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PAL Questions: 1 - Garden Tools:
Can dormant oil or lime sulfur sprays help to control or prevent apple anthracnose? Is there any other way to prevent it besides cleaning up old leaves and disinfecting pruning tools between trees?
There are varying opinions on the best approach to controlling apple anthracnose (Cryptosporiopsis curvispora). British Columbia's Agriculture Department suggests that cultural controls (i.e., good garden hygiene) are key, and fungicides have proven ineffective. Here is an excerpt:
"Spores from new cankers are spread by rain or overhead irrigation during the late summer and fall months, and initiate new infections that appear as cankers during April through July of the following year. Cankers that are allowed to overwinter produce airborne spores during the following spring and summer that can initiate new infections at a distance from the source. The airborne spores function mainly to initiate new infections, while the water-borne spores serve to intensify the disease in trees that are already infected.
Prune out and remove all cankers during winter pruning. Prune out any new cankers that develop on limbs and trunks as soon as they are discovered, and remove them from the orchard. Developing cankers often girdle 1-year-old wood; remove any shoots that wilt or die suddenly during April through July as soon as they appear.
Nursery trees should be examined carefully for symptoms of the disease at planting and again the following spring. Trees with cankers should be returned to the nursery for replacement or discarded.
The cultivars Elstar, Empire, Gala and Sinta are very susceptible to anthracnose canker.
There are no fungicides registered for control of anthracnose and perennial canker in Canada, and fungicides have not proven to be effective."
Although Oregon State University's Online Guide to Plant Disease Control lists several chemical controls, they too indicate that chemical control alone is ineffective.
According to The Apple Grower by Michael Phillips (revised and expanded edition, Chelsea Green, 2005), anthracnose typically follows environmental stresses like cold, drought, or pruning injury. The best control is removing and burning infected parts of the tree. "Bordeaux mixture applied immediately after harvest and again two weeks later can help prevent spore germination in orchards with a severe problem. Any developing cankers the next growing season can be roasted alive using a propane (plumber's) torch."
I'm not sure if you feel comfortable getting out the blowtorch. Bordeaux mixture is one type of lime/sulfur combination, and it has its risks (to plants, the environment, and health), and possibly only limited benefits. Below is more information about this from University of California, Davis Integrated Pest Management.
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March 22 2017 13:26:25