We do our dormant oil and copper spray generally
around April 10. The best time to spray is at silver tip....when the
buds have that silvery/gray tinted fuzz on them. You can use the chart
below to determine the growth stage your trees may be at.
As I mentioned earlier, now is the time to do our
fire blight copper spray and our horticultural oil spray. We want to
get the copper on the trees before they reach full 1/4” green and the
horticultural oil can be sprayed at the same time in a tank mix or done
as a dormant,silver tip,green tip, or 1/4” green spray. In other words,
your oil can be sprayed at any time from silver tip through 1/4” green
providing you are using a mineral oil based product such as Superior Oil
70sec or an off the shelf Horticultural Oil such as Bonide’s All Seasons Horticultural Spray Oil.
Your copper spray should be done before the trees reach 1/4” green to
avoid any phytotoxicity issues. For your copper spray you can also use
an off the shelf brand such as Bonide Copper Fungicide RTU (Ready to Use). Both of these products should be available at your local hardware store or garden center of from Amazon.com.
A
dormant oil and copper spray should not be done until we get at least a
24 hour period that is above freezing at night. The oil cannot freeze
on the trees, but it pretty much dries within about 24 hours. Once dry, there is no chance of it freezing. We usually
get at least one 24 hour period above freezing at night before the
trees get to 1/4" green.
This
“window of opportunity” for dormant sprays for fruit trees depends on
the bud stage of your target fruit tree. You can follow these
guidelines:
Apples: swollen bud to 1/4” green
Pears: swollen bud to cluster bud
Peaches/Nectarines: swollen bud to pre-bloom
Apricot: before bloom
When applying, spray trees just until they are dripping to get good application on all the stems and crevices at the buds. If you are using horticultural oil alone, use a rate of 2% (mixed in water) for best results or follow your chosen product’s label rate. For situations where aphids have been real problems in the past, consider adding an insecticide (such as acetamiprid, etc.) to 1.5 - 2% oil or use one of the Bonide RTU (ready to use) pre-mixes for insect pests.
