6.2 Diseases of Stone Fruits

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6.2 Diseases of Stone Fruits
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• PEACH AND NECTARINE
• ANTHRACNOSE
• BACTERIAL SPOT Xanthomonas pruni
• BROWN ROT Monilinia fructicola
• FUSICOCCUM CANKER
• LEAF CURL
• LEUCOSTOMA CANKER
• PLUM POX VIRUS (PPV)
• POWDERY MILDEW
• PRUNUS STEM PITTING VIRUS
• RHIZOPUS FRUIT ROT
• SCAB
• VERTICILLIUM WILT
• X-DISEASE
• YELLOWS Witch's broom

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• CHERRY
• BACTERIAL CANKER
• BLACK KNOT
• BROWN ROT
• LEAF SPOT
• POWDERY MILDEW
• PRUNUS NECROTIC RINGSPOT VIRUS
• SILVER LEAF

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• PLUM
• BACTERIAL SPOT Xanthomonas pruni
• BLACK KNOT
• BROWN LINE or CONSTRICTION DISEASE
• BROWN ROT
• PLUM POX VIRUS (PPV)
• PRUNE DWARF VIRUS

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• 6.2.1 Brown Rot of stone fruits
• Introduction
• Brown rot is one of the most important diseases
  of stone fruits in the mid-Atlantic region.
• Field losses of nectarines can be extensive if
  conditions favorable for disease development
  occur during the blossoming or preharvest and
  harvest periods. Losses of peach vary with
  susceptibility of the cultivar.

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Symptoms

• Typical disease symptoms include blossom and twig
  blight, cankers, and a fruit rot. T
• he fungus often produces conidia profusely on
  sporodochia on infected areas.
• The first indication of the disease in the
  spring is the rapid death of blossoms which, as
  they turn brown, often become affixed to the twig
  in a gummy mass, later becoming covered with a
  grayish to tan(??) spore mass.

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Symptoms

• Frequently, following colonization of the
  blossom, the fungus enters the shoot where it
  causes a canker on which spores are also
  produced.
• Shoot blight symptoms will occur if the fungus
  girdles the shoot. Leaves on such shoots turn tan
  to brown and may remain attached for several
  weeks.
• Cankers formed following blossom or fruit
  infection appear as brownish, sunken areas, that
  are often covered with gum.

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Symptoms

• Usually, the tree is able to restrict cankers to
  small oval areas at the junction of the shoot and
  the infected blossom or fruit. Cankers and killed
  shoots may be colonized by other aggressive
  canker-causing fungi such as Leucostoma spp.
• Brown rot on ripening or mature fruit typically
  develops as a rapidly spreading brown necrosis.
  Under optimum conditions for the fungus, entire
  fruit may be rotted within 48 hours of infection.
  The infection produces a soft dry rot, although
  occasionally the skin remains firm.

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Symptoms

• On nectarines, brown rot sometimes occurs as
  quiescent infections which can be detected as
  small, circular, necrotic lesions on immature
  fruits.
• Immature or mature fruit with brown rot
  infections will sporulate profusely, shrivel, and
  become tough grayish-black mummies.
• Decaying fruit in cold storage or transit may
  appear black with little or no sporulation.

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Peach brown rot (2005.06,Meixian,Shaanxi )
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Plum brown rot (2005.06,Meixian,Shaanxi )
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Cup-like apothecia which produce ascospores
formed on fruit mummies
Twig blight
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Disease Cycle

• Monilinia fructicola
• overwinters in orchards as mycelium on mummies,
  fruit stems, blighted blossoms and twigs, and
  cankers.
• Sporodochia develop under cool, wet conditions
  during the winter and early spring.
• Occasionally, cup-like apothecia which produce
  ascospores can be found on fruit mummies under
  the tree, but they are not usually common in
  mid-Atlantic commercial orchards.

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Disease Cycle

• In years when apothecia were common, severe
  blossom blight was noted in peach and apricot
  orchards, but severe blossom blight also can
  occur in the absence of apothecia.
• Generally, conidia from mummies and cankers on
  stone fruit trees and other sources (for example,
  flowering ornamental plants of plum or quince, or
  wild plantings of plum) are believed to be the
  primary inoculum sources.
• Conidia are generally formed during late spring
  when temperatures range from 13-25 C.

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Disease Cycle

• Conidia are disseminated by wind and rain and
  germinate rapidly under favorable conditions.
• Optimum temperatures for blossom infection of
  peach range from 22-25 C. Between 0-30 C,
  temperatures above or below the optimum range
  delay germination but do not inhibit it.
• Inoculum concentration also interacts with
  temperature and wetness duration to influence
  incubation period and disease incidence and
  severity.

