Appearance, impact and biology
Chocolate spot describes the reddish-brown-coloured spots that occur on leaves and also on stems and pods. These typically enlarge to have a dark brown margin surrounding a paler brown lesion (see photo, supplied by PGRO http://www.pgro.org/index.php ) and can be distinguished from leaf and pod spot caused by Ascochyta fabae, which produces black specks within the lesion (these are pycnidia, which produce spores), while chocolate spot does not have any pycnidia within the lesion.
The disease is encouraged by high humidity and leaf wetness,
often appearing following wet conditions in late winter and spring on
autumn-sown crops and in mid-spring on spring-sown crops. The disease develops fastest in mild, damp
conditions with high humidity and temperature between 0-30°C. The optimum temperature is
around 15-20°C. In the UK, severe
epidemics have been correlated with high rainfall in the months April-July.
Younger leaves are less susceptible than older leaves for most varieties.
Significant yield losses can occur and this is more common
on autumn-sown crops, particularly if the canopy is dense. Severe infections on the stem may cause stem
collapse and on flowers, may cause complete flower loss or abortion of pods.
The fungus sporulates in humid conditions on the surface of
infected tissues as lesions appear, 4-5
days after infection. The spores can be
spread by wind and also by rain-splash to cause new infections. Survival structures called sclerotia are also
produced inside stems of dying plants these and can survive in the soil, so
disposal of diseased material is important.
Dry weather checks the progress of disease but alternating wet and dry
weather allows lesions to expand by producing new rings of growth around the
original lesion and prolonged wet and humid weather encourages rapid lesion
expansion. Spores can also survive in
dry conditions for at least a few weeks and are able to infect when favourable
Pre-season disease risk can be reduced by using certified seed and by choosing more resistant varieties (none are particularly resistant at present), ensuring at least a 4-year separation in crop rotation, avoiding planting in fields adjacent to last season’s crop and avoiding damp sites. Risk is also reduced by decreasing sowing/plant density and this promotes reduced canopy humidity. The disease has been reported to be encouraged by stress factors affecting the plants, particularly potash and phosphate deficiency. As the disease also occurs on common vetch, it has been suggested to avoid sites where this plant is common in field margins, or used in green manures.
Foliar fungicide application is best done following regular monitoring of disease progress and considering forecast weather. Tebuconazole, azoxystrobin,
metconazole, chlorothalonil + cyproconazole and Signum are effective. Up to two sprays are suggested if weather is frequently wet over the flowering to pod-set stages. According to PGRO, early fungicide treatment is essential if the
crop shows symptoms at first bud or early flower. A second spray may be
required 3 to 4 weeks later if damp conditions persist. Fungicide trials by PGRO have produced yield responses to fungicides of up to 3
t/ha in winter beans. Broad-spectrum fungicide mixtures or products with at least
two modes of action perform best, e.g. triazoles plus either chlorothalonil or