Delayed Dormant Period

Winter is when many fruit and nut trees and cane fruits enter dormancy. This is an excellent time to prune and train trees and canes. It is also a good time to apply anti-pest and anti-disease treatments. But some of those treatments should be applied when a plant is in full dormancy while others should be applied during the delayed dormant period. Let’s find out more about these two time frames and how to make the most of them.

​Life in the year of a tree

Summer is a riot of leaf, flower, and fruit development. Ample warmth and moisture combine to allow trees and canes to invest all their resources into procreation. As days begin to shorten and temperatures start to drop, deciduous trees, grape vines, and bramble fruits pull resources from leaves, as seen by the changing colors and ultimate leaf drop. During the coldest part of winter [December and January if you live in San Jose, California], most fruit and nut trees are in full dormancy. Sometime around February, things start moving again. Sap starts flowing. Buds start swelling. This is called the delayed-dormant period and it continues until spring, when the tips of the buds start to turn green.

Timing tree treatments

Horticultural oils, fixed copper, Bordeaux mixture and fungicides can be used to suffocate pest eggs, thwart fungal diseases, and break many other disease triangles. But the timing of those treatments is critical for them to work properly. Spray too soon and rain will wash it away before it ever comes into contact with a pest or pathogen. Too late is, well, too late. Also, coverage must be complete to the point of it dripping from every surface.

Generally speaking, these treatments need to be done before buds start to swell. Applying horticultural oils during summer, for example, when trees are more likely to be water-stressed, can lead to severe leaf loss and sunburn damage, reducing crop size and making trees susceptible a number of other pests and diseases. Applying treatments during freezing weather can be just as bad. Ideally, tree treatments should be applied on cool (50°F - 70°F), slightly overcast days, when rain, fog, and wind are not expected for at least 24 hours.

Different species have different ideal “windows” of treatment opportunities:

• Apple - green tip stage
• Apricot - up to first bloom
• Avocado - does not enter winter dormancy and should not be given winter treatments
• Cherry - white bud
• Citrus - does not enter winter dormancy and should not be given winter treatments
• Peach/nectarine - pre-bloom
• Pear - cluster bud
• Walnut - should never be treated with oils in winter

Timing also depends on the specific pest or disease. Full dormancy is the best time to treat for San Jose scale and peach leaf curl. Either full dormancy or the delayed dormant period can be used to treat for aphid eggs, European fruit lecanium nymphs, fruittree leafrollers, peach silver mites, and peach twig borer larvae. You can also wait until blossoms appear to use Bt to treat for peach twig borers.

The delayed dormant period is the best time to apply treatments for these specific problems:

• Anthracnose (blackberry, grape and raspberry)
• Black rot (grape)
• Brown mite eggs
• Cane blight (blackberry and raspberry)
• Downy mildews (grape)
• European red mite eggs
• Fireblight
• Leaf spot (grape)
• Mummy berry (blueberry)
• Obliquebanded leafroller larvae
• Pear psylla eggs
• Phomopsis cane and twig blight (blueberry and grape)
• Septoria leaf spot (blackberry and raspberry)
• Shothole fungus
• Spur blight (raspberry)
• Twig blight (blueberry)
• Yellow rust (raspberry)

Overlapping treatments

In some cases, your tree, vine, or canes will need more than one type of treatment. Dormant oil may be needed to combat certain pests, followed by a sulfur treatment to prevent fungal disease. It is very important that at least 30 days separate those two treatments. Also, sulfur should not be applied on days when temperatures will go above 75°F.

Keep in mind that treatments should not be given as a matter of habit. They should only be used when they are needed, as evidenced by infestations of infectious the previous year. Use a hand lens or magnifying glass to inspect buds for signs of aphid or other insect eggs. If your trees do not need treating, don’t do it. This is especially true for fixed copper treatments, as copper can build up in the soil to reach levels that are toxic to valuable microorganisms.

​Whichever treatments you decide to use, ALWAYS apply them exactly as package instructions state and wear protective clothing and goggles. Using these products incorrectly can harm you, your trees, and groundwater supplies.

Other actions you can take to ensure the health of your fruit and nut trees during delayed dormancy include:

• Remove mummies to interrupt navel orangeworm and brown rot.
• Remove deadwood and twigs with cankers and lesions.
• Remove fallen leaves which may contain pest eggs or disease pathogens.
• Whitewash trunks and exposed branches to reduce frost and sun damage.

February may seem like a quiet time for gardeners, but it is the perfect time to get outside and take a closer look at stems, twigs, bark, buds and spurs. Identifying potential pest and disease problems ahead of time, and treating your trees at the ideal time to combat those problems can make the rest of your year that much easier and your trees more productive.