BIOLOGICAL CONTROL OF THE CROWN GALL DISEASE

M. N. Schroth, A. H. McCain and J. Harrison

ABSTRACT

The use of the bacterial strain K-84 to control crown gall was a remarkablediscovery. However, in California, many soils contain resistant strains of

the disease causing agent. A project was initiated to find new biocontrolstrains to protect walnuts from all of the strains that occur in California

soils. A number of strains were found. One in particular, number JH-I,showed great promise. It produces an anti-biotic that is bactericidal and

in greenhouse tests it controlled strains of crown gall bacteria that wereresistant to K-84. Tests are now being run to examine it in the field andto determine if it can be used with K-84.

OBJECTIVE

1. To find new biocontrol strains that will inhibit the strains of

agrobacteria that are resistant to K-84, the currently used biocontrolstrain.

2. Develop an inoculum of biocontrol strains that will protect walnutsfrom the different strains of agrobacteria that occur in Californiasoils.

PROCEDURES

1. Bacteria from soils suspected or known to be suppressive to the crown

gall disease are isolated and tested as possible biocontrol agents.

2. The tests are done by wounding walnuts and indicator plants, swabbing

the wounds with a soil suspension, and then isolating for bacteria that

have the ability to colonize the wound site, but not cause galls.Bacteria with this ability have the characteristics necessary tocompete with crown gall bacteria.

3. Bacteria isolated from wounds are tested in vitro for antibiotic

production against crown gall bacteria. The positive strains are then

tested by coinoculating plants with crown gall bacteria.

4. Field and greenhouse tests are made with potential biocontrol strains.

RESULTS

A number of bacteria were isolated that were inhibitory to crown gallbacteria, agrobacteria, on the basis of antibiotic production. Tests onindicator plants (tomato) showed that a number of strains could reduce

crown gall infection. However, they were not any better than the standardK-84 strain (Table 1).

101.

Table 1. Effect of new potential biocontrol strains on preventing crown

gallon tomato seedlings. when inoculated with strains resistantto K-84

New Strains (108 suspension)3B-27A-l3A-la3A-33B-32B-lKerr's 84Inoculated Control

Galled Seedlings (t)2977554534373670

.TOmato seedlings were inoculated by snapping the leaf to create

a wound perpendicular to midrib, 10 leaves wounded/plant, 1 plantequals 1 replication, 6 plants/treat8ent

In another series of experi~nts (Table 2 representative of data) using adifferent indicator plant (chrysantheaum) and S088 different strains,several antagonists were considerably more inhibitory to the crown galldisease than strain K-84.

Table 2. Effect of bacterial antagonists on preventing galls on

chrysanthemum* by a K-84 resistant strain of Agrobacteriuatumefaciens

Antagonists (108 suspension)lB-23A-l3A-37A-l78-2Kerr's 84Inoculated control

Galled rooted cuttingS (,)13661o2685

.Chrysanthemum leaves injured in Sa8e manner as t088to leaves

above, 10 leaves/plant, 1 plant equals 1 replication, 5plants/treat8ent

In another series of tests, a strain called JB-l was found that appeared tobe far superior to all others. Data in Table 3 are indicative of the kindsof results that have been obtained.

102.

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Table 3. Effect of the potential biocontrol strain JH-l on preventinggalls on tomato seedlings. by K-84 resistant strains ofAgrobacterium tumefaciens

WTwowounds/seedling; 6 seedlings/treatment; 1 seedling equals1 replication

The above results were considered to be very encouraging because a highconcentration of crown gall bacteria were placed into the wound sites.Such concentrations would not occur in the field.

CONCLUSIONS

A number of bacterial strains exist in soils that are capable, to somedegree, of suppressing crown gall infection. Soils with good populationsof these organisms tend to be suppresive to the disease. Two soils werefound where it was difficult to cause crown gall infection even wheninoculating susceptible plants with the crown gall bacteria.

A potential new biocontrol strain named JH-l was found that produced anantibiotic against all agrobacteria strains that were tested. In planttests it proved to be highly effective in inhibiting crown gall infectionsby bacteria thatare resistantto strainK-84, the presently usedbiocontrol strain. This strain should be tested on walnuts in the field.It is hoped that it can be used in conjunction with strain K-84.

103.

No. Ga11s/l2 WoundsK-84 Resistant Pathogen vs. JH-l Control

1 0 62 1 83 2 114 1 8