UNIVERSITY OF CAU~RtlIA -

COL.LEGE OF AGRIC'(J~~iJ~,'f.r," ~" AGRICULTURAL EXPERIMENT STATION

BERKELEY, CALIFORNIA

THE CONTROL OF DOTHIORELLA ROT ()N

AVOCADO FI~UI1~S

W. T. HORNE AND D. F. PALMER

UHS()

BULLETIN 594 JULY, 1935

IlI!lll 11111111111111111111 1;;;1111

JARt

UNIVERS1TY OF CALlPORNIA BBRl(SLlW, CALIFORNIA

Cause.

Loenl natlll'e of the trouhle

Tilll(' of tlliVeiopIll£'llt of rnt

The chlll"llder of (lnthiorelh 1"ot

S()ur!le~ nf illfc'c !.ion.

C'( )Nrl'E~rI'N

:Meihod of pClletration by the fUUgllS

The Fuel"te flV!l('Utlo [lJl(i duthiorelh rut

Attemllj,!,! to !Jrl1vent clothiorelll1 rot by menUR other than by spraying

Experimental methmls .

Sllray experiments for the cllut.roillf dothiorellu rot" 1931 to 1934

Suggested spray progl'am iur Llothiorella rut.

Slln,y 1,reatmm1t. ,vithout Inrnig:1t.illll

Rpmy trel1t.ment with fumigation

Combin[l,tion treatment· for uothiorelll1 rot anu mottle-leuf

Summary.

Acknowledgments

3

4

4

4

6

o 7

8

8

9

14

14

14

14

15

10

I t "J'

I t \

THE CONTROL OF DOTHIORELLA ROT ON AVOCADO FRUITS 1

,2

W. '1'. HORNE' AND D .. ~'. PALMER4

A DEOAY OF AVOOADU ]'RUI'l' caused hy the fUllgus c1otlliorella." wus early observed by H. S. Fawcett and others, hut ito; importance lJecame evi­(lent only with t.he grO'wth of the California avocado indmltry. It 1S not, the most widespread, nor is jt the mOflt rapidly destructive or most. offen­sivE' form of spoilage. However, cf';rtain featnres, as pointe<l (lut further on, have madc it, peculiarly troublesome. It has at timoR occasionecl se­rious ecollomic losses ill the affected c1istricts, both by JOSHes 01 fruit and throngh discrimination against fruit from certain districts l'egardleHs of whrthrr it. was affected or not. Early efforts at control baked on experi­ellce with citrml fruits wel"(~ not successful.

CAUSE

Duthiol'l'lla is a, comparatively common ItUlgu!:! or wille distrihution. It was studied hy n. s. I~'awcett in eonneei ion with a large canker on trunks and Ilmhs of BllgliHh walnut in California, and the disease which it prodnees wus culled melaxuma (black sap). In this case, cankers ill the wild willows growing lH'arby were found to he an important souree or inrection. The fuugus is also frequently found in dead twigs of the walunt. In New York it 'was studied as the canse of a sel'ions disl'aRC of cultivated red currants, which proiluced caue blight..

It is sometimei:'i importa,ut ill moi'>t seasons or sihmtions, causing a stem-end rOot of citrus fruit'l, with tW(} other fnngi which produce simi· lar effects, Diplodia natalrnsis Evans, und Plwmopsl.q citt'i FaWCf"itt. In t'll,e avocado the dothiorella funguf! is found on deeaying' fruits, ill the dead areal-l Df tip burned l£'H.Vfi:1, and ill tIll' bark nr c1t>u(1 twigR. -__'_---

t Reeeivotl fo).' pUblieation May 8, 1935. 2 Paper No. 320, University Ilf CaIifol'llia Citrus Experiment Station antl Grnd­

uate School of Tr6pieal Agricultul'e, Riverside, Californiu. 3 A ssociate Professor of Plant Pathology and Associrlte Plant Pathologist. ill the

Citrus Experiment Station. ~ Agricultural Inspector, San Diego County. 5 Botryo.~phaeria ribis o7iro'nwgena Gl'QBsenba('her and Duggar, 1vhich before the

llisllovery of the perfect stage WaS called ])othio'i"rllu pre!}ttrill, Sace., unO. which has been referred to, probnlJly incorrectly, as J), ·ribi,q.

