ACG Conference 2008

Winter Yellows -A closer look at this physiological disorder and other issues

Sandra Hardy, NSW DPI Pat Barkley, ACG

ACG Conference 2008

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OutlineWinter Yellows:

Typical symptomsWhat we knowPrevious instances

Reasons why?Weather patternsCarbohydrates

- leaves- roots

Summary Other issues

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Typical symptoms

Yellowing of leavesleaves stay yellow throughout winter

Autumn

Usually latest flush affected

Younger trees (2-5yrs)little/no crop

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Other symptoms include….

Sometimes:whole tree affectedleaf fallout-of-season flowering

Spring recovery

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Previous workAfter 1952After 1952--1954 outbreaks1954 outbreaks……

Affected leaves: Affected leaves: •• low in calcium low in calcium (1(1--2% , 3.52% , 3.5--4.5% in healthy leaves)4.5% in healthy leaves)

•• high in starch high in starch

•• RootsRoots -- starch depleted starch depleted →→ death smallest roots 1stdeath smallest roots 1st

•• Applying N aggravated symptomsApplying N aggravated symptoms

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Previous recorded occurrences - MIA

Plant disease surveys 1952-1986

1953 – widespread (+ coast & lower Murray)1955 – young trees1956 & 1963 – scattered trees, mild symptoms1973 – severe (+ Narromine) 1977- young blocks2005 – widespread

2008 - widespread

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1952 weatherGriffith Weather Data 1952

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1953 weather - WYGriffith Weather Data 1953

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2007 weatherGriffith Weather Data 2007

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2008 weather - WYGriffith Weather Data2008

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Climatic triggers?

Good summer rainfall

Sudden cold snaps in early-mid autumn

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Carbohydrates (CHO)• Solar energy stored as chemical energy

in form of CHO & other compounds

• Leaves manufacture CHO

• Accumulated: leaves (1st), branches, trunks, roots (last)

• Tree CHO levels variable: seasonal demands of competing developing vegetative & reproductive sinks

• Starch main CHO

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CHO - sinks & storage

• Actively growing organs – strong/priority SINKS

• Storage: roots (highest) > leaves/branches> trunk (lowest)

• Allocation to storage – lower priority thanneeds of actively organs

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CHO – sinks & storage• Reserves affect flowering, fruit set and yield

following year

Large crop loads (e.g. “on” year ):• depletes CHO → severe tree collapses/dies e.g. Murcott• reduces summer/autumn flush

CHO accumulation:• most rapid between 5 – 15°C• affects photosynthesis/CHO transport

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LeavesLong lived: 2+yrs if N adequate

Photosynthesis rates relatively low

Photosynthesis optimum 25-30°C, → reduced - at high temps >35 °C & low temps

Up to 45% dry matter – mostly cell walls complex CHO

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Leaves & CHO

Age affects CHO storage & export

No CHO export until FULLY expanded(1-2mths)

Significant importer prior to export

New flush – strong sink

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Leaves & CHO

Older/shaded leaves export greater portion of CHO& more rapidly

Shaded leaves– significant contributorto autumn & winter CHO reserves

Starch not readily mobilised for export

No sinks → chloroplasts packed with starch →cell membrane damage

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Movement of carbohydrates

Fruit source CHO principally from leaves of same growth cycle(Source: Goldschmidt & Koch, adapted from Powell & Krezdorn 1977)

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Root growth

3 factors:- shoot growth*- soil temperature- soil water

• Growth alternates with shoot growth- greatest when shoots not growing

* Major factor controlling intensity of root growth

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Root growthValencia on Rough lemon Valencia on Carrizo citrange

Source: Bevington & Castle 1985.

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Root growth –temperature & water

Soil temperature:• most intense at > 27°C• limited < 22°C• none below 14°C

Soil water:• Little growth < - 50 kPA (-0.05MPa)

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Roots & CHO reserves

• Highest CHO reserves usually in root

• Starch accumulation in roots mostly autumn & winter - dependent fruit/vegetativeflush demands

• Build up greatest in absence of veg/fruit sinks (e.g. off year)

→ C depletion high under a heavy crop load

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Summary - why leaves turn yellow

• Latest leaf flush – able to produce CHO – but can’t export (leaves too young)+ also a sink for CHO

• If no/little fruit – no sink competition for CHO → flush

• CHO movement to roots restricted

• Sudden low temperatures → photosynthesis slows + CHO accumulates

So …..

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Summary - why leaves turn yellow

• Youngest leaf flush → loaded with starch→ normally re-greens following springSevere - permanent damage to leaf structure

• Roots starved of CHO → die

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Reducing impact of winter yellowson young trees

Avoid promotingvigorous autumnflush

Leave somefruit on

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Other reasons for leaf yellowing

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→ root curling

Don’t plant root-bound treesCheck young tree roots before planting

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Other reasons for leaf yellowing…

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… root death

Choose soil type carefullyManage irrigation to avoid over watering

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Other contributors to tree collapse

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→ collar rot

Remove/check tree guards

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ReferencesBevington,K.B. and Castle,W.S. Annual root growth patterns of young citrus trees in relation to shoot growth,

soil temperature and soil water content. 1985. J.Amer.Soc. Hort. Sci. 110 (6): 840-845.Cameron, S.H. Starch in young orange tree. American Soc. for Horticultural Science. Pp 110-114.Castle,W. S. 1978. Citrus root systems: their structure, function, growth and relationship to tree performance.

Proc.Int.Soc.Citriculture. 62-69.Fraser, L.R. and Barkley,P. Virus and related diseases of citrus in New South Wales. Dept. of

Agriculture,NSW.Goldschmidt,E.E. and Koch,K.E. 1996. Citrus in Photoassimilate distribution in Plants and Crops Source-sink

relationships. Ed. E.Zamski and A.A.Schaffer.pp 797-823 Goldschmidt, E.E. 1999. Carbohydrate supply as a critical factor for citrus fruit development and Productivity.

HortScience, Vol34(6) October. pp. 10201024Lenz,F. 1978. Photosynthesis and respiration of citrus as dependent upon fruit load. Proc.Int.Soc.Citriculture.

pp 70-71Purvis,A.C. and Yelenosky,G. 1983. Translocation of carbohydrates and proline in young grapefruit trees at

low temperatures. Plant Physiol 73, pp877-880.Schaffer,A.A; Liu,K.; Goldschmidt,E.E.; Boyer,C.D. and Goren,R. 1986. Citrus leaf chlorosis induced by sink

removal: startch,nitrogen and chloroplast ultrastructure. Plant.Physiol. 124: 111-121.Syvertsen,J.P. and Lloyd,J.J. 1994. Citrus in Handbook of environmental physiology of fruit crops. Vol 2

Subtropical and tropical crops. Ed. B.Schaffer and P.C.Anderson. Pp65-99.Vu,J.C. Photosynthetic responses of citrus to environmental change. In Handbook of Plant and Crop Stress.

Ed M. Pessarakli. Pp 947-961Yelonosky,G. and Guy,C.L. 1977. Carbohydrate accumulation in leaves and stems of Valencia orange at

progressively colder temperature. Botanical Gazette 138 (1): 13-17