1996-1997 New England Apple Pest Management Guide


OTHER APPLE DISEASES

1996-1997 New England Apple Pest Management Guide, pp.18-24

BITTER ROT, caused by the fungus Glomerella cingulata, is a summer disease of apple fruit. Extensive damage can develop rapidly during a period of prolonged warm wet weather if inoculum sources are present. At the optimum temperature for infection (79 F), infection can occur with a wet period as short as 5 hours. Fruit lesions appear as brown, slightly sunken spots. Bitter rot lesions expand most rapidly at a temperature of 86 F. Spore producing structures (acervuli) develop in concentric rings, beginning at the center of the lesion when it is 1/2 to 3/4 inch across. Severely infected fruit become shriveled and persist on the tree as mummified fruit. Control of bitter rot is based on sanitation and protectant fungicide sprays. Removal of mummified fruit and dead wood on the tree or on the ground is important. Shoots killed by fireblight can be colonized by the bitter rot fungus and serve as an inoculum source during that same growing season.

The EBDC fungicides are the most effective materials for preventing bitter rot, but the long preharvest interval makes them unavailable for use late in the season when there is the greatest threat from bitter rot. Of the other apple fungicides, only captan has a "good" rating for bitter rot prevention. If bitter rot is apparent, to prevent additional infections use captan at 2 week intervals unless the weather turns dry.

BLACK ROT
is one of three apple diseases caused by the fungus Botryosphaeria obtusa. The name black rot refers to fruit rot or wood canker symptoms, while leaf-spot symptoms are called frogeye. In addition, the fungus can cause a core rot around the seed cavity in developing fruit.

Frogeye symptoms first appear as small (1/4") spots about 1 to 3 weeks after bloom. The typical black rot lesions on fruit near maturity are black with a reddish border, and may enlarge to become irregularly-shaped, firm lesions. While the center of the lesion is typically black or dark-brown, the edges may have concentric rings of alternating dark brown and tan. The core rot is typically "moldy" around the seed cavity, and may cause fruit to redden and drop 3 to 6 weeks early. Wood cankers begin in the bark as slightly sunken, reddened areas. In weak, stressed trees these cankers may develop rapidly and kill limbs. Fire-blight lesions and cold-stressed tissue is also rapidly colonized by the black rot fungus.

The economic impact of the disease varies from orchard to orchard. Fruit rot and frogeye depend on warm (65-75 F) wet periods with a minimum of about 10 hours duration to cause significant infection. Significant black rot cankers usually develop as a result of other stress or injury. Inoculum overwinters in cankers or mummified fruit in the orchard, and can also be produced on many kinds of reservoir hosts around orchards. Removing cankered wood, mummified fruit, and chopping or removing pruned wood will reduce inoculum in the orchard.

Captan is somewhat more effective than EBDC fungicides against frogeye and black rot fruit rot. If there has been a history of fruit rot or significant defoliation from frogeye, treatment with one of these fungicides should be made every 2 to 3 weeks, starting at tight cluster and continuing through summer sprays. There is no information on whether fungicides have any effect on cankers.


CALYX END ROT (blossom end rot) is occasionally a problem in some orchards. It appears to be associated with several contributing factors: wet periods during a prolonged bloom period, spring frost injury (between halfinch green and petal fall), heavy nitrogen fertilization and sod ground cover. Infection is most likely to occur when fruit are wet for 23 days around petal fall and the temperature is 59-77 F. Fungi isolated from infected fruit include Alternaria sp. and Sclerotinia sclerotiorum. Benlate and TopsinM are the only fungicides with good activity against this disease. Where calyx end rot has been a problem, include Benlate or TopsinM (in combination, see Part II: Notes on Fungicides) in the bloom, petal fall, and first cover sprays if weather conditions are favorable for infection.


CANKER AND DIEBACK DISEASES OF APPLE. Apple trees are susceptible to many wood rotting fungi. These canker and dieback fungi are usually "stress pathogens", and the infections caused by them are also related to environmental stress or previous infections by other pathogens. Typical wood decay fungi include Stereum, Schizophyllum and Coriolus, while other fungi such as Botryosphaeria can cause both fruit and wood infections. One to two year old wood may be susceptible to canker fungi such as Phomopsis and Cytospora. Symptom expression will depend on the type of stress which predisposes the tree to fungal infection. For example, mid-winter cold stress may result in heartwood rot caused by Stereum or Schizophyllum. Summer drought may result in twig infections caused by Cytospora or Phomopsis.