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Disease Cycle

• Although blossom blight(??)can be severe enough
  to reduce the crop, early sporulation on even a
  few infected blossoms provides more inoculum for
  later fruit infections.
• The subsequent invasion of shoots also enables
  the pathogen to survive in the host for long
  periods. In some areas, infections of flowers may
  result in active or quiescent infections that
  either cause decay of green fruit or become
  active prior to harvest.

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Disease Cycle

• Quiescent infections of peach and nectarine have
  not been reported in the eastern U.S., perhaps
  because blossom infection is less common here
  than in other locations
• Under optimum temperature conditions, fruit
  infections can occur with only 3 hours of wetness
  when inoculum levels are high.
• Longer wet periods during infection result in
  shorter incubation periods so symptoms develop
  more rapidly.

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Disease Cycle

• Large amounts of inoculum with highly favorable
  environment produces a high potential for heavy
  losses.
• Sporodochia of M. fructicola on infected blossoms
  and shoots may produce viable conidia throughout
  the remainder of the growing season, although
  sporulation from infected blossoms tends to
  decline over the summer.

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Disease Cycle

• Insects (beetles and honey bees) also can be
  important as vectors of the fungus during fruit
  ripening, carrying conidia to injury sites
  produced by oriental fruit moth, beetle, and
  other insects that injure fruit.
• Wounded fruit are infected much more readily than
  nonwounded fruit.
• At harvest, apparently healthy fruit usually are
  contaminated with spores which, under favorable
  conditions, may later decay during storage and
  marketing.

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Monitoring

• During or after pruning (before the pink stage),
  monitor a minimum of 20 sample trees per block
  for the presence of fruit mummies and cankers.
• A total of one to ten mummies and/or cankers, and
  more than ten mummies and/or cankers represents
  levels of moderate and high risk, respectively,
  for blossom infection under the appropriate
  environmental conditions.

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Monitoring

• Before bloom, monitor the orchard floor under
  sample trees for the presence of apothecia of the
  brown rot fungus.
• These are more likely to occur in the wettest
  areas of the orchard on mummies partially buried
  in soil and/or among weeds.
• Finding any apothecia represents a potential
  high risk for blossom infection.
• Remove cankers surgically if possible or prune
  out the entire diseased area. Monitoring for and
  removal of cankers is best done at the same time.

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Monitoring

• At shuck fall, examine ten shoots on each sample
  tree for the presence of blossom infection.
• A total of one to ten blossom infections and
  greater than ten blossom infections represents
  moderate and high risk, respectively, for fruit
  infection during the preharvest and harvest
  periods.

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Monitoring

• Fruit susceptibility to brown rot increases
  rapidly as fruit begin to color.
• Monitor ten fruit on each sample tree for
  disease incidence. Greater than two infected
  fruit per ten acres (eight trees sampled)
  represents a high risk for a brown rot outbreak
  at this time.
• Monitor approximately every three to five days
  during the preharvest period.
• Insect, bird and hail damage to ripening fruit
  can result in wounds which can be quickly
  colonized by the rot fungus.

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Management

• Cultural practices.
• Sanitation is essential if your orchard is to be
  considered a low risk for a brown rot epidemic.
  The practices listed below, if followed, should
  minimize brown rot spore populations and limit
  the likelihood of an epidemic when conditions are
  favorable for rapid disease development.

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Management

• Remove all remaining fruit from the tree after
  the final picking.
• This practice limits infection of fruit
  peduncles(?) and twigs thus reducing the number
  of brown rot cankers.
• In addition, this practice prevents the situation
  where overwintered mummies within the tree would
  be immediately adjacent to susceptible blossoms
  in the spring.

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Management

• Furthermore, removal of remaining fruit after
  final picking separates the practice of removing
  mummies from spring pruning.
• Where these practices are separated, the grower
  has more latitude to selectively prune (following
  a severe winter with high bud mortality(???), for
  example) without increased risk of blossom
  infection.

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Management

• Fruit thinning practices influence the carry
  over of brown rot during the summer months and
  into the fruit ripening season.
• In general, fruit thinned before pit hardening
  decompose rapidly whereas, fruit thinned after
  pit hardening become infected on the orchard
  floor and serve as spore sources for the disease.

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Management

• In spring, monitor for blossom infection and
  prune out any cankers and infected shoots.
• In spring, just prior to and during the blossom
  period, examine the orchard floor for apothecia.
  Their presence requires that blossoms be
  thoroughly protected with fungicide sprays during
  wet periods.
• Prune to avoid excessive overcrowding of branches
  to increase air circulation, promote rapid
  drying, and increase light and spray penetration.

