In many African countries, legumes are a major source of protein. Dry beans have high nutritional value and offer dietary and health-promoting qualities for humans. Nationally, the economic impact of dry beans plays a vital role in efforts to control trade imbalances. Therefore, a sustainable production of this globally important crop is critical.
Angular leaf spot disease occurs sporadic and is caused by the fungus Pseudocercospora griseola, previously known as Phaeoisariopsis griseola and Isariopsis griseola. Pseudocercospora griseola has several strains that can cause yield losses of up to 80% in common dry bean production, mainly manifesting as a reduction in seed size and pod mass. The disease also has a negative impact on seed quality and marketability. High levels of seed infection causes serious seed health issues.
Disease cycle and epidemiology
Pseudocercospora griseola can survive in the form of spores for at least twelve months on crop residue, ‘volunteer’ plants and on seed to serve as a primary source of inoculum for the follow-up crop. Apart from dry beans, crops such as cowpeas, soybeans, Desmodium species and Dolichos lablab have also been identified as suitable hosts for P. griseola. This fungus can be spread by wind or, over long distances, by infected seed. The disease is favoured by moderate temperatures (16 to 28 °C), and high humidity (spore germination, infection, and production of symptomatic spots) alternating with dry times (spread of the spores released from the spots on leaves by wind).
A temperature of 24 °C with high humidity, rainfall, and the presence of dew are the environmental conditions necessary for successful infection and symptom formation since this fungus can only penetrate and establish on the leaf surface in the presence of free moisture. Infection is often latent for 7 to 14 days, and symptoms do not appear until later growth stages. Continued high relative humidity and rainfall will facilitate secondary infections and development of the disease. Premature leaf fall may occur during severe infestations, which leads to a decline in plant functionality and a resulting yield impact.
Symptoms
Initial symptoms start at flowering and appear as small, water-soaked spots similar to beard stubble on the underside of leaves, which then enlarge and turn brown, dark brown or black. The specific symptom manifestation type depends on the infected leaf. On trifoliate (three leaflets) leaves, angular spots up to 3 mm wide can be found, but they are limited by leaf veins to spread. These spots have an initial grey appearance that turn light brown and can be surrounded by a yellow halo. On single leaves spots are up to 15 mm in diameter and often possess rings that cause the spots to appear like targets.
Lesions on the leaves turn yellow around the edges to form a yellow (chlorotic) halo, which are best visible when holding the leaves up against sunlight. This yellow halo is the most distinguishing symptom of angular leaf spot. Lesion colour varies with the age of the spot and the host. Lesions will often cause the leaves to dry and curl. Severe infection of the leaves can result in defoliation (Photo 1).
Although pod and stem lesions are less common than leaf lesions, it can occur in significant numbers during wet seasons or when the host is susceptible to this disease. At first, the spots will appear superficial with virtually black margins and reddish-brown centres. Under humid conditions, dark grey to black synnemata, consisting of conidia, appear. The spots may coalesce and cause the entire pod or stalk to be infected. Seed beneath the spots on the pods become infested. Lesions are dark, sunken and oval or circular, varying in size, and often lead to discolouration, pitting, and distortion
(Photo 2).
Disease management
Detection and inspection
Monitor fields regularly throughout the dry bean growing season and seek for first symptoms that appear on the primary leaves (Photo 3), which will become more visible during late flowering. Pods should be inspected for the reddish-brown, round spots with dark borders.
Host resistance
Resistant dry bean cultivars and varieties planted in fields with a history of the disease, should reduce yield losses. However, keep in mind that the possibility of resistance breakdown due to pathogen adaptability and the development or emergence of new races of the pathogen exists, and that may render previously resistant hybrids susceptible. Dry bean fields should, therefore, be continuously monitored.
Cultural control
Practice a minimum of two-year crop rotation with a non-host crop such as cereals and root crops where dry beans are planted every third or fourth year. This way, initial inoculum levels can be lowered by enabling infected stubble to break down over time. Do not plant beans after broad-leaved crops such as potatoes or other leguminous crops such as soybeans. Choose a suitable planting date to avoid cold, wet conditions at planting or hot, dry conditions during the growing season.
Plough in dry bean debris thoroughly after harvesting dry bean plants or remove from the field as fodder for domestic stock. Remove volunteer bean plants that occur in follow-up crop plantings and keep dry bean plantings weed free. Do not hoe fields or move irrigation pipes while plants are wet. Hoe uninfected or young plantings first and then move on to older or infected plantings. Clean tools and machinery where possible before moving on to the next field. Wash clothing and clean shoes after having worked in an infected field.
Disease-free seed schemes
Plant only disease-free and certified seed that can be obtained in small or large quantities from seed companies. Discard old retained seeds from previously infected fields to prevent spreading of the disease to new fields.
Biological control
Trichoderma and Bacillus species possess multiple mechanisms of action to control plant pathogens and promote plant growth. Using these products affords the producer an environmentally friendly management tool that requires little extra effort. Before using biocontrol agents, however, producers should enquire about efficacy data from companies that market such products. If the biocontrol chemical proves to be ineffective, it can be costly in addition to the economic loss of income due to lower yield.
Chemical control
Several effective fungicides are registered on dry beans as foliar applications or seed treatments to control angular leaf spot. In the case of a seed treatment, seed germination can be increased while reducing angular leaf spot and the occurrence of seed-borne infections. However, seed treatments will not be sufficient when applied to severely infected seed. Foliar fungicides are often utilised to treat angular leaf spot and improve grain quality and yield. Most foliar fungicides prevent spore germination, but have a limited, curative effect once plants are infected. Established field infections may require follow-up fungicide applications.
Integrated disease management
Angular leaf spot can be effectively controlled when implementing an integrated disease management programme that includes disease-resistant varieties, biocontrol agents, fungicides, and appropriate cultural practices.
Dry bean producers are requested to contact their nearest plant pathologist to assist in developing an integrated disease management system based on the producer’s specific needs. Producers are advised to contact the ARC-Grain Crops in Potchefstroom at 018 299 6100 for advice and to report any disease symptoms in their fields. The pathologists will then arrange to collect infected samples. These samples will assist the pathologists to collect isolates from various localities that will be used for research purposes.
This article was co-authored by the late Dr Bradley Flett.
