Chocolate Spot in UK Broad Beans

VANCOUVER - Apr 6/11 - SNS -- There have been further reports of chocolate spot symptoms in winter broad bean crops in the United Kingdom, according to ProMED-mail, the Program for Monitoring Emerging Diseases, is a program of the International Society for Infectious Diseases.

The diseases instances were reported to the organization by the United Kingdom's PGRO (Processors and Growers Research Organisation). The outbreaks are blamed on "the harsh winter (which is) predisposing the plants to disease. Damage is mainly on older leaves, showing up as blackened areas, with more typical chocolate spot symptoms tending to be on the younger leaves.

"Fungicides for chocolate spot control are protectant and so will not cure existing disease but as soon as new growth becomes more apparent over the next 2 or 3 weeks, and where chocolate spot is developing on the new growth, a fungicide application will provide protection for new foliage," says PGRO.

ProMed explains that Chocolate spot caused by the fungus Botrytis fabae or the more virulent B. cinerea. It is cnsidered an important disease of broad bean (Vicia faba; also called winter bean, horse bean, Windsor bean, field bean, faba bean) and other leguminous crops. For example, it is one of the most destructive diseases of pulses in tropical African regions. B. fabae is prevalent throughout the season, while B. cinerea affects crops intermittently and is usually confined to flowering time.

Symptoms start as brown spots on lower leaves, and this is considered the 'non-aggressive' phase thought to have little effect on crop yield. However, if warm, humid conditions persist for several days, the disease will spread quickly and be termed 'aggressive' with symptoms of leaf blight, defoliation, and loss of flowers and pods. Stems can weaken and collapse, and flowers and pods may also develop lesions although affected seeds are still edible. A plant toxin produced by the pathogens is thought to be responsible for the damage to the host tissue.

A strong relationship exists between disease incidence and seed yield, with an average of 3 percent yield loss per percent of crop infection for B. c. and 0.5 percent loss per percent of B. f. infection. Disease management includes cultural and phytosanitary measures (such as burning of plant residue at the end of cropping, wide spacing of plants, limiting the amounts of nitrogenous fertilisers, crop rotation), use of clean seed, control of volunteer crop plants, and fungicides. Host cultivars with varying levels of tolerance are being developed.