[3]

4 UNIVERSl'l'Y OF OALIFORNIA-ExPERIMENT STATION

LOOAL NATURE OF THE TROUBLE

Dothiorella rot is only one of a number of decays which attack the avo­cado, anll except in certain places where it is severe, it is of no greater importance than various other types of rot. It probably occurs through­out Call£onlia wherever avocados are gro,"'l1, but merchants have expe­rienced serious losses only from fruit grown in certain coastal areas. A

, ,

Fig. 1.~Lower end of a Fuerte avoeado showing the light dots or :flecks and spee.kles. This fruit does not have dotbiorella rot or other aggressive form of decay.

moist climate and tipbm'll or the leaves appear to be highly important predisposing conditions.

TIME OF DEVELOPMENT OF ROT

Dothiorella rot does not usually develop on sonnd fruits hanging on the tree or on the fruit after it is picked until softening hegins. In typical cases with Fuel'te fr'uits at the stage known as "breaking" (the first de­teetable softening), small dark spots with vague boundaries appear. Not all the dothiorella spots develop at the 'breaking stage, but new areR,s continue to make their appearance as softening progresses. The fact that the firm fruit shows no sign of the trouble alld that no method is available to distinguish those fruits which are to develop the decay irom those which are not destined to develop it, gives opportunity for seri(}us trouble between grower and merchant. .

'l'Im CRAltAOTlt"8, OF DO'rmORE!.LA ItOT

Dothiorella rot spots may be seen when less than % inch in diametex:. : At this stage they a.re light umber, rathC1' v~ely bounded, and not

BUL.594] DO'L'HIORELLA ROT ON AVOCADO 5

sunken nor distinctly marked. They often spread out from a speekle or other blemish as a somewhat paler margin. Figure 1 shows speckles, none of which are developing dothiorella spots, while figure 2 shows dothiorella spots, some of which are sp~'eading out from speckles. As the spot becomes more distinct, it darkens slightly but is not black in the early stages." Speckles might be confused with dothiorella rot; they are

Fig. 2.-A Dorothoa fruit with typical dothiorella rot; notice eye·rot at the stem attachment.

dark and sunken and rarely more than Y16 inch in diameter, and, unles."l some complication appears, inv(}lve only the skin of the fruit. The dothi­orella spot, by contrast, may become 1J2 inch in diameter within 3 or 4 days. The latter is cireula.r, not sunken, and is rather uniformly eol­ored. If the sudace is scratched, it is round to be softer and more easily brokeu than the sound surface. If (lut, most of the rot will be found to

. be in the skin, and i£ the flesh is at all affected, only the outermost part 'lshows a slightly watery .condition.

\From this stage the spot spreads more rapidly, becomes soft and sOInewhat sunken and uneven, while a watery rot spreads rather slowly intCl the flesh. A rank odor develops. Sometimes the rotting fruit fades out to a greenish gray. The surface settles and becomes uneven. Later, the whole fruit shrive1s up and becomes dark. If a bit of the decaying flesh is put in a drop of water, it is easily crushed, and under the (lom·

e Oolor of the rather well-established spots on Fuel'te fruits by Ridg~ay's color chart is approximately mummy-brown to bone·brown. The later, softer phase may be raw umber to light brownish olive or lighter. ·

6 1 iNlnmi:-nTY IH' CALU'OUNIA-ExPERaIENT STATION

ponnd Inier()i'\(~Dpe :;;hows strong brallehing fungus thrE'arls of varying breadth. 'rhe ~epd is llot (lrdinaril~' affectt'd.

At a. wonnd or at the stf'm attachmellt, dothi()r~lla rot may ap!)flUr (~ul'lipr than on other parts of the fl'nit Hnd penetrate more deeply aml rapidly. ,Yhen i1 atttld,s tlH~ litem aHae1unent, it prodnces a form or eye-rot' (fig. 3), Whilf' tilt' rot HhoWll in figure 2 is mainly superficial, Home orInI' ",.ill hnyc penetrated the flesh, and utilization aJter this stage will be disa111inintillg.

SOURCES OF INFEOTION

,Vhell 1.he decayed fruit:;; heeome old and dark, the surface develops nu­merom; small pimples at the tOPfl of whieh mjnute spore masses may ap­jwur 1I:; !lrnps Ol' ef)il~. Thei-le- ,;pore maSSPH emcrge from fungus pockets (pytmidia) lind Himilm' HtructnreR are Tormed in the clpud. areas of tip­lnlrlwd lea.nfl fmel ill the 1)(11'k of dead twigs. If the surface of an af­i'eeted tiyig if:; lightly i:\hav(~(l flO as to eut off the top~ of ihc prominences, white dotH may lw seen, eudl dot surrounded lly a black ri.ng. In raiuy nr wet timrs. I-jJlOl'E'S emN'g'p from thPH(l pycmic1ia am1 are freely washE'cl and spattered ahollt. Sport'S of another Hort art~ also fonned which may be :.;Ilot to a tlistlluee of al)ollt % inch. 'While thrse R])Ol'es arc compara­tively larg'e and heavy, tllCY still may be carried considerable distances in a c1riving rainst()rm. It. is hr lievcd that. the l;pore::; are mainly of rathrl' local diHtrilmtioll. and that tIley drift very litth~ in dry air. in this way heing unli)~e tlw f'xeessi vely light and dusty spores of citrus hlue molrl.