Symptoms can also vary. Silver leaf, caused by Stereum, is characterized by a white metallic luster on leaves, showing first soon after petal fall. Affected leaves are often found on only one branch of the tree. Heartwood of a diseased branch has a dark discoloration. Symptoms may appear only after a tree has been infected for a year or more.

For several years, young McIntosh, particularly the Marshall Mac, have shown an occasional rapid dieback in spring and early summer. Typically, in these cases, dead leaves and/or petioles on branch tips are retained into the next growing season and bark may be abnormally red-brown. Trees will start growth in spring, but branches will dieback over several weeks, and in extreme cases trees will die. This dieback has been associated with various causes including Erwinia (fire blight), Schizophyllum, and cold stress.

Trees that are nutritionally healthy and managed well horticulturally will recover best from environmental stress. Avoiding stress where possible (for example, by using irrigation or pruning carefully) will reduce canker and dieback problems. Where cankers develop, for whatever reason, they should be pruned out as soon as possible so that no discolored wood remains. Healthy trees can then "wall off" the area, preventing spread of the disease. In many cases, dormant application of a high rate of a copper fungicide have greatly reduced the canker and dieback seen on young McIntosh.


CROWN, ROOT and COLLAR ROTS.
These diseases are caused by a group of soil fungi including Phytophthora cactorum. The above ground symptoms of these diseases are similar to those of volegirdled trees, i.e. reduced vigor and growth, sparse foliage. Fruit may be small and color prematurely. Adding more nitrogen to trees showing poor growth and foliage color because of Phytophthora infection may worsen the problem.

Crown rot (infection of the underground rootstock portion of the trunk and base of primary roots) is the most common of these diseases. Underneath the bark, infected tissue is reddish brown. Root rot (infection of the roots away from the crown) also occurs. Collar rot (infection of the trunk above the rootstock-scion junction) is less common. Severely infected trees will die. Occurrence is sporadic and is much more likely where susceptible rootstock is planted on a poorly drained site. Virtually all nursery stock contains Phytophthora inoculum. Water saturation of soil for 24 hours is sufficient to initiate infections. Aliette, Ridomil, and some copper fungicides can be used to prevent infection. But because of the sporadic nature of these diseases, it may be difficult to economically justify routine application. Because of the irreversible effect of girdling, successful chemical control of crown rot or collar rot is less likely than saving a tree with moderate root rot.

Important preventive measures are: rootstock selection, site selection and preparation, and maintaining soil drainage. MM.106 is particularly susceptible to crown rot.

See Table 6 Characteristics of Apple Rootstocks for more information on rootstock susceptibilities.

If susceptible rootstock is already planted in wet soil (either because of poor drainage or unusually wet conditions), or if adjacent trees have the disease, then protective fungicide treatment should be considered. Ridomil can be applied as a drench around the trunk (best for preventing crown and collar rot) or as a banded treatment within the drip line (best for root rot). Ridomil drench can be applied in the spring before growth begins, or in the fall after harvest but before the ground freezes. Alternatively, Aliette 80WDG can be sprayed on the foliage of nonbearing trees at 30 to 60 day intervals beginning at tight cluster. Ridomil and Aliette dosage rates are discussed in Part II: Notes on Fungicides.

Table 6 Characteristics of Apple Rootstocks (Adapted from Paul Domoto, Iowa State University).

Rootstock
Size
Hardiness
Soils
Crown and
Root Rot
Fire Blight
MM.111
80-85%
Moderate
Drought tolerant but does not tolerate wet soils
Tolerant on well drained soils
Tolerant
MM.106
70-75%
Very susceptible early; hardy later
Avoid poorly drained soils
Very susceptible
Moderately susceptible
M.7 EMLA
60-65%
Moderate
Adapted to most soils except heavy clay
Susceptible under wet conditions and poorly drained soils
Tolerant
M.26 EMLA
55-60%
Good, but may be susceptible early
Best in well drained soils
Moderately susceptible on poorly drained soils
Very susceptible
Ottawa 3
50-55%
Good
Best in well drained soils
Resistant under most conditions
Susceptible
M.9; M.9 NAKBT 337; M.9 EMLA
40-45%
Moderate-Good
Best in well drained soils
Resistant under most conditions
Very susceptible
Bud.9 (B.9)
35-40%
Good
Avoid poorly drained soils
Very resistant
Susceptible
P.2
35-40%
?
Best in well drained soils
Resistant
Moderately susceptible
M.27 EMLA
2530%
Moderate
Best in well drained soils
Resistant under most conditions
Susceptible




FIRE BLIGHT
is not a common disease in New England, but occasionally it can be destructive on susceptible cultivars (see Table 7). The disease is caused by a bacterium, Erwinia amylovora. It can cause damage to flowers, fruit, shoots, branches and trunks. Infected tissues turn black and may emit a watery ooze on humid days. Infections of suckers at the base of the tree can spread into the rootstock and kill it within a year. This is especially true of M26 EMLA and M9 rootstocks. See Table 6 - Characteristics of Apple Rootstocks for more information on rootstock susceptibilities.

