Citrus Postharvest Decays and their Control

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Citrus Post-harvest Decays and their Control
UF Course 5115C 2008
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Citrus Post-harvest Decays

• Post-harvest decays of citrus are those that
  developed during harvesting, transit, grading,
  packing, and transportation of citrus to market
  during storage and during the various handling
  operations required to move citrus fruit from
  packinghouses to retail stores and finally to the
  consumer.
• Post-harvest decays actually continue to develop
  while the fruit are in possession of the consumer
  but are stored at room temperature or under
  refrigeration until the moment of actual
  consumption or use.

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• Economic Impact of Citrus Post-harvest Decays
• Post-harvest decay is one of the most important
  factors affecting values and the marketing of
  fresh citrus.
• The losses of fresh citrus by decay is more
  costly than those losses
• incurred at pre-harvest stage because of the
  added costs for harvesting,
• post-harvest handling, treatments, shipping and
  storage.
• The losses of fresh citrus due to post-harvest
  decay could be up to 100 depending on the
  post-harvest treatments and handling.
• The control of post-harvest decays of citrus is
  vital to maintain fruit quality and shelf-life in
  a market.

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Post-harvest Decays of Florida Citrus Fruit

• The common post-harvest decays occurred on
  Florida citrus fruit Green mold, blue mold,
  stem-end rots (Diplodia stem-end rot, Phomopsis
  stem-end rot and Alternaria stem-end rot), sour
  rot, anthracnose, and brown rot.
• Post-harvest decays can be separated into two
  categories based on the time of infection (1)
  decays manifested from latent infections include
  Diplodia stem-end rot, Phomopsis stem-end rot,
  anthracnose, and Alternaria stem-end rot and (2)
  decays manifested shortly after infections
  include green mold, blue mold, sour rot and brown
  rot.
• The types of decay and severity are dependent on
  the climate, varieties, grove practices and
  post-harvest handling.

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Green Mold

• Pathogen Penicillium digitatum (Pers.Fr) Sacc.
• Infection and Symptoms It is identified by the
  mass of olive-green spores produced on infected
  fruit and their prolific production ensures that
  this fungus is found wherever fruit is present,
  including field, packinghouses, equipment,
  degreening and storage rooms, transit containers
  and in the marketplace. Infection takes place
  only through wounds where nutrients are available
  to stimulate spore germination and fruit decay
  begins at these infected injury sites. The early
  infection area appears as a soft watery spot. As
  the lesion progresses, white mycelia develop and
  these produce the green spores. The white
  mycelium develops into a broad zone surrounding
  this sporulating area. Within a few days the
  entire fruit can be covered with green spores.
  Spoilage of fruit, caused by the spread of spores
  from diseased fruit onto adjacent fruit, can
  occur within the shipping container, but green
  mold spores will only infect damaged fruit in
  packed cartons.

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Blue Mold

• Causal organism Penicillium italicum Wehmer.
• Infection and symptoms P. italicum infects
  citrus fruit via injuries to cause blue mold.
  Blue mold is recognized by the mass of blue
  spores produced in decayed fruit. Initial lesions
  are similar to the lesions of green mold, but the
  spores are blue in color and are surrounded by a
  narrow band of white mycelium encompassed by
  water-soaked rind. Blue mold develops less
  rapidly than green mold under ambient conditions
  so that the green mold is often observed in mixed
  infections. Blue mold is more common in fruit
  held in cold storage for summer, and it can
  spread in packed cartons more readily than green
  mold. It occurs in all citrus-producing regions
  of the world, although it is not as prevalent as
  green mold under Florida conditions.

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Sour Rot

• Causal organism Geotrichum citri-aurantii
  (Feraris) Butler.
• Infection and symptoms It has been reported in
  most areas where citrus is grown occurring on all
  cultivars, but is particularly troublesome on
  fruit that is stored for long durations. The
  fungus only infects fruit through injuries and in
  particular deep injuries that involve the albedo
  tissue. Sour rot develops more frequently on
  mature to over-mature fruit with high peel
  moisture. The initial symptoms are water-soaked
  lesions, light to dark yellow and slightly
  raised, with the cuticle being more easily
  removed from the epidermis than lesions caused by
  green or blue mold. Decayed fruit tissue has a
  sour odor that attracts fruit flies and these can
  spread the fungus to other injured fruit during
  storage. The fungus is present in soil and can
  reach the fruit surface from wind-blown or
  splash-dispersed soil and by fruit-soil contact.
  Fruit on the lower portion of the citrus tree
  contains higher populations of the fungus and
  soil from the field or from diseased fruit can
  contaminate drenching equipment, soak tanks,
  pallet bins, washer brushes, belts and conveyors.
  Packed infected fruit allows the disease to
  spread to sound fruit in the container. The
  disease develops rapidly at warm temperatures,
  with an optimum at 27 oC.