Spores on fallen leaves and twigs probahly do not readily get np into the tree RO that in om opinion it is unncceRsary to remove the mulch. An additional light. eovpr of straw or other material should make it entirely sare. Dpad i wig!'! HhOl.lld 11(' removecl periorlically, which can heflt he done at the time of tIl(' principal leaf drop in the Rpring and again in the autumn.

METHOD OF PENETRATION BY THE FUNGUS

Once a spore if:; deposited on the moist fruit, it germil1atf's hy Rending out a t.ulle-like thread; this grows down through one of the openings in the skin hy 'which tlw fruit is :mpplied with air, and into one of the air spaces below. 'rheRe airspaces are marked by the yellow flecks oT the B.yocado skin, and in them the fungus makes considerable growth, but is unable to attack anu rlestroy the living skin until the time or sortering of the fruit.

PreparatiOI1H £01' microscopic studyB clearly show the fungns in the

7 Eyo-rot is any rot which spreads from the point of attachment of the fruit stem' it may be caused by 1'arioUA organisms, but most of tllese are lesa eapabl(l of beMm:­ing established all the sound skin than is dothiorella.

8 Prepal'ations used W!ll'e made by Dl'. Ira Ayel', of C~I1'lablld, California.

BUL.594] DU'l'HIOHEJ.JLA Ito1' ON AVOCAlJ(_) 7

open spaceH below the surface of the Akin of Round avocado Irllitf; (fig'. 8). Protection of tl!p Iungus in theHC spaceii will explain the difficulty of preventing the rot hy treating the sl1l'facp of harv(,l'ted :fruit with fun­gicides.

THE FUERTE AVOCADO AND DOTHIORELLA ROT

]i'uel'tes ill the coaHtalarea bloom through a long period; in fact, Rome flowers can b(' found Ht almoRt any time. 'I'he principal time for hloRsom-

Fig. 3.-A vertical section of the surJ'aet> of a Fum·to fruit showing a stoma or breathing pore with the s1l1'l'ollnding eclls more OJ' leSE! hroken clown (a) and the' air spaees in the tissue llelow (11, Ii). Mycelium of dothiol'ella fung\lS has grown into the npr.ll passage of the hreathing pore and into the air spa('.(l helow, (Figure drawn somewhat diagram­matically from a photomkl'ogl'aph kinuly lJ1ade hy Dr. Ira, Ayer, of Carlsbad.)

lug and set.ting frnit. is from 1\-1 arch to May, but mall~T fruits are set at other times; these are "off-blooms." 'l'hey tenrl to be small and ill-shaped and do not mature with tlw :princil1al crop. 'rhe most skillful growers can now judge fairly well the state of maturit-y of a Fuerte fruit, if it can be seen in a good light, llUt selective picking is troublesome and diffi­cult. Ma,ny off-blooms in the past have become excessively Renile, so that they failed to soften normally when picked. In these cases, dothiorella rot often developed early, and the whole surface was rotted before the inner flesh was fully soH. The foregoing statement is based on general observations, hnt careful study of the softening proces,q and its failure with senility of fruit is lacking. There is a growing opinioJl that it would be hetter to cut down and (lestroy the off-blooms at about the time the main crop sets. There is reason to he1i~we, IrGm experience, tllat fruit at

8 UNlvERsI'rv OF CAr.U'OR,NI.A-EXPERIMENT STATION

its earli{'.~t satisfactory stage for harvesting softens more perfectly, and that dothiorella J'ot develops later and less actively on it than on fruit whi(Jh }las become gray with age and has lost its resilience. Specific evi· dence is furnished by the difference in behavior of the two lots in the spray experiment of 1934 (page 12).