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Management

• Fertilize to maintain optimum nitrogen/
  potassium balance.
• Avoid dumping rotten fruit in one location, which
  could become the starting point for disease and
  insect outbreaks in the following season.
• Pick and handle fruit carefully to avoid
  injuries remove field heat from the fruit
  promptly after harvest by hydrocooling or forced
  air cooling use clean containers keep packing
  areas clean.

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Chemical management

• Fungicides are recommended generally in a
  protective program for a complex of diseases,
  including brown rot, scab, and powdery mildew.
• Fungicides are to be applied prior to fungal
  infection that occurs during rain periods.
• Blossom infections are controlled with two or
  three fungicide sprays during the bloom period,
  with the number of sprays often varying from year
  to year depending upon the weather, the
  susceptibility of the stone fruit species, the
  length of the bloom period and the type of
  fungicide. 

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6.2.2 Peach leaf curl
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Significance

• Peach leaf curl is a fungus disease that, under
  the right conditions, can cause severe early
  defoliation and crop loss on nearly all peach and
  nectarine cultivars.
• Because of weather factors and good grower
  management practices in most years, however, the
  disease often causes little or no significant
  damage or loss.
• For this reason, the destructive potential of
  leaf curl is frequently underestimated to the
  point where important control measures may be
  forgotten or delayed.

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Symptoms

• Infected leaves are severely deformed and often
  display a variety of colors ranging from light
  green and yellow to shades of red and purple.
• The fungus causes the meristematic cells at leaf
  margins to proliferate quickly and randomly,
  which results in the leaves becoming variously
  wrinkled, puckered, and curled

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Symptoms

• As these infected leaves mature, naked asci
  containing ascospores of the pathogen are
  produced on the surface giving them a dusty
  appearance, after which the leaves turn brown,
  shrivel, and drop from the tree.
• Many infected fruits drop early and go unnoticed
  those that remain may become crooked at the stem
  end like a small yellow squash, while others
  develop reddish to purple, wart-like deformities
  on the fruit surface .

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Pathogen
Taphrina deformans(Berk)Tul
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Disease Cycle

• The pathogen occurs commonly almost wherever
  peaches are grown, and overwinters as
  blastospores in protected crevices in the bark
  and around the buds.
• Primary infections are the most damaging and
  occur during the early spring from bud swell,
  when the bud scales loosen, until the first young
  leaves are fully emerged from the bud.
• Infections on young peach leaves occur at
  temperatures of 50 to 70 F (10-21 C). Little
  infection occurs below 45 F (7 C).

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Disease Cycle

• The incidence of infection is greatest when rains
  wash the overwintered spores into the bud and
  cool temperatures lengthen the time that the
  emerging leaves are exposed to the pathogen,
  before they are fully expanded and can resist
  penetration by the fungus.
• When temperatures following bud swell are warm
  and early leaf development is rapid, infections
  rarely become established, even when spring rains
  occur.

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Management

• Non-chemical control
• Collect and dispose of infected leaves before
  the bloom of spores appears. This is useful
  because it limits the number of spores
  overwintering on the plant.

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Management

• Erect a polythene or glass structure to cover the
  top and front of the tree between January and
  mid-May.
• Trees so sheltered from rain and dew show only
  very mild symptoms in comparison with unprotected
  trees.
• Ensure the sides are left open so that
  pollinating insects can enter. You should also
  carry out hand pollination.
• If a tree suffers leaf loss, boost its vigour
  with an application of fertiliser and make sure
  it is kept well watered and mulched.

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Management

• Chemical control
• Applications of fungicide must be timed
  carefully. A copper fungicide (Vitax Bordeaux
  Mixture, Murphy Traditional Copper Fungicide) or
  mancozeb (Dithane) should be applied as the buds
  begin to swell and then repeated 14 days later.
• Bud swelling normally occurs in late January or
  early February but may be delayed in colder
  districts.
• Spraying must be completed before flower buds
  open. Another application at leaf fall may also
  be beneficial.

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6.2.3 Peach scab
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Symptoms

• The most notable symptoms of peach scab occur on
  the fruit, where small, greenish, circular spots
  gradually enlarge and deepen in color to black as
  spore production begins.
• Fruit lesions are most common on the shoulders
  of the fruit, but can occur anywhere on the
  surface. Where numerous, they often coalesce and
  may lead to cracking of the skin as the fruit
  enlarges, allowing rot organisms to enter.
• The overwintering twig lesions are clearly
  visible during the early season as small,
  grayish, more or less circular, slightly sunken
  lesions on the previous season's shoot growth.

