Articles
METHODS FOR EVALUATING NEW CHEMICALS FOR THE CONTROL OF INFECTION OF APPLE BY ERWINIA AMYLOVORA (FIRE BLIGHT)
The development of new bacteriocides to control fire blight will require the screening of a large number of compounds by efficient, yet effective means.
Work on the development of reliable methods for evaluating a bacteriocide’s ability to control fire blight will be discussed.
The techniques for field evaluating spray materials to control fire blight have been developed by S. V. Beer (In Zehr, E.I. ed., Methods for Evaluating Plant Fungicides, Bactericides and Nematicides, pp. 46–50. Amer.
Phytopath.
Soc., St.
Paul, Minn.). The critical feature of the test method is to produce uniform disease pressure throughout the test plot by artificially inoculating with E. amylovora.
Although the method allows for the reliable evaluation of spray materials, damage from fire blight to test site trees can be severe.
To optimize the limited availability of test sites, economical methods of preliminary evaluation are needed to determine if a test compound has sufficient merit for field testing.
In vitro test methods, in which the effect of a chemical compound on the growth of E. amylovora on artificial media is observed, cannot be used to evaluate reliably a bacteriocide’s efficacy.
Tests in which the chemical compound is applied to apple shoots in the greenhouse which then are inoculated with E. amylovora and the resultant percent of the shoot blighted is observed, are not efficient in screening chemicals for the control of blossom blight.
The method of Koike, et al (Journal of Pesticide Science 3:433–436) for screening chemicals for their ability to control soft rot diseases has been modified to screen chemicals for their ability to control fire blight.
Green pear fruits are cut into uniformly sized cubes of approximately 1 cm3 and dipped into solutions of the test chemical.
Five to six similarly treated cubes are placed in a petri dish with moist filter paper.
The pear cubes are then inoculated with .01 ml of an 18 hour broth culture of E. amylovora.
The pear tissue cubes are incubated at 28°C for 48 hours and evaluated for disease development according to the rating scale: 0 – no signs of disease development, 1 – slight ooze, with little or no discoloration, 2 – much ooze produced with discoloration and watersoaking.
The mean rating obtained for five or six plugs in one petri dish is the unit of replication.
Four replicates are done for each chemical concentration and chemicals are tested at four concentrations at 5X increments.
One concentration is selected at or above suggested field rate and three