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The Garden Plot for April 25, 2008
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Disease Prevention”
This is the time of year to prevent the plant diseases we see in the summer. Sunken cankers on branches, blackened new twigs, and dark blotches on apple and crabapple leaves last year were probably caused by a disease called Fireblight. We saw a lot of Fireblight infection last year. Prune out blackened twigs on a hot, dry day. Sterilize your pruners with bleach in between cuts. If more than 10% of your tree’s branches are affected, you might want to spray with a disease-preventative material. The least-toxic disease preventative spray for bacterial diseases, such as Fireblight, is copper or Bordeaux mix. Bordeaux mix is a combination of copper and lime. If fungal diseases caused spots on leaves last year, Lime-sulfur is the least-toxic disease-preventative option. Wait until day time temperatures are between 55 and 65 F before spraying copper-containing or lime-sulfur sprays. Don’t spray if temperatures are over 75 F, and spray early enough in the day that disease preventative sprays have a few hours to dry before temperatures drop below freezing at night.
If you can avoid it, don’t mix oil sprays with lime sulfur or copper-containing sprays, especially if the weather is humid and temperatures are greater than 65 F. These sprays can burn sensitive new leaves and emerging buds, especially when sprayed in combination. Injured leaves and buds may turn yellow and drop prematurely.
There is another new way to think about preventing plant disease. Leaves may look like peaceful, green solar collectors, but on their surface, there is a battle raging between good and bad microbes. These leaf microorganisms are competing for space and nutrients. Gardeners may be able to help the good guys and thus prevent disease causing microbes from taking over. Iron is a limiting factor for microbes growing on leaf surfaces. Many disease-causing and beneficial leaf microbes have evolved sophisticated ways to compete for this limited iron. When iron is applied as an easily available iron chelate, some disease microorganisms loose their competitive advantage. So, several applications of foliar iron chelate to fruit trees, such as cherry, might help prevent fungus diseases, such as shot-hole fungus, from invading. Shot-hole fungus disease causes dark-colored lesions on cherry and apricot leaves and fruit. Another preventative nutritional spray, calcium, might make some trees less susceptible to disease. Researchers have found that foliar calcium chloride sprays reduced brown rot fungus disease on peach trees in some situations. Calcium is thought to prevent disease by increasing the levels of cell-wall-bound calcium. Calcium stabilizes plant cell walls and protects the plant from cell-wall- degrading enzymes produced by the fungus disease to attack plant tissue. It is a bit like applying a coat of paint to metal so it will not rust; or, perhaps more like taking vitamins to ward off disease in humans.
The best disease prevention is to utilize several tools simultaneously: enhance plant nutrition (including foliar calcium sprays), spray preventative least-toxic fungicides just as buds begin to break in the spring, and discourage foliar disease-causing organisms by keeping water off of leaves, and perhaps adding a foliar iron chelate spray when leaves fully emerge.
Call the Missoula County Extension Pest Alert phone line at 258-3820 to find out if and when disease infection periods are occurring this spring.
Helen Atthowe's new short program of gardening tips
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