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Symptoms
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Pathogen

• Fusicladium carpophilum,also called Cladosporium
  carpophilum,
• Sexual (Venturia carpophilum).
• Peach scab is caused by a fungus which can be
  extremely damaging to trees throughout the
  mid-Atlantic region because of the typically
  warm, wet weather during the day through the
  mid-season period. The disease appears to affect
  all cultivars of peach and is known to occur on
  nectarines and apricots as well.

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Disease Cycle

• The fungus overwinters in twig lesions.  
• In the spring, spores are produced on the lesion
  surface (primary inoculum) and are washed or
  splashed primarily by wind-blown rain to fruit,
  leaves, or new growing twigs.  
• On peaches, fruit are first susceptible starting
  5-7 days after shuck split.  

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Disease Cycle

• Periods of rainfall with temperatures of 65-75 F
  are optimal conditions for infection. 
• Although fruit remains susceptible until harvest,
  the 4-week period following start of shuck split
  is the most critical period for infection to
  occur.  
• The time from infection until lesions are visible
  may be 4-6 weeks.  Thus, infections occuring
  within 4 weeks of harvest do not result in
  visible lesions.

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Management

• All varieties are susceptible to scab, with some
  being more severely effected than others. 
• Generally, scab is most severe the first year the
  trees bear fruit (usually 3rd growing season). 
• This results from the large numbers of twig
  lesions that develop during the first two growing
  seasons.
• Low-lying planting sites should be avoided and
  trees should be properly pruned to allow good air
  circulation, thus promoting rapid drying of
  fruit, twigs and leaves.

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Management

• Fungicides such as captan and wettable sulfur
  provide adequate control of this disease if
  applications are properly timed. 
• On peaches, sprays should begin about 1 week
  after petal-fall. 
• Sprays on nectarines and apricots should begin at
  about petal-fall.  
• Subsequent sprays should be applied at 10-14 day
  intervals until approximately 1 month before
  harvest. 
• During the month before harvest, sprays applied
  for brown rot control will help reduce late
  season scab infections on the fruit, twigs and
  leaves.

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6.2.4 Crown gall
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Significance

• Crown gall is worldwide in occurrence, attacking
  140 plant genera in 60 different families.
• Plants most commonly damaged in Texas by crown
  gall are pecan, peach, blackberry, grape, apple,
  pear, willow, pyracantha, euonymus, rose, fig,
  and crabapple.

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Significance

• Damage to infected plants results from
  interruption of water and nutrient movement up
  the stem.
• Galls also interfere with normal growth and
  development, therefore, infected plants may be
  stunted and unthrifty.
• With many plants, the amount of damage depends on
  where the gall or galls are located and how many
  are present.
• Death can result if galls girdle the primary
  trunk or stem. Infected plants are more sensitive
  to winter injury and drought stress.

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Symptoms

• Crown gall first appears as small round
  overgrowths on stems and roots.
• As they enlarge, the galls become woody with a
  rough and irregular surface.
• Aerial galls can develop but most are found at or
  just below the soil line. Galls range from
  pea-size to larger than 1 foot in diameter.

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Pathogen

• Agrobacterium tumefaciens

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Disease Cycle

• Crown gall bacteria infect plants through wounds,
  such as those arising from cultivation,
  transplanting, wind damage, insect injury, etc.
• Wounds that have healed beyond a certain point
  are no longer susceptible to invasion.
• After establishing itself in the wound, the
  bacterium transforms normal plant cells to tumor
  cells.

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Disease Cycle

• Once this has taken place, the tumor cells are
  able to reproduce without the bacterium being
  present.
• Although crown gall of plants is very much like
  cancers in humans and other animals, there is no
  relationship between crown gall and animal
  cancers.
• Crown gall has been studied extensively by
  scientists in their search to understand
  cancerous growths.

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Management

• Control is primarily dependent on prevention.
  Pruning off galls is not effective since the
  bacterium is systemic and gall tissue can
  reproduce itself.
• Chemical control with antibiotic drenches has
  shown promise however, they are not practical at
  this time.
• Inspect plants for crown gall before purchasing.
  Plant only crown gall-free trees and shrubs.
• Remove and destroy heavily infected and weakened
  plants. Dig up as many roots as possible.

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Management

• Replace with a more resistant type plant if
  possible.
• Avoid wounding plants while mowing, cultivating,
  etc.
• Keep plants in an active growing state with
  proper fertility and watering.
• Heavily infected nursery fields should be planted
  to a grass crop for three years before planting
  susceptible nursery stock.

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Management

• Control root feeding insects.
• Dip grafting and pruning tools regularly in a
  disinfecting solution, such as 70 percent
  alcohol, 10 percent sodium hypochlorite (common
  bleach) or potassium permanganate solution (1
  ounce in 2 gallons of water).

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