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Kentucky Apple Integrated Crop Management Manual
APPLE DISEASE CONTROL STRATEGIES AND THE SIGNIFICANCE OF DISEASE SCOUTING
Cultural Control Strategies
Where disease resistant apples ill fit into the market and into the orchard layout, they provide an economical way to control diseases. Even cultivars that are partly resistant, or at least not highly susceptible to diseases will be easier to manage. There are many apple cultivars that are resistant to scab; many are also less susceptible to fire blight, cedar rust, and powdery mildew. Use these cultivars if possible. Plant rootstocks that tolerate collar rot and fire blight diseases.
Disease-causing fungi and bacteria often survive from one year to the next on diseased leaves, fruit, and branches. Reduce the activity of the apple scab fungus by chopping fallen leaves into tiny pieces with a power mower in fall and winter. Remove and destroy fruit "mummies" and cut out dead and diseased twigs and branches during annual tree pruning. Do not leave piles of tree prunings in the orchard. Remove and destroy any abandoned and unsprayed apple or pear trees near the orchard.
Use only disease free nursery stock when planting a new block of trees. The apple grower needs to be aware that some disease-causing microbes can be kept out of the orchard, or can be kept from spreading to a tree or block from one nearby. Soil contaminated with the collar rot fungus should not be moved about, and pruning tools contaminated with fire blight bacteria should be disinfested before using in the next tree, especially during early summer.
Remove and destroy nearby cedar trees to break the cycle of cedar rusts on apple.
Chemical Control Although chemical control strategies may vary for each disease, they are generally classified as being protective, or reactive (where the control measure employed is in reaction to the development of a problem), or a combination of the two.
Protective strategies are based primarily upon prior disease histories and the likelihood that a disease will show up year after year. With the more predictable diseases such as apple scab, rust, and the "summer diseases", as well as fire blight, control measures are generally in place prior to known periods of infection. The primary role of scouting in these instances is to provide insight into where a control program can be adjusted to provide for more effective disease control in subsequent seasons.
Scouting also allows reactive control strategies to be implemented into protective programs. That is, scouting may detect lapses in the effectiveness of the protective program due to one reason or another. These lapses may indicate ways to correct a problem before it gets out of hand. This is especially true of diseases such as apple scab and powdery mildew where the development of secondary inoculum, if left unchecked, can soon result in poor disease control.
When properly implemented, a protective spray program will effectively control most of the apple diseases encountered on a yearly basis. However, many of the chemicals used in a protective program require that they be present on the plant surfaces prior to the arrival of the disease organism. Unfortunately, this is not always possible because of the unpredictability of the weather in Kentucky. Many times it is simply impossible to apply spray materials prior to periods of infection. Fortunately, the development of a new breed of fungicides, and a modified method of application have provided us with means to circumvent this logistics problem. Chemicals are now available which show substantial eradicant, or "kick back" action against certain diseases. These chemicals have the ability to stop disease activity after infection has taken place. This is very desirable in situations where plant tissue is left unprotected during periods of infection, because of rapid growth and/or lengthened spray intervals. Sometimes they are included early in the season to assure the grower of eradicating infections that might have escaped protectant fungicides. Consequently, these chemicals add a certain degree of flexibility to a spray schedule and ease the demands of a strictly protective spray schedule. In most cases, growers having orchards with a history of diseases are best served with a strict protective spray schedule, or better yet a program using a combination of protective and eradicant fungicides.
In orchards where apple scab has been kept under control, growers can reduce early season fungicide use. Scab infections, determined using Mill's Table as explained earlier, can be controlled using "eradicant" or "kick back" fungicides. Scab fungicide use can also be reduced by using a four spray schedule at tight-cluster, pink, petal- fall, and first cover. An eradicating fungicide such as myclobutanil (Nova), used in the four-spray schedule, will also control cedar rust and powdery mildew. Beginning at first cover, traditional calendar sprays are then used.
Weather monitoring involving leaf wetness measurements not only are helpful for scab management, but also for sooty blotch and flyspeck management. It is known that late spring and early summer wetness affect the timing of the appearance of sooty blotch and flyspeck. If spring and summer weather are dry, fungicide application for protection against this disease complex may be made later than in a season where wet weather predominates. Thus, with both scab and sooty blotch and flyspeck diseases fungicide applications can be reduced or at least be more targeted when weather monitoring is used. Scouting the orchard should also entail collecting data from whatever weather monitoring equipment is used.
Although the above discussion is very general in nature, it should assist in the development of control strategies based upon individual needs. It is also meant to tie together the role of scouting in the development and implementation of an effective spray schedule. For more specific information concerning diseases and control recommendations, please consult the current Commercial Tree Fruit Spray Guide (ID-92) or the Midwest Tree Fruit Handbook (ID93) which was developed in cooperation with the University of Kentucky College of Agriculture and the Kentucky Cooperative Extension Service.
More detailed information about symptoms, causal organisms, disease cycle and epidemiology, and control can be found in the Compendium of Apple and Pear Diseases. This book is available from The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121.
Apple IPM web site created by Kerry Kirk - 2001 - Maintained by Pat Dillon