SUNSCALD OF FRUIT

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SUNSCALD OF FRUIT

• Preston K. Andrews
• Department of Horticulture Landscape
  Architecture
• Washington State University

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HISTORY

• First referred to as sunscald of beans by
  McMillan in 1918
• Sunscald of apples described by Knight in 1922
• Exposure to sunlight most important determining
  factor (Brooks Fisher 1926)
• Shade fruits with foliage or artificial covers
  (Moore Rogers 1943)

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SUNSCALD

• All injuries to plant tissues resulting from
  exposure to sunlight (Walker 1952, 1957)
• Types (Barber Sharpe 1981)
• Heat injury sunscald
• Ultra-violet radiation sunscald
• Photodynamic sunscald of heated tissues

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SUNSCALD

• Heat injury absorption of infrared radiation ?
  cooked appearance
• Ultra-violet sunscald common on fruits at high
  altitudes
• Photodynamic sunscald absorption of visible
  light energy by photosensitive pigmented cells
  with temperature-induced chemical lesions

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SYMPTOMS

• Discoloration silvering, yellowing, browning or
  blackening (apple, tomato, pepper)
• Surface droplets (squash)
• Water-soaked blisters (tomato, pepper)

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SUSCEPTIBILITY FACTORS

• Solar absorptivity
• Infrared radiation - surface color
• Visible radiation - pigments
• Interception of solar radiation
• Shading by foliage
• Transpirational convective properties
• Acclimation
• Light lower soil moisture
• Temperature tolerance

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SUSCEPTIBILITY FACTORS

• UV tolerance
• Mature fruit more susceptible than green fruit
  (pigmentation)
• Pigment photostability
• Chlorophyll-caroteniod complex
• Green tissue impair of photoprotection by
  carotenoids ? photooxidation products
• Red tissue presence or absence of Chl (muddy
  appearance)

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FRUIT MATURITY
Tomato, 30-hr exposure
Retig Kedar, Israel J Agric Res 1777, 1967
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Rabinowitch et al, Scientia Hortic 199, 1983
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ENERGY BUDGET EXPT

• Sweet pepper (Capsicum annuum)
• California Wonder - dark green, pendant fruit
• College Gold - ivory-yellow, erect fruit
• Sunscald frequency (40oC air temp)
• Cal Wonder 100, College Gold 8
• Sunscald initiation
• Cal Wonder - surface 49oC for 15 min in full sun
• College Gold - max surface 41oC (no sunscald)

Barber Sharpe, Agric Meteor 8175, 1971
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ENERGY BUDGET

• Assumptions
• Heat production from respiration negligible
• Light transmission through fruit small
• Absorbed energy stored in organic compounds
  emitted fluorescence negligible
• Dissipation of radiant energy
• Reflected radiation
• Emitted long-wave radiation
• Convective conductive heat loss
• Latent heat of vaporization

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ENERGY BUDGET
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Ivory
Green
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RADIANT HEATING

• I(1-?)cos ? k(?T/?r) hT
• Energy absorbed heat influx heat loss
• I irradiance
• ? reflectivity of surface
• ? incident angle of irradiance
• (where cos ? is only)
• k thermal conductivity
• ?T temperaturesurface air
• r radius
• h heat loss (radiative, convective, latent)

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Thorpe, J Appl Ecol 11755, 1974
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SUNSCALD TOLERANCE

• Tomato fruit, Rehovot 13
• Fruit exposure
• Sun-exposed
• Shaded
• Bagged (black) from 7 after anthesis
• Mature green fruit insolated for 1 day

Retig et al, Scientia Hortic 229, 1974
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SUNSCALD TOLERANCE
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SUNSCALD INDUCTION

• Mature green tomato sweet pepper
• Treatment stages
• Induction 12-18 hr at 45-46oC
• Incubation 60 hr at 25oC
• Light treatment
• 32 full sunlight applied with fluorescent
  incandescent lamps
• or during induction incubation stages

Rabinowitch et al, Scientia Hortic 2265, 1974
2921, 1986
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SUNSCALD INDUCTION
Sunscald ()
Induction

Incubation

Light
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Rabinowitch et al, Scientia Hortic 2265, 1974
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SUNSCALD ACCLIMATION

• Mature green tomato (Moneymaker)
• Acclimated in 2 steps
• 1. Induction pericarp reaches 45oC after 6 hr
• 2. Rest (potentiation) 23-27oC for 15 hr
• Mature green sweet pepper (Maor)
• Acclimated by insolation in black bags up to 5
  days

Kedar et al Rabinowitch et al, Scientia Hortic
383, 1975 2921, 1986
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Sweet pepper
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HYPOTHESES

• Acclimation
• Insolation (light intensity duration)
• Rest (chilling temp?)
• Sunscald damage
• Stage of development (chlorophyll degradation)
• High temperature light
• Photodynamic injury
• Photosynthetic inhibition
• Chlorophyll photosensitized
• O2 visible light ? photooxidation