Disease management of Ascochyta fabae Speg., a seed borne pathogen of Vicia faba L.

Abstract

Ascochyta fabae was present in all the field and broad bean crops surveyed in the Canterbury and Marlborough areas during the 1978/79 and 1979/80 seasons. In broad bean crops, the disease incidence on the flowering nodes ranged from less than 1.0 to 30.0 per cent and the disease severity of the first flowering node ranged from less than 0.1 to 10.0 per cent.

Commercial seed lines examined between 1977-80 were infected with A. fabae. The mean disease incidence over the three year period was 6.9 per cent with the highest value of 8.0 per cent in the 1979/80 season. The mean percentage disease incidence in broad bean seeds was much lower than that for field beans.

The incorporation of a decimal scale principle in a modified growth stage key was found to increase its usefulness in the field. The use of detailed standard area diagrams of whole leaves rather than leaflets was found to increase the accuracy and speed of disease assessment of A. fabae in the field.

Seed treatment with an MBC plus captan mixture, and foliar spray with chlorothalonil were the most effective in reducing disease incidence, severity, pod and seed infection under field conditions. The amount of pod infection during crop growth was related to the degree of seed infection at harvest. Yield losses increased with increase in disease. Investigation of the optimum timing for disease control measures indicated that greatest yield was obtained by reducing early disease epidemics, the yield component most affected being the number of pods per stem.

Different amounts of initial seed infection, obtained by varying the proportion of diseased to healthy seeds, were highly correlated with disease development in the field. Differences in degree of disease establishment were reflected in the yield at harvest, such that greatest yield was obtained from sowing seeds with the minimum amount of seed infection.

The disease transmission rate from seeds to seedlings ranged from 15 to 47 per cent of the infected seeds sown. The increase of infection from sown seed to harvested seed ranged from less than 1 to 8- fold with the greatest increase from seedlines with the least amount of seed infection.

The investigations contained in this thesis have shown that under New Zealand conditions a threshold level of 0.1 per cent seed infection (e.g. a test value of nil in 1000 seeds) should be recommended for all categories of field and broad bean crops. Only seedlines with infection levels below the maximum acceptable standard should be used for further multiplication purposes. Seed treatment should be used on all seeds as a cheap insurance against the build up of disease.

Foliar sprays could be applied to seed and processing crops to protect pods and seeds from infection. Adoption of the above recommendations should ensure the most economic control of the disease.