Argentina's battle with stem canker is gaining reinforcements in the form of resistant varieties.
This soybean disease has caused serious problems throughout the country since 1996. That has led breeders to concentrate on producing varieties resistant to the stem canker pathogen Diaporthe phaseolorum var meridionalis (Dpm).
Since the principal stem canker control strategy is the use of host resistance, an assessment of the soybean genotype reaction to Dpm is needed. To determine both soybean host reaction to Dpm and to evaluate pathogenic differences among isolates, researchers used the toothpick method.
The host soybean genotype is inoculated in the hypocotyl using toothpick tips colonized with a Dpm isolate. Plants are then incubated in moist chambers for 72 hours and kept in the greenhouse until 30 days under permanent light at 25-30 degrees C. Plants are considered healthy if the canker lesion is limited around the wounding point, or diseased if they die or have running cankers along their stems.
Varieties such as Tracy M, Crockett and Hutcheson, plus local varieties rated as resistant in the field, also tested resistant by the toothpick method. Susceptible-in-the-field varieties such as Asgrow 6443 or RA 702 rated susceptible with the method.
(Nicanor Lorenzo, Silvia Vallone and Laura Giorda, Marcos Juarez, Cordoba, Argentina)