NEW APPROACHES TO CONTROL WALNUT BLIGHT

Milton Schroth, Bill Olson, Art McCain, Joe Osgood, Mavis Date-Chong Hendson,Yung-An Lee, and Beth Teviotdale.

ABSTRACT:

The addition of iron chloride to fixed coppers greatly affects the susceptibilityof blight bacteria to the toxic action of copper. In addition, by lowering the

pH at the leaf and nut surfaces, iron causes much greater amounts of free copperions to become released from the fixed form, thus enabling them to kill bacteria.

Copper and copper-iron mixtures were tested for the second year in the field.

The copper-iron mixture. reduced the number of blighted leaves by 177%. Differentforms of iron also were tested. The combination of iron chloride with the

insoluble iron oxide caused some phytotoxicity. However, the results suggestedthat combinations of different irons, because of solubility aspects, could be

used to develop improved formulations with fixed coppers for persistence on

foliage and for improved toxicity to blight bacteria. Data also are presentedshowing the extent that iron increases the release of free copper ions from fixed

coppers. This is shown in water solutions and on the leaf surfaces. Other ~ield

research suggested that a late winter spray could markedly reduce the percentageof buds colonized by blight bacteria. The copper and copper-iron mixtures also

were tested for efficacy against nut blight. Although the copper-iron was 120%

better than copper alone, the great amount of variability among replicationsprevented the results from being statistically significant.

INTRODUCTION:

This year's research focused on field testing iron-copper formulations. Previous

laboratory and greenhouse data showed that the iron increased the susceptibilityof blight bacteria to the toxic action of copper. Moreover, the addition of

ferric chloride to fixed coppers lowers the pH, thus increasing the amount of

free copper ions. However, this has not been tested on plant .surfaces, anobjective of this year's research. Another added benefit of the iron is that.

the iron-copper combination kills copper tolerant bacteria whereas the normalfixed coppers do not.

As with any new product it will take several years before the optimal formulationcan be worked-out. As is, we do not know how much ferric chloride to add to the

fixed coppers to obtain greatest efficacy without also increasing the amount of

phytotoxicity. In addition, a number of growers have been using ferric sulfate

as an amendment to coppers. This needs to be tested experimentally as it is muchmore insoluble than ferric chloride and may not produce the desired results. It

is possible that combinations of the two forms of iron may ultimately prove tobe the. best material.

OBJECTIVES:

1. Test different forms .of iron to add to fixed coppers in order to improve thecontrol of walnut blight. Determine the role of iron in lowering the pH at theleaf and nutlet surfaces which in turn increases the amount of free copper.

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2. Determine by use of a copper electrode the best concentrations of copper andiron to use in the field in relation to persistence and the availability of thefree copper ion on walnut foliage.

3. Monitor the populations of blight bacteria throughout the year to determinewhen best to apply bactericides--fall, winter, and prior to bud break.

4. Investigate the significance of copper tolerance in blight bacteria withrespect to their persistence and infectivity.

PROCEDURES:

1. Forms of iron--There are many different forms of iron and they differ greatlyin solubility. Ferric chloride is very soluble, ferric sulfate is slightlysoluble, and iron oxide is very insoluble in water but becomes slowly solublewhen on plant surfaces. Much work needs to be done testing these different formsand combinations thereof. This year we field tested ferric chloride and acombination of ferric chloride with ferric oxide. This was done by sprayingtrees at normal intervals and then evaluating nuts and leaves for blight lesions.The normal fixed coppers were used as standards.

2. Effect of iron in increasing the amount of free copper ions on leaves-- Thiswas done using English walnut seedlings grown in the greenhouse. Leaves weresprayed with standard concentrations of fixed copper or fixed copper.plus ferricchloride. The leaves were allowedto dry. Twenty-fourhours later, the leaveswere moistened by a light water spray, the droplets on leaves were collected, andthe amount of free copper i?ns analyzed with a copper electrode. This providesinformation about the availability of copper ions to kill blight bacteria.