ATTEMPTS TO PR.EVENT DOTHIORELLA ROT BY MEANS OTHER THAN BY SPRAYING

Early in these studies, avocado fruits were submitted to various treat· ments which should destroy spores on the surface of the fruit. None of these was effective. Fruit from dothiorella areas may be wet with strong a,kohol and flmner1 without preventing' the rot, and pieces of the skin planted ill culture m(ldia shortly after treatment will give cultures of the fungus. Treatment with nitrogen trichloride gas (Ag'ene) conducted WIth the aid of IJ. J. Klotz, and with and without the vacuum process, tailed to I)re'Vcnt the l'ot, although some applications were severe enough to speckle the fruit. Prevention of this rot is evidently a problem unlike that involved with blue and gl'een.lllOld rots of citrus fruits.

Ooniroll)y preventing tip burn and by removing dead twigs presents Cliffienlties eoncerning the evidence secured, so that we do not have fig. nres to present, although measnres of this .kind are doubtless of the greatest importance.

EXPERIMENTAL METHODS

We ourselves sorted the fruit at the Calavo Growers packing.house, usually assisted by an experienced member of the Calavo staff. Injuries and blemishes due to physiological breakdown were disregarded. These include cut':!, bruises, wounds, scald, speckles, carapace spots, and others. Many doubtful eases were studied further at the laboratory. Decays other than dothiorella rot were comparatively few so that error from misjudging the kinds of decay spots was not important. Many spa.ts of true dothiorella rot appear late and develop so slowly that some of them have done no serious harm until the fruit is spoiled by oversoftness (natural overripening) ,

Most of the fruit.s classed as "clean" which were st.udied further, later developed some dothiorella, but usually this was not of much practical importance. Fruits from the check plots dassed as lCelean" suffered more on longer keeping than ~ruits from the sprayed plots. In general, the advantage of t.he better lots of fruit increased on keeping. This is probably a true result and related with the manner ~f dothiorella infec­tion.

Experhnents with Fl.lertes from the Citrus Experiment Station at. Riverside, where dothiorella is so rare as to be negligible, showed that

BUL.594] DOT HIORELLA RO'l' ON A VOOADO 9

some infection might be caused by spores getting on the £l'uit at harvest, but that the number of infections is so small as to be unimportant in ordinary marketing. In such experiments aR those in which young fruits were disinfected and bagged at different stages to find the earliest size at which infection occurs, the results were believed to be approxi­mately correct. However, if all possible error is to be avoided, special methods of handling would need to be employed. The infections pro­duced by inoculating after picking al1peared late and developed rather slowly.

'1'he good quality of fruits ill t]1e better lots, espeeially ill ] 934, and other results difficult to express in figures, indieate thfLt the cvitleuce given by the tables is not uuduly favorable to the treatments.

SPRAY EXPERIMENTS FOR THE CONTROL OF DOTHIORELLA ROT, 1931 TO 1934

With the completion of four years of experimental work on the control of dothiorella rot, enough uata have now been collee,ted so that.certain conclusions of a practical character may be drawn. The experiments were carried on in a commerciallllanller, in that the orchards used were of an average grade in regard to their condition of thrift and care. The spraying was done by commercial operators. The fruit was picked in a commercial manner and shipped by the usual common carrier to the Calavo house at Los Angeles. Here it was handled in the same way as other shipments except that it was allowed to soften in the packing­house instead of being distributed while firm to the various markets. Determinations were made on the fruit in an early-soft condition.

These experiments were conducted upon trees of the Fuerte Variety ranging in age from four to eight years. They were started in 1931, at a time when very little was known about the disease. The first problem was to determine whether or not it could be controlled by the use of a fungicide. For this reason, a maximum unmber of applications on plots

,8-1 allelD-1 were made the first year (table 1) . . The first application was made before the bloom had opened and at .

two-week intervals, or aftel" each rain or sprinkle until the bloom was over. Thereafter, applications were made monthly nntil. the fruit was matured and picked. These plots received ten applications in all, the first one havh;tg been made April 30, 193L

Plots 8-2 and D-2 were treated before the first bloom had opened and at two.week intervals, or after each rain o-r sprinkle Until the bloom was over. Plots .8-3 and D-3 were treated once only, that treatment being giveri af~er the bloom. Each of these plots was composed of :three trees.

10 UNI\'ERSl'l'¥ OF CALUrORNIA-Rx_PERIMENT STA'rWN

All fruit was picl;:pd 011 Noveml)('r :)0, 1!J:11, the earliest clate at which tIl!' fruit had attainrd a sati:-;fat'tory oil content.

By the time tllP 1!la2 experiment (tahle 2) ,va>; laid ont, it WaI-; ap­parl>nt that tIll' inseet prohlem wonlc1 be of considerable importance

TABLE 1 J!I:ll EXPERIMENT FOR CON'rROI, OF DOTHlOlmLL,\ HO'f --------- -----r---

~rtlterilll 11'Iot Nil.

