Ascochyta disease of broad beans

Abstract

Ascochyta fabae Speg. which causes leaf, stem and pod spots on Vicia faba (faba and broad beans) is a potential cause of low yield especially in broad bean crops where blemishes on pods and seeds present marketing difficulties. Varying the duration of various pretreatment from 1 to 4 minutes for hydrogen peroxide and mercuric chloride and 2 to 10 minutes for sodium hypochlorite did not affect the results of the agar plate tests on faba beans. Mercuric chloride and sodium hypochlorite were the most effective in reducing the saprophytic and other fungi colonies.

Commercial broad bean seed lines produced in New Zealand were infected with A. fabae ranging from 1.5 to 16.5 per cent in an agar plate test. Screening out infected seeds visually reduced the incidence of A. fabae infection from 11.0 to 3.0 per cent for one seed line. The method of plating out whole seed may under estimate the level of A. fabae infection.

There were indications that at a low pre-germination temperature (5°C) and high nitrogen level, the incidence of A. fabae on broad beans were highest. Overhead irrigation appeared to enhance spread of the disease. Four isolates of A. fabae and one isolate of Botrytis fabae from faba bean seed and stem respectively were able to be cross-inoculated onto broad bean plants. It appeared that A. fabae is able to infect seeds within two weeks after inoculations on the pods.

Seed treatment with benomyl/captan and RH 2161, and foliar sprays with chlorothalonil significantly reduced disease severity and percentage incidence of pod infection under field conditions. Benomyl/captan may more effective than RH 2161.