3. Do fall, winter, or early spring sprays reduce the incidence of buds infested

with blight bacteria?--This was tested by spraying trees at different times withfixed coppers and then monitoring large numbers of buds for the incidence of

blight bacteria. The process calls for the sampling of hundreds of individual

buds on special media in the laboratory. The colonies of blight bacteria are"

then easily recognized.thereby enabling an evaluation of the number of buds that

have been colonized by the bacteria. The trees were sprayed in November and

again in late January or early February. Some trees were treated only in late

January or early February. The buds were sampled j~st before bud break.

4. Incidence of copper tolerant blight bacteria--This work was not done in 1991

because of the amount of effort devoted to the other objectives.

RESULTS AND CONCLUSIONS:

This year's research shows conclusively that the addition of iron to coppergreatly increases its efficacy. The best data were obtained by evaluating leafblight. Table 1 summarizes data from two field plots and indicates that a 177%

reduction in percentage of blighted leaves occurred when comparing iron-copper

to copper alone. This was statistically significant at P-O.OS. As exp~cted, boththe iron-copper and copper (Champion) substantially reduced leaf blight compared

to the control. This demonstrated the superiority of iron-copper to copperalone.

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Table 1 . Percentage of walnut leaflets infected with Xa..n.tb.Am.a.ncampestris pv. ~ on walnut trees with different treatments.

Treatmentswx first surveyYz second survey---------------------------------------------------------------ChampionChampion+FeClaChampion+FeCla+Fe20aControl

9.5a6.6a5.6a

19.7b

12.2a4.4b5.5b

27.4c---------------------------------------------------------------w100 ppm Fe3+ of FeCI3 were applied In the second treatment (Champlon+FeCI3).

50 ppm Fe3+ of FeCI3 and Fe203 (total 100 ppm Fe3+) were applied In the thirdtreatment (Champion+FeCI3+Fe203)'

xChampion were applied at the rate of 2 Ibl100ga!. All treatments were applied to theAshley variety, beginning on 16 March1991, and sprayed 6 times at intervals of10-14 days. First samples were collected on 18 April 1991, and second samples wer.ecollected on 10 June 1991.

YValues are the mean number for each treatment replicated six times In completelyrandomized design. 150-200 leaflet for each replicate were assayed for the Infectionof X. C.pv juolandis. .

zMeans with a column followed by the same letter are not significantly different (P-0.05), least significant diff~rent. 4.6 (first survey) and 6.0 (second survey).

The mixture of ferric chloride with insoluble iron oxide resulted in veryinteresting findings. Although the amount of blight was reduced to approximatelythe same extent of that obtained with iron-copper, phytotoxicity was noted. Thiswas surprising in that the iron oxide is relatively insoluble. Possibly the ironoxide remained on the leaves (resisting washing-off by rainfall) and the slowrelease of copper resul t:ed in internal copper accumulation by the leaves. On theother hand, the amount of free copper ions on the leaf surfaces immediately afterspraying was half since only half the amount of ferric chloride was used. Thesepeculiar and unexpected results suggest that there are great opportunities tomake various kinds of iron-copper formulations which differ in persistence,phytotoxicity, amount of free copper ions, etc. Ideally, the optimum formulationwill be one that has the greatest amount of free copper ions available to killblight bacteria and one that is not easily washed-off leaves during rain fall.In addition, there should be a sl~w release of free copper ions to make-up forthose that become bound to plant surfacesand plant exudates. Lastly, thereshould be low to no phytotoxicity.

Potency of free copper ions on leaf surfaces. Aside from the fact that ironsomehowaltersthe physiology of. blight bacteria causing them to become moresusceptible to the toxi.c acti~n of the copper ion, ferric chloride greatlyincreases the amount of ~ree copper ions on leaf surfaces. By use of the copperelectrode,water dropswere c'ollected from leaves that were sprayed with copperalone and copper with varying amounts of iron. As shown in table 2, iron causeda marked increase in the amount of free copper ions. The data are presented two

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ways: one column shows the release of free copper from copper hydroxide in awater solution whereas another column shows the amount released on walnut leaf

surfaces. As can be seen, the extent of free copper ions increased with added

concentrations of iron. Thus, this again shows the importance of doing extensive

field testing with different combinations of iron and copper to determine whichformulation is best to control blight.