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Disease cycle of Sour Rot
G. citri-aurantii completes its life cycle in
soil
Drencher, brushes, and belts, etc.
Wind, splash, and contact
Spores attach to fruit surfaces
Spores germinate and infect fruit via injuries
Sour rot developed
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Diplodia Stem-end Rot

• Causal organism Diplodia natalensis Pole-Evans
  (Syns. Lasiodiplodia theobromae (Pat) Griffon
  Maubl. and Botryodiplodia theobromae Pat
  teleomorph Botryosphaeria rhodina (Cooke) Arx).
• Infection and symptoms D. natalensis is a
  saprophyte that completes its life cycle on
  deadwood of citrus trees in grove. Water
  transmits fungal spores from deadwood to the
  surfaces of immature fruit where the fungus
  colonizes dead tissue of the button (calyx and
  disk). These fungal colonizations remain latent
  and do not cause fruit decay before harvest.
  Infections develop after harvest especially under
  conditions of high temperature and relative
  humidity. The pathogen usually infects fruit from
  the button at the stem-end of the fruit. It
  proceeds through the core more quickly than the
  rind, leading to development of soft brown to
  black decay symptoms at both ends of the fruit.
  The pathogen usually develops unevenly in the
  rind, forming finger-like projections of black to
  brown discolorations at the lesion margin between
  the segments. Decay develops rapidly during and
  after excessive degreening and can be observed in
  fruit at the packinghouse. It is often observed
  at market arrival or shortly thereafter. This
  decay does not spread from infected fruit to
  healthy fruit in packed cartons. It is a serious
  post-harvest decay in humid subtropical and
  tropical areas.

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Disease Cycle of Dipolodia Stem-end Rot
D. natalensis completes its life cycle and
produces spores on dead twigs on citrus trees.

Pre-harvest stage
Rain and overhead irrigation
Spores are deposited over fruit and under the
calyx of fruit
Ethylene and high temperature
Infection and decay development
Post-harvest stage
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• Phomopsis Stem-end Rot
• Causal organism Phomopsis citri H. Fawc. Non
  Sacc. Traverso Spessa (teleomorph Diaporthe
  citri F. A. Wolf).
• Infection and symptoms The fungus grows on tree
  deadwood, where it produces spores that spread by
  water to immature fruit during rainfall or
  irrigation. Infection of the young fruit
  produces small pustules. The fungus also becomes
  established in dead tissue of the button, where
  it lays dormant until harvest. As the button
  deteriorates during storage, the fungus grows
  from the surface into the base of the fruit
  through natural openings that occur in the
  abscission zone. Decay progresses evenly through
  the rind and core until the entire fruit is
  completely rotted, with no spread to adjacent
  fruit. This type of stem-end rot is dark to light
  brown in color and more prevalent in late-season
  non-degreened or cold storage fruit of all types.
  Ethylene degreening has no effect on Phomopsis
  stem-end rot. It is a serious post-harvest decay
  in humid subtropical and tropical areas.

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• Anthracnose
• Causal organism Colletotrichum gloeosporioides
  (Penz.) Penz. Sacc.
• Infection and symptoms C. gloeosporioides grows
  and sporulates in deadwood on the trees, with
  water transmitting spores to the immature fruit
  surface where the fungus forms infection
  structures known as appressoria. These
  appressoria remain latent and do not cause decay
  prior to harvest. The appressoria germinate and
  form infection hyphae when fruit is treated with
  ethylene during the degreening process.
  Anthracnose lesions are initially silvery gray
  and leathery, being similar in firmness and
  elevation to adjacent healthy rind tissue. The
  infected rind becomes brown to grayish black and
  softens as the rot progresses. Lesions vary in
  size and are irregular in shape. Pink spores may
  form on the lesion surface in humid environments.
  Decay may also develop on injured rind of any
  type of fruit, producing firm, sunken dry
  lesions.
• Anthracnose only occurs in a few citrus growing
  regions (such as Florida) and is a major cause of
  decay in tangerines that are harvested early in
  the fall when long periods of degreening are
  required to enhance fruit appearance. It is a
  minor problem on other citrus varieties.