8-t* B!)rll(,Hl1x ·t-4-,jO H-2

8·3

!l·l ]\.'lrmohnlrnt{'d ('OpPP'T 'iulfu1 p , 1J·2

D·3

Nfl trratmt'ont (ha ... in irri,l.!;atiou) "hcrk

No trl.",:\tmcnt (nV(;,l'helld lIrign- ( check

Number of treat· mcnt~

10 4 1

10 4 1

0 tinn' ,-, ,- I "heck 0

___ ~ _______ 1 _______

Total nUIJlber lIffruits

flS 101 141i

llR 103 104

154

54 40

Fruits "ith slight, or un

dothiore!l£1

9R 93

104

111 77 3S

80

11 11

Fruits with pro­nounc(~d or severe

dothioroll<t

U 8

42

7 26 1;0

74

4~

20

Por cent of fruits

with dothiorella

0 8

20

Ii 25 03

50

80 70

" In plot!'\ lettpTPd "~," tlw tr(lutment waH applied.us u,Rpra~r: in plotl:llet.tered ltD." a.9. a dust.

'l'ABLE 2 1 D3:l l£XI'ERIMEWf Jo'OR UON'rlWL OF DO'l'IIlORZLLA RO'f

~.:._.::_::---:_;:--=---~----- .:::_=-_=-.....=...=:=..:__ -::_-=-=_-=--==-....:=..:....=::::_

Fruits Fruit, Per ('ent Number 1'ot.al with wit,h pro- of fruits

Mllterhll PInt No. of treat- number slight. nounced with ments Df fruit. or no Of severe dothiorella

dothiorella dothiorelia ---------_-- - ------- --_------ ---"-----~ -------_.

( 8·1" i 07t 67 ~O 31

I ,,·2 f Bordeaux 4.4·50, ..

1 ~:~ 2 3(; 26 l() 23

6 4 2 33 H-5 21 IS 3 14 S·1l 6 4 2 33

Zinc sulfate. R lh; ... , r 1;-7 110 70 40 36 Hydrnted lime, 8 lb •. " R-8 152 B4 68 45 Flotation sulfur. 6 lba. i R-O 62 54 S 13 Casein spreader. 1 lb." S·10 n9 85 14 14 Water. 100 gals. ,_ l 8-11 94 76 18 19

Copper Bulfate, 8 lbe." , - - 1 Hydrated limo, 8 lb~ ...

) f 8-12 } Flotatlon sulfur. 61bs, S-13 23~ 23 0 ()

Casein .preader. 1 lb. , .. l 8-14 Water, 100 gaL! ..

No tr09.tment, '.. , cheek 0 97 :;·1 43 44

• ·'S"=Trlll'.tIllent applied lUI a spray. t The fruits from plots 8-1 and 8-2 were combined. t The fruit. from platH 8·t2, &-13, and 8-14 were ~ombined.

BUL.594J Du'rmORELLA l:to'r ON A VOUADO 11

in avocado-growing districts. Where latania scaleu was present, fumiga­tion with hydrocyanic Hcicl gas was IOllmlneccssary. As tllis material is not compa.tihle with bordeaux or t.hE' copper compounds, the necessity for a copper suhstitut!' was apparent. For this reason attempts werE' made to subst.it.ute zinc sulfate for copper sulfate, sillce somB investiga­t.or!> reported good. results from the use of this material as a fungicide.

I n districts 'where the avocac1o nd mite is a vest, it is necessary to use fmlfur in somA form for its control. l~or this l'eaS011 sulfur was acldecl to several of the formulas.

During this season tlH're was a demand on the part. of the growers for a Rimple method for det,('rmining in the field the approximate amount of dothiorella t.hat could. lIP eXIJ!'cte(l to show up ill the fruit at tlle time of softening. It was found that hy making fill insllcctioll of an orchard all(l paying careful attention to the following points, a V{'ry elose cRtimat(> could be made: (1) amount of tiplml'll, (2) nnml)('r of d.eac1 twigs in t}H~ trees, (3) amount. of dothiorella infectioll in tJw tipbnrneclleaves and dead twigs, (4) amount of off-hloom, (5) methocl of irrigation, (6) insect and other injuricl'l, (7) fungiei(lallllatrrial mied, and (8) maturity oE the fruit.

From a eom.mcreial :->tallclpoint, we deeided t.hat. it. would not be prac­tical to make more than three applieatiolls of a fungicide ill It single sea­son, and for that reason three applicatioll:-l were selected as a maximum numher tn be used. All fruit waH pieked as S0011 a.<; a. satisfactory oil con­tent had been attaine(l, which was on N ovem her 16, 1932.