Table 2. pH values and free copper ion concentrationsof Kocidesolutionsandwalnut leaf surfacessprayedwith Kocidesolutionsamendedwith differentconcentrationsof iron.============================================================

Kocide (11b/SO gal)---------------------------------------------------------------------water solutions on leaf surfaces

AddedFe3+ (ppm)

------------------------------- -------------------------------

pH [Cu2+] ppm pH [Cu2+] ppm--------------------------------------------------------------------------------------------------------

o20405080

7.96.16.05.95.8

0.0325.8060.4080.70

101.86

6.7%0.16.5%0.26.4%0.16.2%0.25.9%0.3

0.79%0.34.20:i:1.0

13.50:i:3.522.60:i:5.128.50:i:5.1

--------------------------------------------------------------------------------------------------------

What about dormant sprays to reduce blight bacteria? It is difficult to

determine the importance of dormant sprays by relying solely on nut blight data.This, in part, is because nutlets drop-off. An alternative method is to

determine if dormant sprays reduced the amount of inoculum. This is done by

assaying individual buds for populations of the bacteria after spray treatments.The rationale is that the less bacteria present, the less amount of blight may

occur. However, this may not be true if weather, conducive for multiplication

of bl ight bacteria, persists 'for long periods of time. In these cases,multiplication of the bacteria can be so fast that the amount of inoculum is not

of such importance.

The field test (table 3) showed that late winter sprays substantially reduced the

number of buds infested with blight bacteria well over 100X. The key factorseemed to be the time of application. The November spray had no affect on the

incidence of infested buds as evaluated in ~arch. However, the mid-January and

February sprays markedly reduced the amount of blight bacteria. These resultswere unexpected; the experimentation needs to be repeated in succeeding years for

purposes of validation.

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Table 3 . Percentageof walnutbuds infestedwith~campestris pv. l.um.andii.on walnut trees sprayed with Kocide 101at different time.

Date of sprayingx percentage (%)yz----------------------------------------------------------11/7/90

-1/17/912/14/9111/7/90 +1/1 7/91 +

1/17/912/14/91

56.5a22.0bc28.5ac24.8bc20.3bc

-----------------------------------------------------------xKocide 101 were applied at the rate of 2 Ibl100gai. All treatments were applied to

the Ashley variety. The samples were colledted on 1 March 1991.YValuesare the mean number for each treatment replicated four times in completely

randomized design. 60 buds for each treatment were assayed for the infestionof X. ,-. pv juolandis.zMeans with a column followedby the same letter are not significantlydifferent"(P.. 0.05), least significant different- 30.5.

Effect of iron in reducini nut bliiht: A major effort was made to determine the

effect of iron-copper compared to copper alone in red~cing nut blight. This wasdone by tagging nutlets, counting those that were blighted, and counting t~e

number that fell-off trees. Unfortunately, there was great variation among thereplications making it impossible to obtain statistically significant results.

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One of the problems was that the experiment included a number of trees thatserved as controls the previous year. We suspect that the great amount of carry-over bacteria contributed to the erratic results. Th~s, even though the iron-

copper treatments resulted in a 1201 improvement over the copper alone, resultswere not significant at the P- 0.95 level (table 4). This year larger plots willbe used.

Table 4 . Percentage of walnut nutlets infected with Xanthomonascampestris pv.~ on walnut trees with differenttreatments.

Treatmentswx percentage (%)Yz------------------------------------------------------------

ChampionChampion+FeCl3

Champion+FeCI3+Fe203Control

12.3a5.6a

10.2a34.6b

-------------------------------------------------------------

w100 ppm Fe3+of FeCI3 were applied in the second treatment (Champlon+FeCI3).50 ppm Fe3+ of FeCI3 and Fe203 (total 100 ppm Fe3+) were applied in the thirdtreatment (Champlon+FeCI3+Fe203)'

xChampion were applied at the rate of 2 Ibl100gal. All treatments were applied tothe Ashley variety, beginning on 16 March1991, and sprayed 6 times at Intervalsof 10-14 days. Nutlets were surveyed on 10 June 1991.

YValues are the mean number for each treatment replicated six times In completelyrandomized design. 80 nutlets for each replicate were assayed for the Infectionof X. ,. pv luolandis.

zMeans with a column f6110wedby the same letter are not significantly different(P- 0.05), least significant different- 9.48.

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