Integrated Management of Apple and Pear Diseases
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As is the case for the post-infection program, it is not recommended in orchards that had greater than 2 percent fruit scab the previous year or late-season scab on leaves. The first fungicide application is delayed 1 to 3 weeks; a total of four applications of an SI fungicide e. Optimal protection is achieved by mixing a protectant e. Including a protectant with the SI should also delay development of resistance to the SI. Because this is a minimal spray program, spray coverage must be nearly perfect. Thus, the program is not suited to blocks of large trees where near-perfect coverage is impossible.
The timing of the four applications is dictated by the weather spraying when conditions will allow optimal coverage and timing of insect and mite control rather than by infection periods. However, if intervals greater than 10 days occur e. Monitoring for Scab: Losses from scab often occur when conidia, formed in primary infections, infect young, developing fruit. Therefore, orchard blocks should be monitored for scab lesions starting at petal fall and continuing through first cover. Examine both surfaces of spur leaves and fruit.
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If scab is detected, the safest recommendation is to apply an SI at the full recommended rate plus a protectant through second cover to suppress further development of lesions and to protect susceptible fruit. However, dodine applied after bloom can cause russeting. Note that after pre-bloom, the EBDCs may not be used at rates greater than 3 lb per acre. Powdery Mildew Powdery mildew, caused by the fungus Podosphaera leucotricha , can seriously reduce the vigor and productivity of apple trees.
Powdery mildew is usually not as severe a problem as scab in the Midwest, but on highly susceptible varieties Table 1 the mildew fungus may deform, stunt, or kill twigs, leaves, blossoms, and fruit. Infected fruits may become severely russeted. Gray to white felt-like patches occur on the leaves and on 1-year-old twigs.
Leaves are narrow, crinkled, and folded lengthwise, and they become thickened. Disease Development: The powdery mildew fungus overwinters in vegetative or fruit buds infected the previous season Figure 3. Infected terminals may have a silvery gray color, stunted growth, and a misshapen appearance and are more susceptible to winter injury than are noninfected terminals. Thus, disease pressure from powdery mildew is usually greater in growing seasons following mild winters.
As buds break dormancy, the powdery mildew fungus resumes growth and colonizes developing shoots, causing primary infections. Primary mildew infections may occur on vegetative shoots and blossoms and thereby cause a reduction in yield. The powdery white appearance on infected shoots consists of many thousands of spores, which are responsible for secondary infections. Secondary infections usually develop on leaves and buds prior to terminal bud set in midsummer and may reduce the vigor of the tree. Young fruit may become infected from about the pink stage of flower bud development up to 1 to 3 weeks after bloom.
Fruit infection results in a weblike russeting on the mature fruit. Infected buds are the primary means of overwintering for the fungus. The powdery mildew fungus produces masses of small black fruiting bodies called cleistothecia on infected leaves and terminals in the late summer and fall. Although cleistothecia contain ascospores, their role in the disease cycle is not clearly understood.
The conditions required for infection by the powdery mildew fungus are very different from those required by the scab fungus. Unlike the apple scab fungus, the powdery mildew fungus does not require leaf wetness for infection. In fact, powdery mildew spores will not germinate if immersed in water. The high relative humidity that often occurs before and after wetting periods is conducive to powdery mildew development.
Under optimal conditions, powdery mildew will be obvious to the naked eye 48 hours after infection. About 5 days after infection, a new crop of spores is produced. Non-germinated powdery mildew spores can tolerate hot, dry conditions and may persist in the orchard until conditions are favorable for germination and infection. Control of Powdery Mildew: Apple cultivars vary in relative resistance to powdery mildew Table 1. Because powdery mildew does not need a wetting period to develop, control measures may be needed even during dry weather.
The critical period for powdery mildew control is from about tight cluster to pink through first or second cover. The SI fungicides that are used to control apple scab e. Another SI fungicide, Bayleton triadimefon , is a good mildewcide but is less effective in controlling scab. Thorough spray coverage, including the tops of trees, is essential for control of powdery mildew with fungicides. Rust Diseases Three rust diseases commonly infect apples in the Midwest: cedar-apple rust Gymnosporangium juniperi-virginianae , quince rust G.
Pears are susceptible to quince rust and hawthorn rust but not to cedar-apple rust.
Starting an Orchard: Apples, Cherries, Peaches, Plums and Pears
All three rust fungi require infection of eastern red cedar or related species of Juniperus to complete their life cycles. The rust diseases are usually kept in check by fungicides aimed at scab. Where fungicide use is minimal e. Quince rust causes fruit lesions but rarely affects leaves of apple. Hawthorn rust causes leaf lesions but rarely affects apple fruit.
Cedar-apple rust leaf spots initially develop on the upper leaf surfaces shortly after bloom. Lesions are pale yellow, but as they expand, they turn orange, which distinguishes them from other types of leaf spots. Later, small black dots fungal fruiting structures called pycnia appear in the orange rust spots. By late summer, tiny fungal tubes aecia emerge from the lower leaf surfaces.
Extensive leaf infection may cause early defoliation, which in turn weakens the tree. Fruit infection by cedar-apple rust is most common near the blossom end. The yellowish orange lesions are similar to but much larger than those on leaves.
Apple Crop Load Management - Alternate Bearing
The tissue beneath the lesions is brown and spongy, and necrosis can extend to the core. Disease Development: The disease cycles of the three rust fungi are similar and relatively complex. Cedar-apple rust involves two hosts, three types of fruiting structures telia, pycnia, and aecia , and requires 2 years to complete its disease cycle Figure 4. The fungus overwinters in reddish brown galls on cedar trees.
The teliospores germinate to form basidiospores which are carried by wind to apple trees. Basidiospores germinate if temperature and wetting requirements have been met Table 5. Fruit are most susceptible for 2 to 3 weeks starting at bloom; leaves are most susceptible when 4 to 8 days old. Once release of basidiospores from cedar trees has ceased generally by second or third cover , there is no further infection of apple tissues.
Site Selection and Pre-Plant Practices
Unlike apple scab, cedar-apple rust lesions on apple leaves will not produce spores that reinfect apple leaves and fruit. Instead, fungal mating occurs in rust lesions which result in the formation of aecia. Aecia release aeciospores which are carried by wind to cedar trees where they infect and complete the disease cycle.
Galls start to develop on cedar shortly after infection but do not exude telial horns until the second spring after infection.
Control of Rust Diseases: Some apple cultivars are resistant to cedar-apple rust Table 1. Removing cedars within a 2-mile radius of an orchard will disrupt the disease cycle, and fungicides may not be needed. Many of the fungicides directed at scab will also control rust, although captan, dodine, and benomyl do not control rust diseases. The basidiospores that infect apples are produced and released from cedar galls starting at about the pink stage of flower bud development through first or second cover; this is the most critical time for control with fungicides.
Fire Blight Fire blight, caused by the bacterium Erwinia amylovora , is a serious disease of apples, pears, and related ornamental plants.
go Incidence and severity are influenced by cultivar and rootstock susceptibility Tables 1 and 6 , the weather, and the amount of succulent tissue present.