University o~ Cal1~ornia Agricultural E:z:tension Service San Diego County

AVOCADO TREE DEFOLIATION

AYO.CADO AND CITRUS

NOTES JUNE - JULY 1977

Defoliation of avocado trees, young and old, is of great concern to growers this spring. The condition is fairly general from San Diego County to Santa Barbara County. Many calls have been received as growers want to know what is causing the excessive leaf fall and what can be done about it.

The cause of defoliation appears to be a combination of things, such as rainfall pattern, blos­som period, wind, soils, and climatic conditions between .August 1976 and May 1977. The rainfall In September and October had a lulling effect on growers. Growers told me that they felt there was enough rain so there was no need to irrigate for another two-four weeks. Dur­ing the period from November to March there was no measurable rain and plenty of dry, warm easterly Santana-type winds. Also, the westerly and southwesterly winds blew more than usual. Between the poor rainfall pattern, salts in the soil, drying of the soil with a concentrating of salts in the rootzone, and the abundance of drying winds, the avocado trees could not tolerate this onslaught. Adding to nature's a.ttack on the orchards, the grower for­got to check his soil for moisture. The amount of leaf tipburn was the greatest in four-five years. Orchards suffering the most damage were young--one to four-year-old trees--wlth the Hass variety mostly affected, planted on shallow, coarse textured Boil, with a low water holding capacity, and subjected to strong winds.

During the blossom period, the level of nitrogen in leaves decreases as the flowers withdraw the nitrogen from the leaves. When soils are cold and/or wet, nitrification does not occur. A Iso, roots are inactive at this time resulting in no uptake of nutrients to replace those being used by the flowers. As a result, the leaves drop prematurely.

Wind causes physical damage to leaves and leaf removal when the velocity is high. Tran8~

piration (water loss from leaf surface) Is higher with increased wind movement over leaf surface. A tree cannot replace this moisture 108s if the roots are inactive due to wet and cold soil, or if the surface soil is warm but dried out. It is important, therefore, to know the soil moisture content in the first foot of soU and irrigate just enough to wet the soil that has become dry. It is even more important to watch trees a[~d the soil moisture on the coarse and shallow soils where so many avocado.s are now planted. Generally speaking. defoliation has been more prevalent on these thinner soils. Extra water will be required in high wind areas.

Now--What can you do? If defoliation is excessive and all that remains on your trees are blossoms, there may be a need for a whitewash spray or hand application to prevent sun­burning of branches. Use caution in applying material, as too heavy a deposit on branches may inhibit bud initiation and development. (See list of publications in last paragraph. )

COUNTY OF SAN DIEGO, BLDG. 4, 5555 OVERLAND AVE., SAN DIEGO, CALIF. 92123 PHONE: (714) 565-5385

The University of California', Agricultural Extension Programs are available to all, without regard to race, color, or national origin

Co-operative Extension Work in Agriculture and Home Economics, U.S. Department of Agriculture, University of California and County of San Diego Co-operating

V.Then trees lose the older leavei:J p,~ematurely, a zinc defic.i.ellcy usu911y occurs. The new growth is si:ubbYt due to sho rt .;,1::C1'llOdal growth, and the 1"' ;"'i 813 remain small. A foiiage spnLy of zir;.c is recommen(~u:i ,. ')\ itl8 as the new leaveD cle . • ,·top Hud expand. (See below)

Mulch materials, such as f~t; ' [1"W, wood chjps, mn7dl~f; ::~ 'i'8.ce !i.'[~ c:_" straw, !y-ell rotted ma­nures, etc., should be app.~i.cd to soLis in your,g (IJ.'chal'd:j, r'i.'lee mulch on soil from the tree trunk outward beyond the d d p .. linc of b~ec. .A WJod -rule to fo).1c;l'! i.s to apply mulch over en­tire area wetted by s~ittor, baSin, sp:ri"nkler, or eIIlUtfJl'. A mulch does a number of things: 1) prevents sealing of surface soil due to the mechar.lieal action of water droplets hitting directly onto tae soil~ ~:) prevents top soH honi drying out, 3) rnaintains a lower surface soil temperature d11rlng hot days, and 4) heI}H3 reduce weed growth.

Nitrogen application can be reduced until foliage reappears on the tree.

h'1:igate carefully! Controll weeds I Be tough on squirrels, rabbits, ants, and gOp!lCI'S! !

rnRIGATICN OF AVOCADO AND CITRUS THEES Irrigation is the most important part in the care of avocado and citrus tl'ees. When spI'in­kling, be sure to get the area wet where roots are growing. In young trees this wHl be th:3 ball of the tree. A s roots grow into the surrounding soH, the area of wetting should be ),[1-

crea sed.

Avoid overwatering. Do not keep the :::oil wet around the trunk, Check the soil to a depth of 2-4 feet and determine the moisture cO:"1tont. During winter and early spring both citrus and avocado trees suffer from excessive leaf burn, pl'cmatu:ee dl'opping of leaves, and (Sen-, erally look poor. This is a erHical time for trees, so ched,. -Jle soil, read tensiometers o ftet1 , and appl.y water as the Lrees need it. Concentrate Oi: :1oing a better job of irrlgaUon.

]'requent irrigations, wetting tho first two feet, are neC(jSC~' <r for avocados. Periodic leaching irrigations every sb: weeks is advised to move acer , ' Llu,~Cl;ted salts hom the root­zone. The poorer tho quality of water used l the greater the i:j-,~; ed for leaching irri.gation,s with greater aI110unts of water.

