SITE OF INOCULATION AND STAGE OF PLANT DEVELOPMENT DETERMINE SYMPTOM TYPE AND EXPRESSION IN BRASSICA JUNCEA FOLLOWING INFECTION WITH ALBUGO CANDIDA
P. Kaur, K. Sivasithamparam, M.J. Barbetti
Investigations were carried out firstly to determine if there were any differences in symptom expression following spot inoculation of the cotyledons, leaf lamina or the growth apex at different stages of plant growth and, secondly, to determine the specific reproductive stage during development of Brassica juncea at which the flowers have to be inoculated with Albugo candida to maximise development of pod hypertrophies. Spot and growing point inoculations performed at the cotyledonary or true leaf stages caused clear differences in disease progression and severity of disease occurring at subsequent stages of plant growth. In this test, inoculations of only the growing points at the true leaf stage [GS 2.1] resulted in systemic pod hypertrophies. However, among the different reproductive stages tested, only inoculations of flower buds just changing colour [GS 3.3] or just opening flowers [GS 4.1] resulted in pod hypertrophies. Earlier or later inoculations of flower buds or flowers failed to result in pod hypertrophies. These studies have defined, for the first time, the particular reproductive stages of plant development at which infection results in pod hypertrophies. They have also shown that, in relation to infections of shoot tissues, it is the inoculation of the growing point at the first true leaf stage that leads to systemic development of the disease, rather than inoculations of cotyledons or leaf laminae. The findings of this study indicate that fungicide applications, targeting these critical stages of early plant development, and the associated plant sites, and also flower buds changing colour and/or just opening, present the best opportunities for arresting infections that lead to systemic spread of the pathogen within the plant, thus minimizing the extent and severity of both leaf disease and pod deformations. These studies provide a basis for subsequent identification of the mechanisms controlling the systemic spread of A. candida within B. juncea. Once identified, such mechanisms could be exploited in developing and deploying new cultivars with improved resistance against both white rust leaf disease and pod hypertrophies.