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Disease cycle of Anthracnose
C. gleoesporioides completes its life cycle on
dead twigs. The fungus produces acervuli with
spores on dead twigs
Pre-harvest stage
Rain and overhead irrigation
Spores on fruit surface develop to appressoria
Ethylene degreening
Appressoria germinate and infect fruit rind
Post-harvest stage
Decay occurs over fruit surface
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Brown Rot

• Causal organisms Phytophthora palmivora, P.
  citrophthora, P. nicotianae, P. hibernalis and/or
  P. syringae, occurs in several citrus-growing
  regions. In Florida, brown rot is caused by P.
  palmivora or P. nicotianae.
• Infection and symptoms Brown rot occurs in both
  pre-harvest and post-harvest stages. Phytophthora
  species persist in the soil and are spread
  through rain splashes to fruit hanging on the
  lower canopy of the trees thereby infecting the
  fruit. Most infections develop on the tree within
  3 to 4 feet of the soil surface although they
  might be found in higher locations as a result of
  wind-driven rains. Initial infection shows as
  light discoloration on any area of the fruit
  surface. As the decay develops, the lesion
  becomes light brown, firm and leathery. Under
  humid conditions, decayed areas spread rapidly
  and white mycelia may form on infected areas.
  Fruit with brown rot has a characteristic rancid
  odor. Brown rot spreads in packed containers from
  infected to healthy fruit.

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Alternaria Stem-end Rot (Black Rot)

• Causal organism Alternaria alternata (Fr.Fr.)
  Keissl.
• Infection and symptoms The disease occurs
  primarily as a stem-end rot on fruit stored for a
  long time. The decay can develop at the stylar
  end of the fruit, particularly in Navel oranges
  and cause premature fruit drop. The pathogen
  grows on dead citrus tissue or other substrates
  and produces airborne conidia. Latent infections
  are established in the button or the stylar end
  of the fruit. The disease can develop further
  when the button becomes senescent, as in
  over-mature fruit and during long-term storage.
  It is an important problem in commercial storage
  of lemons in California. Alternaria black rot can
  also be a problem for the processing industry by
  contaminating the juice. Black rot occurs
  throughout all citrus growing regions, but is
  rarely abundant enough to cause economic losses
  in Florida.

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Post-harvest Disease Control

• The effective control of citrus post-harvest
  decays could maintain fruit quality, enhance
  fruit shelf life, reduce the losses, and increase
  growers returns.
• Occurrence and severity of post-harvest decays
  depend on many factors including the growing
  regions, fruit varieties, tree conditions,
  cultural practices, pre-harvest treatments,
  harvesting methods and post-harvest handling
  practices.
• The control of one or more diseases by using a
  single treatment method is not always effective.
  More effective control could be achieved using
  an integrated approach to prevent, reduce, and/or
  eradicate pathogen infections and disease
  development during pre- and post-harvest stages.

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Pre-harvest Control Practices

• Cultural practices
• Adequate fertilization, pest control, and
  reducing deadwood which results in lowering
  pathogen populations and disease pressure
  particularly those of C. gloeosporioides, D.
  natalensis and P. citri.
• Pruning low-hanging branches can minimize the
  incidence of brown rot.
• Brown rot can also be reduced by mowing and use
  of herbicides to reduce wetness, which is
  required for inoculum production of Phytophthora
  species.
• Post-harvest green mold can be reduced by
  removing fallen fruit under citrus trees that
  might be infected with P. digitatum which can
  produce air-borne spores, contaminating fruit
  surfaces in the tree canopy.