A deeper rooting plant, citrus trees reqnire water to be applied in greater amounts at aI]Y one irrigation, but the i.nterval between itTlgations should be extended. A t 1.'Jo-wcek interval is a 1l8ual pattern, u[~ing enough water to penetrate :3-·4: feet into the f)On. Tensiometers are helpful in determining the time for irl'ir~;n.tiGl'fl.

If your orchard iG in a windy area, extra water will be requIred, both i.n amounts and fre­quency of apPUcrltion. This is espccially true with orchards on hilltops :tnd hHlsicles whel'e Boil may be shallow and coarse with a low water holding capacity. vVest winds in spring and summer, and east winds in fall and winter have a drastic effect on water use of plants. The effect is more th3~1 most growers realize. Tests with grass plants grown ill areas where winds were dry (10-30 relative humidity) show \vater UBC was three tim(~s the normal llse on clear, calmer days.

In Sari Diego County, avocado and citrus trees exposed to strong, desiccating winds suffer damage from water stress. This is not uncommon. during fall and winter Santana wI-ods. However, strong westerly winds that occur in some areas of the county regularly probably cause more drying than is norm[llly expected. These wInds during the critical time of blos­som can have an adverse effect 01.1 fruit set.

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AVOCADO FERTILIZATION July is the time to apply fertilizer to avocado trees. All orchards require nitrogen fertil­ization each year in order to maintain maximum y5.elds and tree vigor. The amount of nitro­gen required for avocados will vary as to variety and age. Up until thy fjfth year, commer­cial fertilizers such as ammonium nitrate or urea will be applied frequently in increasing amounts. The first year tho trees should receive a tablespoon of material each third irri­gation; the second year, an eighth of a pound of actual nitr ogen per tree for the year, applied monthly between February and August; and during the third and fourth years, the tree should receive a quarter of a pound of actual. nitror~el1 for the year. Mature trees, 15 years and older, should receive 1 1/2 pounds of actual nitrogen per tree per year, applied in February and July. For mature trees, this amount applies to Fuerte, Bacon, and Zutano. The Hass variety must receive half again to twice as much nitrogen as the Fuerte. After the fifth year, leaf analysis may be used as a 'guide for nitrogen application. The amount of nitrogen in a leaf rangi.ng between 1.6% and 2% indicates the nitrogen fer d azation program is adequate. The source of nitrogen should be changed each year. For example, use the following materi­als, one each year: ammonium nitrate, urea, calcium nitrate, ammonium phosphate or a mixed N - P - K fertilizer. Phosphorus and potassium materials should be applied periodi­cally. A leaf analysis in the fall is recommended to determine the trees' need.

FERTILIZER USE WITH DRIP IRRIGATION The amount of -fertilizer to apply to avocado and citrus trees when using drip irrigation is outlined in the following table.

Pounds of Material Per Tree Per Month Tree Age II, Urea Ammonium Nitr::>..te ~33% N)

1 ,I I .028 .038 2 . l..'l .056 .076 3 f 3") .092 .126 4 ' (. .1~9 .189 5 • :l .278 .379

THE NEED FOR ZINC INCREASING

Calcium Nitrate (15% N) _--=-----­

.086

.172

.287

.431

.862

Zinc is an important micronutrient needed for vigorous growth and good production of avo­cados. The fastest way to get zinc into a tree is by foliage spray, either by ground rigs or airplane. Timing of leaf application should be when the new leaves are expanded. Soil appli­cation is satisfactory on the acid soils of San Diego County and can be done now where spitter or sprinkler irrigation is used. Zinc should be applied as an individual material and not used with other nutrient elements. The follOWing formulas can be used for spraying on the leaves:

Ground Rig Application: Formula 1: 3 Ibs. zinc sulfate (36% metallic zinc). plus 2 1/2 lbs. of hydrated spray lime

or soda ash in 100 gallons of water. Formula 2: 2 lbs. of zinc oxide in 100 gallons of water. Formula 3: 1 lb. zinc sulfate (36% metallic zinc) in 100 gallons of water.

Aerial Application: 8 - 12 lbs. zinc sulfate (36%) in 20 - 50 gallons of water per acre. This concentration provides the same amount of zinc per acre as does H con­ventional spray using 800 gallons per acre with 1 lb. zin~ sulfate per 100 gallons of water. J ') S .' " f /)

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AVAII~ABLE PUBLIC1\ TrONS 1977 Citrus Orchard Development Costs 1977 Orange (Valencifl & Navel) Produc~i()n Co?ts 1977 Lemon (Eureka & Lisbon) Production Costs .. 1977 Grapefruit (Marsh White & Hed) ProdlJetion Costs 1977 Lime (Bearss) Production Costs 1976 Avocado Orchard Development Costs 1976 Avocado Production Costs Whitewash for Home Use Some Do's and Don'ts to Prevent Spread of Root Rot Recommendations for Preventing the Introduction of Hoot Rot Organisms in New Avocado

Grove Plantings Zinc Deficiency Treatment for Avocados Tem~iometers - Their Use in A vocado Orchards Mulching Citrus and Other Subtropical Tree Crops Micronutrient Deficiencies of Citrus

Sincerely,

j)Qh$~/V--C. D. Gustafson Farm Advisor

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