FIRE BLIGHT MANAGEMENT
There are three major stages on which to focus fire blight management: dormant cankers, blossom blight and postbloom shoot blight.

Dormant cankers.
Remove any cankered or damaged tissue during dormant pruning. Leaving a large stub when pruning out active infections during the previous growing season will help identify areas of infection. Make dormant cuts back to the next healthy branch.

If fire blight has been a problem in a block in either of the previous two seasons, dormant application of a high rate of a copper fungicide plus oil may reduce the chance of fire blight. The treatment is intended to kill bacteria on bark and bud surfaces. Therefore use dilute applications. Full block treatments will reduce bacteria on the surfaces of all cultivars, and prevent dispersal from the unsprayed trees to the sprayed trees. Applications after green tip may russet fruit.

Late pink and bloom. The greatest risk of fire blight is during bloom. Apply streptomycin (+ surfactant) just before an anticipated infection event when infection risk is predicted to be high. Infection risk is defined by: 1) the presence of susceptible host tissue (e.g. apple blossoms), 2) a population of the disease causing bacterium, and 3) suitable weather conditions for infection to occur.

Table 7 Susceptibility of Selected Apple Cultivars to Fire Blight*

Cultivar/Sport

Rating**

Cultivar/Sport

Rating

Red Chief Del. (Campbell)

2

Golden Delicious

4

Liberty

2

Imperial Gala

4

Red Max

3

Fulford Gala

4

Empire

3

Royal Gala

4

Marshall McIntosh

3

Stark Gala

4

Freedom

3

Red Fuji

4

Prima

3

Red Fuji #4

4

Scarlet Gala

3

Jonagold

4

Pioneer Mac

3

Northern Spy

4-5

Akane

3

Paulared

4-5

Gravenstein

3-4

Red Fuji Nagano

5

Macoun

3-4

Spigold

5

Spartan

3-4

Fuji

5

Mutsu

4

Braeburn

5

Red Cort

4

Idared

5

       

* Data from 1991 Michigan fire blight outbreak, and from 1996-1997 Pennsylvania Tree Fruit Production Guide by J.W. Travis, Plant Pathology Coordinator; E.G. Rajotte, Coordinator.
** Cultivars are listed in order of rated resistance, from most resistant to most susceptible using a
5 point scale developed at Geneva, New York. 1=very resistant; 2=moderately resistant; 3=intermediate; 4=moderately susceptible; 5=very susceptible.

An orchard with highly susceptible cultivars or rootstocks; or with previous history of fire blight, has moderate risk. An orchard for which both statements are true should be considered at high risk.
Weather conditions required for infection to occur are:

The MaryBlytŠ computer program, available commercially, uses weather data and bloom dates to estimate if weather conditions are suitable for fire blight infection. If conditions permit blossoms to absorb streptomycin spray, an application within 24 hours after a wetting event begins can also prevent fire blight infection. A streptomycin application provides 3 days protection. After that, if site characteristics and weather conditions indicate risk of infection, protection would require another application. Do not apply streptomycin on an alternate row middle basis. Avoid high volume foliar sprays for other purposes during bloom when infection risks are high because these may trigger an infection. Do not exceed 4 antibiotic applications per year.

Postbloom.
If the possibility of fire blight infection is a concern, monitor orchards extensively for early blight symptoms. MaryBlytŠ is useful for estimating when symptoms from infections will become visible. Remove fire blight symptoms before extensive necrosis develops using the "ugly stub" method. Several trips through may be necessary. Streptomycin is not effective at this time, and will only encourage the development of resistance. Avoid extensive cutting as it may stimulate vegetative growth and prolong the time shoots are susceptible.

The "ugly stub" procedure is used for postbloom fire blight suppression. The blight pathogen can often be found in apparently healthy tissue 3 ft. or more in advance of symptoms. In addition, simply cutting into colonized but otherwise healthy limbs breaches the tree's normal defense mechanisms, and may lead to an overwintering canker. These cankers will develop regardless of whether the tools are sterilized. Cankers can produce inoculum the next year.