Owing to a short crop of Fnertes ill IB~~2, the number of fruits har­vested in this experiment WUI> not l:lufficiently large t.o he satisfactory for drawing very definite conclusions. However, the 19:32 experiment strongly indicated thai the addition of snlfur increased t.he fungicidal value of bordeaux and that zinc sulfate ha.d some promise as a fuugi­<lide in the control of dothiorella rot. For this reason, these material~ were included in the formulas used in the 1$)83 expedment (table 3) .

.... , The fruit in this experiment .vas 1101., picked until I!'ehruary 12, 19:34, !-lince it had not attained a satisfactory oil content until that date. The 1933 Fuerte crop was again light, and the quantity of fruit examined was rather small.

The lime-sulfur which was used as an insecticide for the control ()f aV(l{Jac1o red spidel' showed marked fungicidal value. For this reason flotation sulfnr applied as a fungicide wa..c;; tried in the 1934 experiment (table 4).

9 Latanil1 scale, ASltidiottl8 lataniue Sign.

12 UNlVERSI'l'Y OF CAI,IFORNIA.-ExpERIMENT STA.TION

Since the addition of sulfur to bordeaux had given 100 per cent con­trol of dGthiorella rot in 1932 aml 1933, it was again used in the 1934 €'xperiment.

In an attempt to finel ont when infection i1l the sense of penetration of frnit actually takes place, 100 fruits which had attained a diameter of

TABLES 19;)31 EXPEl(III[ENT l'OR COWI'Ror, OF DOTHlORE[,LA ROT

=r==--= Fruits Fruita Per cent

Number 'fotal with with pro- of fruits lIfutorial 1'1,,1, No. of treat- IlUlnbor Blight nounced with

ment" of fruits or no or t-lGvere dnthiorella r1othiorolla dothiordIn.

-----------_-- -.~------- .. _--Zinc ~u1fa.te cry.tal., 8 lb ....... ,. lo're..h hydra.ted lime. 8Ibs ....... , Wettable sulfur, Glbs ... " .......... l::i-l' 43 32 11 256 Casein ppreader, lIb .. " ....... Water, 100 gals ..........................

Zinc sulfate crystals, 16 Ibs ...... 1 Fresh hydrated lime, SIbs ...... Wettable auUux, 6 lb •... ".n.""" ( 8-2 110 U~ 16 14.5 CWlein spreader, lIb .. "...... .. ... I Water, 100 gills. .... . ...............

Co_" •• , ..... ".,.I> ...... ) Commeroial bordeaux, 161bs .... 8·:l 118 lUi 2.5 Casein spreader, 1 lb. ........... .. ... Water, 100 gala .......................... ,

eo.. ....... _~, >oJ,,"'_] Wettable sulfux, 6 lbs............. .. S-4 84 84 0 0.0 Casein spreader, llb .......... ., .. , ... Water, 100 gals ............... , ..........

Co_ ."'M., ",. ........... ""'J Commercial bordeaux. 16Iba, '" Wettable sulfur, 6 lb ................. 8-~ 48 47 2.0 Casein spreader, 1 lb ... , .... , .. " ....... Water, 100 gala..... .. .... , .......... , •

Dry lime-sulfur, 6lbs •. " .......... } Casein apreader. 1 lb .................. check 292 244 48 16.4 Water. 100 gaIs, ...... ., ............ , ........

• "S"""Treatment applied lIS a .pray. t Fruit not pioked until Feb. 12, 1934.

IYz inches were dipped in 70 per cent alcohol and each covered with a paper bag to prevent further infection. These fruits were bagged on August 2.

The fruits in this experim.ent were picked in two separate lots, the first lot on November 17, and the second lot 41 days later. The object of this was to determine differences in the amount of infection in early­

" harvested :fruit as com.pared with that picked at a later date. Fruit from

Bur,,594] DOTBIORELLA RoT ON .A. VOOADO 13

the check plot of the first picking developed 40 per cent of rot, while that or the second picking gave 70 pel' cent of rot.