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Pre-harvest Control Practices--continued

• Pre-harvest application of chemicals
• Thiophanate-methyl (Topsin-M) is an only
  fungicide registered (EPA Section 18) on citrus
  for post-harvest decay control when applied prior
  to harvest. It is effective for post-harvest mold
  and stem-end rot control.
• Control of brown rot can be achieved through
  pre-harvest applications of copper and/or
  Aliette.
• The growth regulator 2,4-dichlorophenoxyacetic
  acid (2,4-D) used to control fruit drop also
  effectively reduce stem-end rot and black rot by
  sustaining juvenility of the tissues.

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Pre-harvest Control Practices--continued

• Harvesting practices
• Harvesting is a major source of fruit injuries,
  which provide infection channels and nutrients
  for pathogens, especially for Penicillium species
  and G. citri-aurantii. Care must be taken to
  minimize fruit damage during harvesting. Fruit
  should not be allowed to come in contact with the
  soil since soil particles cause abrasive injuries
  during fruit handling and harbor pathogen
  inoculum.
• Harvest time should be delayed for several days
  after irrigation or heavy rains to reduce decay
  and peel injuries.
• Spot picking for better natural color or
  delaying harvest to allow better color break and
  development will shorten the degreening time, and
  reduce the incidence of these diseases.

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Post-harvest Control Practices--continued

• Post-harvest drenching
• Drenching fruit with fungicides is a common and
  effective practice in Florida to control
  post-harvest decay especially green mold and
  Diplodia stem-end rot. It usually precedes
  degreening and when the fruit cannot be packed
  within 24 hrs of harvest.
• Thiabendazole (TBZ) and imazalil are commonly
  used in the drench treatment, as both are
  effective for the control of stem-end rot and
  Penicillium decays, but have no activity against
  sour rot. Chlorine is added to TBZ drench
  suspension to control G. citri-aurantii,
  Phytophthora spp. and TBZ-resistant strains of
  Penicillium spp. Recommended concentrations of
  TBZ and free chlorine are 1,000 ppm and 50 ppm,
  respectively. The optimum pH range of the drench
  with chlorine is 6.5-7.5. Chlorine cannot be
  added to imazalil drench since they are not
  compatible. However, a heated imazalil drench is
  used in some packinghouses in Florida it works
  well since heat can enhance the efficacy of
  imazalil and it has some sanitizing activity
  against sour rot and molds.

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Post-harvest Control Practices

• Degreening management
• In tropical and subtropical areas such as
  Florida, early season fruit is often subjected to
  ethylene degreening treatment to reveal the
  orange and yellow pigments. This practice can
  significantly enhance the development of Diplodia
  stem-end rot and anthracnose.
• Excessive degreening can significantly enhance
  decays such as Diplodia stem-end rot and
  anthracnose. The concentration of ethylene and
  duration of degreening treatment are positively
  correlated with the decay incidence and severity.
  If degreening treatment is necessary, fruit
  should be degreened with less than 5 ppm ethylene
  and for the shortest duration possible.

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Post-harvest Control Practices--continued

• Soaking, washing and grading
• - Cleaning fruit and removing surface dirt,
  which harbors microbes and post-harvest
  pathogens, is important in minimizing
  post-harvest decay and is usually achieved though
  spraying sanitizers such as chlorine in water or
  soaking fruit in water to which sanitizers or
  soda ash have been added. Immersion of lemons in
  3 sodium carbonate at 35oC for 30 seconds is a
  common commercial practice for control of molds
  in California packinghouses. Ozone is also used
  in dump tanks to disinfect fruit in some
  packinghouses.
• - Washing is usually accomplished over brush
  beds with the aid of variety of approved cleaners
  or soaps. In Florida packinghouses, the fungicide
  sodium o-phenylphenate (SOPP, which is the only
  registered post-harvest fungicide having some
  activity against sour rot, is often applied with
  soap foam for decay and canker control. Washing
  is followed by a potable water rinse to remove
  soap and residues followed by air drying and
  grading.
• - Grading is to remove defective fruit to avoid
  poor arrivals at market. USDA inspection in
  Florida packinghouses for canker is presently
  required for canker free fruit shipping.

