[NAFEX] Phytotoxicity from Oil Sprays

Jim Fruth jfruth at uslink.net
Fri Apr 21 07:59:40 EDT 2006

This is just out from the Ontario Ministry of Agriculture:

Oil sprays are an excellent early season treatment for mite, psylla, and
scale control but it is difficult to time their applications in some years.
Oil is an excellent penetrant which is why it can cause phytotoxicity
(tissue damage) by allowing unwanted penetration of some products if used
within 10 to 12 days of certain fungicides (Captan/Maestro, Dikar, Bravo).
Oil should not be used within 48 hours of below freezing temperatures. To
apply oil when fruit trees are/were in the right stage this year (see
product label; generally dormant up to green tip) is/was difficult given the
freezing temperatures at night. Oil sprays are timed based on the crop
stage, but remember that oil acts by smothering mite eggs. If mites have
started hatching before oil applications, those that have hatched can drag
themselves out of the oil film and survive.

If you had difficulty timing oil sprays and experienced freezing
temperatures within a day or two of application, you may see some
phytotoxicity. Generally, oil-induced cold temperature damage is relatively
slight and is usually seen as minor leaf tip "burn" from which healthy trees
easily recover. In rare cases where freezing has been prolonged and severe,
the damage can be seen on entire leaves. Even with minor freezing events,
some blossom damage can also occur. Blossom damage may or may not be of any
consequence depending on how healthy the tree is in general, whether ideal
conditions occur at pollination time, and how well the air was mixed during
the cold event (stratification of blossom damage on the tree will lessen
production more than slight damage spread evenly).

Superior oil is a superb IPM product in many ways. It often sufficiently
reduces mite and scale populations early in the year so that no additional
expensive miticides or insecticides are necessary for these pests. In fact,
in many cases, yearly applications are not required and one application
every two or even three years provides measurable pest control benefit. Oil
controls multiple mite and scale problems in one economical, safe, and
environmentally benign application. No resistance will develop to oil
because of its physical mode of action. Given those benefits, anticipating
the weather to avoid phytotoxicity seems like a very small inconvenience. 

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