Fnmigat.ion experimeIlts were cOll(luotect by H, ,T. Qna~Tle to c1etE'r-

TABLE 4 19:34 EXPEnIMEN'l' FOR. CONTEOL OF DO'l'HlOR);:LLA HOT

-) Number

Fruits Fl'uits I Par cont Total witb with pro- of il'uits

MaterIal l'\.o~ No. or nllltt- llllIDb,,-. IDIg'nt llmm~',.o. with ments of fruits or no or severe dothiorella

dotbiorella dothiorella - ---Zinc sulfate erl'BtalB, 16 Ibs,."". ) Fresh hydrated lime, 8 Ibs" ...... I F"'."~ "Ii"" m" ....... " ) S-l" S 200 153 47 24 a Blood albumin spreader, 6 oz .... No.3 ta.nk-mix oil, ~ gal.." ....... Water, 100 go.l ......... " ....... " ........ ""_

"' ... --"", .. ''' ....... ] Cl)pper Bulfate crystals, lIb ..... Fresh hydrated lime, 8 lbs ...... ". Flotation sullm, 6 IbB ... " ........... " S-2 3 211 179 32 15 0 Blood albumin spreader, 6 oz ... No. a tank-mix oil, ~ 1181 ........... Water, 100 gal ................ "" ............ "

zm ... ""., m,.", .. ''' ...... ] Fresh hydrMed lime, 161bs ....... Flotation sulfm, 6Ib .................... S-3 3 200 190 10 8.0

Blood albumin spreade" 6 0 ....

No. a tank-mix oil, ~ gal ......... Water, 100 go.l •• """ ..... " ...................

"'m_.'!h_~, ........ I Flotation sulfur, 6Ib ... " .......... " .. Blood o.Ibumin spreader, 60 .... ' 8-4 3 176 176 1 0,6

No.3 tank-mix Oil, ltii !llll. ... " .... Wllter, 100 sols ....... " .......... " .. "" .....

""""'" ."'~, 10 ~, ... - -"-'J Blood albumin spreader, 6 03". S-5 3 16~ Iii7 7 40 No. a tank-mix oil, ~ gal .... ,,, .... Water, 100 gols ......... "." ......... " ......

No treatment, "." .... " ...... " .. "." ............ Check 0 410 189 221 54.0

Fruit dipped in alcohol and b~lIged when fruit had reached l;i inches Bagged in diameter."" ................. " ..... "",, ...... fruit 0 72 71 1 1.0

. * "S" .. Trentment IIPpUed as II aprl'Y·

mine the compatibility of cyanide with the various materials used in this experiment, The results were as follows:

'1. Zi.1;tc sulfate sprays were found to be compatible ~ith cy~ide. 2. The addition of 1 pound of coppel' sulfate to eljLch 100 gt;tllons Qf.

zinc sUlfate spray did not increase ita incOInpatibility markedly. ,,'

14 UNIVEHRl'l'Y oP CALlPORNIA-Exl'EIUIIIEWI' S'l'A'l'ION

:1, The flotatioll sulfn!' \I'll:'; incompatihle with c,ranide and consider­ahlf'- dmnage l'l'l'lultrrl in rnmigutl(lll OWl' trpt'H RpraYNI with thi!', mute.­rial. It. is thollght that tlw illl'nmpl11.ihilit,r W!lS due to slll'Htances ot.llI'r than tlw l'lulfm 1'l'P!-;pnt ill thi~ particUlar ~Jlra.r proparation. It is also felt that the incompatil,ility IHlllld not 1w oj' a l't'sidnalnuture a:-; iH some­times the cu~{' with the coppel' eomponu(l!, ..

4, It'umigation over tl't'E'S sprayed with hordl:'allX dill not result in as seriollS injury as was l'xJH'(~ted, Howewr. the esperiments 'were not ex­tl'llsivl' eU{}llgh tn be eOHeitl;;ive, Ilwl seriolls injlll'Y might l'E'l:lult from fnrther attc'lllllh; nrHler (liffrrrnt cnnclitiOllH,

SUGGESTED SPRAY PROGRAM FOR DOTmORELLA ROT

8pl'((!J TI'C(1tmc'flt 1nthoul PHm[uafio'/l.-\Vbere no fumigation is con­templated, there' s1lOnlii he two appli(lations witli the following .formula, the fir:;;t appli('.atiOll to be 1ll1H1(~ when Hw frnit i;.; about l1;~ ill'e]!es in diameter, thE' seeom1 application to be made about two months lat!.'l':

COlllll1ere.ial horUl'aux (nr ·1-4-iiO ll(llnemalll) hOl'llmmx) .. ,. .. 16 }loUluls IV !1ttahlll su1fur .,. .,.,. .. ,. ............................... "............................ Ii ]1ounds Blood albulIlin sI)readl'r.. '" ........................................................ 6 ounces Water ,. ......... ,. ....................... ,. ........ ,. ..... ,. .. " ...................... ,. ..... 100 gallons