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Post-harvest Control Practices--continued

• Fungicide applications on packingline
• Fungicide application It is one of the most
  important steps for successful control of
  post-harvest decay. Imazalil and TBZ are used in
  an aqueous suspension before waxing or in the
  water emulsion wax to control stem-end rot, green
  and blue molds effectively. In Florida,
  recommended rates for these two fungicides are
  1,000 ppm for aqueous application and 2,000 ppm
  for wax application, but a range of rates (500 to
  3,000 ppm) is used depending on the packinghouses
  and the citrus producing regions. The higher
  rates of the fungicides used in wax are due to
  the reduced efficacy of fungicide-wax
  combinations. Imazalil has good activity for
  control of Penicillium sporulation, and has some
  activity for control of Alternaria stem-end rot.
• Guazatine It has good activity for sour rot
  control and has been used in South Africa and
  Australia, but is not registered in the USA.
  2,4-D at 100 to 500 ppm, can be added to the wax
  to delay senescence of the button leading to a
  reduced Alternaria stem-end rot.
• New fungicides Three new fungicides,
  classified as reduced risk compounds,
  pyrimethanil, fludioxonil and azoxystrobin have
  been or are being registered by the EPA for
  post-harvest decay control. Studies indicate that
  they have good activity against green mold and
  stem-end rot. New fungicides provide useful tools
  for managing the resistance problem of pathogens
  to commercially available postharvest fungicides.
  

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Post-harvest Control Practices--continued

• Storage and shipping
• Refrigerated storage and shipping of citrus
  fruit is an effective measure for decay control,
  maintenance of fruit quality and extension of
  shelf life. Diplodia stem-end rot and sour rot
  are significantly suppressed by storage at 4oC.
  Florida grapefruit shipped overseas is maintained
  at approximately 10oC during transit. Appropriate
  temperatures of storage for citrus fruit depend
  on many factors including variety, susceptibility
  to chilling injury and production region.
  Recommended storage temperatures for Florida
  citrus fruit is 10-15oC for grapefruit, 10oC for
  lemons and limes, 4oC for tangerines and 0-2oC
  for oranges.

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Post-harvest Control Practices--continued

• Packinghouse sanitation
• - Maintaining a regular sanitation program in
  the packinghouse is vital for reducing citrus
  fruit decay as it lowers the level of inoculum of
  major decay organisms such as green and blue
  molds, sour rot and brown rot. Packinghouse
  sanitation is an important component of the
  overall strategy to minimize decay, reduce losses
  and fungicide resistance.
• - Numerous sanitizers are approved for use on
  fruit and/or packinghouse equipment and chlorine
  is by far the most commonly used. Peracetic acid
  is a new sanitizer approved for fruit and
  equipment sanitization. Quaternary ammonium
  compounds are commonly used to sanitize equipment
  and harvesting containers. Although these
  compounds are not approved for direct use on
  fruit, their regular application in the
  packinghouse contribute to lowering the inoculum
  levels and reducing post-harvest decay.
  Formaldehyde at 1 to 3 is used to fumigate
  packinghouse storage rooms and is effective in
  eradicating spores of green and blue molds as
  well as mycelia of the sour rot causing fungus.

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Integrated System for Citrus Postharvest Decay
Control

• Good cultural practices in groves
• Pre-harvest fungicide application
• Drench fruit with fungicides and sanitizers
• Minimize ethylene degreening treatment
• Wash fruit with sanitizers and cleaners
• Fungicide application of fruit on packingline
• Low temperature shipping and storage
• Packinghouse sanitation
• Minimize fruit injury during harvesting and
  post-harvest handling

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Selected References

• Brown, G. E., and Miller, W. R. (1999).
  Maintaining fruit health after harvest. Pages
  175-192 in Citrus Healthy Management, L. W.
  Timmer and L. W. Duncan, (eds.), APS Press, St.
  Paul, MN, USA.
• Eckert, W. J. Brown, G. E. (1986). Post-harvest
  citrus disease and their control. Pages 315-353
  in Fresh Citrus Fruits. W. F. Wardowski, S. Nagy
  and W. Grierson, (eds.), Van Nostrand Reinhold
  Company, NY, USA.
• Ismail, M., and Zhang, J. 2004. Post-harvest
  citrus diseases and their control. Outlooks Pest
  Manag. 1529-35.
• Ritenour, M. A., Zhang, J. Wardowski, W. F., and
  Brown, G. E. 2003. Postharvest decay control
  recommendations for Florida citrus fruit. Florida
  Cooperative Extension Service, University of
  Florida. CIR 359-A (Revised)1-6.