Plant Pathology Infobytes
January 13, 1999
Start the '99 Plant Health Season with Dormant Sprays
Gardeners and home orchardists know that cold weather can be beneficial. A period of extended cold is needed for orchard and woody landscape plants to reach full dormancy, and frigid conditions help reduce populations of some injurious insects which attack fruit and landscape plants.
Unfortunately, cold weather doesn't have much effect on plant diseases microorganisms which have a nasty habit of hanging around on fruit and ornamental plants from one season to the next.
An important first step in preparing for the coming growing season is through the application of a dormant season fungicide spray to clean-up some of the diseases which are likely overwintering in your orchard and home landscape.
One of the best dormant spray materials for control of fruit diseases is liquid lime-sulfur. Used at recommended rates during the winter months, lime-sulfur very effectively wipes out fungus spores which overwinter on peach, plum and apple trees, grape vines, and other fruit crops.
Lime-sulfur is also recommended as a dormant spray as part of a control program for black spot, powdery mildew, and other rose diseases. Lime-sulfur is particularly effective for rose powdery mildew, since it kills the mildew spores which persist on canes and are present to attack new growth next spring. As an eradication treatment for overwintering fungus spores, apply lime-sulfur before the buds begin to swell. Most formulations of lime-sulfur are approved as a dormant spray for other ornamental plants as well.
While the labels on some brands of lime-sulfur have directions for the addition of a dormant oil (for control of certain overwintering insects) as part of the spray mixture, it's probably a good idea to stay clear of this combination. Dormant oils may be quite phytotoxic and cause injury, particularly when applied at temperatures below freezing. If you should choose to use these materials as a single spray, check the label for use directions to make sure that such combinations don't present plant health risks.
Peach leaf curl is a good example of a disease where a single application of lime-sulfur applied during the dormant season provides excellent control. This fungus disease is frequently severe in Mississippi following wet springs and causes premature defoliation, thus seriously weakening trees. Control peach leaf curl by applying lime-sulfur after trees are fully dormant but before buds begin to swell in the spring.
A related disease on plums, referred to as plum pockets, is also reduced by lime-sulfur application in the dormant season. This disease shows up as small blisters on fruit shortly after fruit set. Blisters enlarge and soon the fruit becomes spongy. The young seed within the fruit stops development and the young plum becomes a hollow cavity, thus the name "plum pockets."
Black knot on plum and cherry is characterized by the production of elongated swellings or knots on the limbs of susceptible tissue. These corky outgrowths appear on twigs and small branches, where they may reach a foot or more in length. Newly formed knots are greenish and soft but become hard, black, and knotty with age.
The most effective control for black knot is to prune away all galls. However, a liquid lime-sulfur spray during dormancy will help control this disease and should be used along with canker removal.
Fire blight of apple and pear is a highly damaging bacterial disease that kills blossoms, fruiting spurs, entire branches and limbs, as well as trees during seasons when this disease is especially severe. The bacteria which cause the disease overwinter in fire blight cankers and spread to healthy tissues in the spring. Therefore, a dormant spray application of a copper fungicide (available under trade names such as Kocide, Bordeaux Mixture, and others) will help control this disease. Make sure that the brand of copper you purchase is cleared for use on apple and pear trees.
Dormant season fungicide applications can play an important role in your orchard and home landscape pest control program. Visit your local garden supply store and read labels on lime-sulfur and other fungicides for additional application information. If you have questions, please feel free to call or visit us at your county Extension Service office.
Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.