Spray 'l'1'eatment '!/lith ]i1umi[/lltion.-\Vhere fumigation is contem­plated, it Hhonld take plalje as socm at; the fruit at.tains the size of lY2 inches in diameter. Ii'umigaJion s11ou](1 11e followed at once with an applicatioll of wettahle s11lfnr used without the horcleaux; at least one subsequent application 01: this spray materIal should be marle about 6 to 8 \Veeks later, 1'I!e formula for use follows:

Wettable sulfur ·· ........ ··· .. · .... · .... · .. ·· .. · .... · ......... e........................... 6 pounds Blood album ill spreaclcr ... " ...... .,................................................ 6 ounces \Vater ........................................... " ............................................. 100 gnllons

CumbinuNvn Tl'catmcnt tOj' Doth iorella Rot an(l Jf ottZe-LeaJ,-rrreat­lllent to Pl'('VE'ut duthiol'ella l'ot. may he combined with treatment for mottle-leaf. The fil'Ht :;pl'a~' :-;houhl be applied when the fruit has at: tained a (litl;meter of nbout 1 % llll'hes, and the :second about two mouths later, If fUItligation is practiced, it should precede the first spraying. The. formula l'ecoll1menc1e(1 for this dual pUl'pOfie .follows:

Zinc sulfate cl'ysials.................................................................... 16 pouncls Copper sulfate ............................................................................ 1 pound Fresh llydr!ltecllim~.................................................................... 8 pounds Flotation sulfur .......................................................................... 6 poullds Blood albumin spl'eac1er.............................................................. ij ounces Water ........................................................................................... 100 gallons

nUL. G94]

SUMMARY

Dothinrclla rot call he completely controlled hy tIl!' UKe oj' fUllgicicles. Liqnid fungicides are more effective than those applied as dusts. Dothiol'ella rot is increased by ovprheatl irrigation.

l[l

Oopper sulfate is more effective than ziuc snlrate ill controlling dothi­orelJa rot.

'rhe fllngicidal value of copper sulfate when mHJ{l as hordea.ux is greatly increased b~T the addition of tlulfur.

Sulfur alone when appliell as a liquicl with Aj)l'l'ac1Pl' is It "pry pffective fungicide against clothiorellu. rot.

'rhe fungicidal value or zinc sulfate is inereaHed hy the addition of a t,lmall amoullt of (wpper Hulfate. Copper snlfate when a<lllell tn zinc sul­fate in small amount)! apparently does not interfere with fumigation.

For the control of o.othiore11a rot, zinc sulfate alone, thtLt is, without copper sulfate, nlllHt be applied at such stroug concentrations as to make it impractical from a. commercial stallupoiut.

It haA been indicated that the fruit is not infected until Home time after a diameter of 1% inches has been reached. 'rherefore, early Sllray­illgS are not as important as ap:plicatiol1s made after the fruit is fairly well developed.

'rhe most effective spray for the control of dothiorel1a rot is the bor­de/tux and Rulfnr mixture pr'eparcd fj(~(Jorc1illg to tlie formula given 011

page 14. The next most effective fungicide is wettable sulfur USE'c1 alone. Either of the first two formulas recolllmended on page 14 also give

satisfactory control of the avocado red spider. If the fruit is picked as S0011 as possible after it has attained a satis­

factory oil content, infection will he greatly reduced, particularly on unsprayed trees.

Spray recommelHlations for the control of dothiorella rot mnst he based upon the comhination of illseets and tlif')eaReS for which control measureH are nec('ssary.

16 UNIVERSll'Y OF CALll<'ORNIA-ExPERIlVlEN'f S~I'Nl'lON

ACKNOWLEDGMENTS

'l'he 'vTiterH deKire to aelmowleuge the helpful interest of. Dr. L. D. Batchelor, Director of the Citrus Experiment Station, and OI Dr. H. S. ll'awcett, head of the Division of Plant Pathology in the Citrus Experi­mellt Station, withont w11o..'>e support the work could not have been car­ried out; and of Mr. H. R. McLean, Agricultural Commissioner of San Diego Oounty, who provided the materials and equipment used in the experiments. :Mr .• Tames D. Hoffman, especially in the earlier years of these studies, assisted generously in making trees and various facilities available. The Oa1avo Growers of California cheerfully furnished fruit, faeilities, and as:;istullce to the full extent of our requests. The Univer­sity Agricultural Extension agent.'l and the agricultural officials, and many growers and. friends of the avocado industry, have helped in va­l'iousways.

18150 tlHlIllnllllltt~Hllllltnllllll

IARl

l1m.a,'SS