Guidelines for "ugly stub" pruning to combat fire blight are:


FLYSPECK and SOOTY BLOTCH
. These two fruit blemishes of apple present difficult disease management decisions, because they do not occur with the same intensity every year, and they occur at a time when growers would like to minimize pesticide applications. The diseases are caused by different fungi, and often occur together on unprotected fruit. Sooty blotch is more easily controlled with fungicides, and in most cases one or two summer fungicide applications provide adequate protection. Flyspeck is more difficult to control.

Growth of the pathogens depends on humidity. If relative humidity is very high ( 95% or above) then the pathogens will grow. In drier air they apparently remain inactive. Optimum temperatures for growth are at 60 F to 75 F, making warm summer nights ideal for sooty blotch and flyspeck development. Many conditions can contribute to maintaining high humidity in apple tree canopies, and therefore increase incidence of sooty blotch and flyspeck. Practices which improve air circulation and drying will reduce the diseases. Summer pruning, thinning to break up fruit clusters, mowing, and cutting dense hedgerows are recommended where flyspeck and sooty blotch are problems.

Cultural practices can significantly reduce these disease problems, but fungicides are generally required to maintain commercial fruit quality in all except the most northern parts of New England. Timing of fungicide applications will depend on rain and the fungicides used. See Table 14 Fungicide Options for Flyspeck and Sooty Blotch, in Part II of this guide.

A prediction model may someday be useful in managing the disease. In North Carolina, the first appearance of flyspeck and sooty blotch has been associated with accumulated hours of wetting. Beginning at ten days after petal fall, the durations of leaf and fruit wetting periods that were at least 4 hours long were accumulated. Flyspeck and sooty blotch appeared between approximately 200 to 300 hours of accumulated wetting. A potential management scheme based on this finding would be: After completing primary scab prevention, the next fungicide spray is a combination including Benlate or Topsin M that is delayed until the accumulated wetting hours approach the 200 hour mark. This scheme has not yet been validated in New England however. For now, the value of accumulated summer wetting hours is limited to use as a warning signal of unacceptable risk if rainy weather predominates during a planned extended interval between postbloom fungicide applications.

POSTHARVEST ROTS. Several fungi, notably Penicillium (blue mold) and Botrytis (gray mold), cause postharvest or storage rots on apple. Typically, these rots are not a significant problem unless fruit are wet going into harvest. Dip tanks or drenches in which the treatment solution is recirculated are a particular problem. Unless such treatments are required for scald control, post harvest fungicides are not generally needed.
If postharvest fungicides are needed, thiabendazole plus captan is the only option at present. Care should be taken to keep the solution circulating, as otherwise fungicides will settle with soil particles in the bottom of the tank. The fungicide should be changed or recharged at least as frequently as the label recommends. Suggested rates are as follows:

                    Amount per 
Fungicide         100 gals. drench 
Mertect 340 F        16 fl. ozs.
     plus
     Captan 50WP     2.5 lbs. 
or   Captan 80WP     1.6 lbs.
or   Captec 4L       1.25 qts.


Besides fungicides, several other practices can reduce postharvest rots.

1. Disinfect contaminated bins or boxes (those that contained badly decayed fruit the previous season) before they are reused. You can do this by washing or soaking them with 10% commercial bleach or other disinfecting compounds.
2. Harvest fruit at the proper maturity level. Smooth orchard roads. Handle fruit so as to minimize bumping and bruising.
3. During harvest, place bins and boxes on sod and not on bare soil. This is to prevent entry of decay organisms from the soil.
4. Cool fruit rapidly after harvest to minimize opportunities for decays to become established.
5. Remove culls and leaf debris from the packing shed daily.
6. Unless you need to drench for storage scald prevention, do not use a postharvest drench. Researchers are testing whether Cortland, Red Delicious, and McIntosh harvested after 200 chilling hours below 50 F have accumulated can be stored without DPA treatment.
7. Keep drench solutions clean. Consider pre rinsing loads with a hose before drenching. You can install a filter with the agitation system.
8. Most drench tanks would benefit from improved agitation.


POWDERY MILDEW
can be a serious disease in other apple growing regions, but is extremely rare in New England. It is caused by a fungus which can damage young shoots and fruit. The first appearance is a powdery white growth and distortion of leaves and shoots. If this appears, growers should apply an effective mildew fungicide, such as Nova, Rubigan, Procure, Benlate, or Topsin M in the last primary season scab fungicide sprays. Special treatment is not required in New England.

On to Insect and Mite Pests


1996-1997 New England Apple Pest Management Guide