If a bean plant is not looking healthy, the cause could be any of a number of factors, including poor or unbalanced nutrition, drought and/or heat, water-logging, frost, sunburn, an overdose of herbicide, pests such as aphids, or a disease.
Here are some tips on how to deal with important fungal diseases.
- Diseases are caused by living organisms that are too small to be seen by the naked eye.
- These organisms could be viruses from bacteria or fungi.
- Some diseases can become more severe when the plants are stressed by factors such as drought, heat or water-logging.
- Many diseases become more severe if the weather is cool and wet, but some flourish under hot conditions.
- More than one disease can be present at the same time.
Diseases spread by seed and plant debris can often be avoided (for example, anthracnose) and the damage caused by most diseases can be limited by good farming practices. In some cases, however, diseases such as rust can be spread by wind.
Others, like wood rot, may already be present in the soil. In such cases the damage is mainly determined by weather conditions. Some cultivars are more resistant to certain diseases than others; for instance, small-seeded beans are generally more resistant to disease than large-seeded beans.
THE MOST IMPORTANT FUNGAL DISEASES IN BEANS
Rust can be recognised by raised rust-coloured lesions (pustules) that give off a fine brown dust (fungal spores). Pustules can be on either or both sides of the leaves and are often surrounded by a small yellow halo or dead tissue. Rust occurs under alternating wet and dry conditions. It is spread by wind but can be avoided by planting resistant cultivars. Several fungicides are available for combating this disease.
Anthracnose can be observed in the form of veins on the underside of leaves, which become black, while dark-brown lesions appear on the stems. In the case of a serious infestation the whole plant becomes red-brown to black and growth is stunted. Pods exhibit sunken, round, brown to black lesions, often with a darker border. Seeds from infected pods may show dark brown lesions of varying size. Although anthracnose is known as a cool-weather disease, it can cause damage under all conditions if seed is heavily infected. The disease starts on one or more plants growing from contaminated seed and is then spread from plant to plant by people, machinery, animals or water. The disease can often be prevented by following good agronomic principles (see page 33).
Angular leaf spot (ALS) result in leaf lesions that are grey and angular with very small black structures, which look like beard stubble, on the underside of leaves. Leaves yellow, curl and
fall off prematurely. Pod lesions are large, flat, circular and red-brown to black. ALS occurs under humid conditions, especially on large-seeded beans, and is spread by wind.
Ascochyta. This disease causes characteristic large, grey lesions with concentric rings on leaves and sometimes also on the pods. Leaves can also become tattered. Ascochyta is spread by wind and water and occurs under damp conditions.
Scab. This disease causes abnormal bending and curling of the upper stems and leaves and also forms light grey, raised, scab-like lesions on older leaves and pods. It affects large-seeded beans, especially during warm, wet weather and is mainly spread by infected seed or bean debris.
Powdery mildew. This fungus causes powdery white, star-shaped lesions on the upper surface of leaves, but these may also cover the whole plant. It occurs mainly on large-seeded beans under moderate to hot conditions.
Fusarium yellows. Plants yellow prematurely, starting with the lower leaves. Under conditions favourable for the disease, the whole plant yellows and dies. Leaves are inclined to remain attached to the plants. This disease is caused by a fungus in the soil that enters the plant and blocks the transport of water in the stem, but it is generally only serious when the plants are under stress, especially in abnormally hot, dry weather.
Charcoal rot. Dark lesions occur on the main stem and branches. These can fuse and are often covered with small black spots. It occurs during hot weather, can be present in the soil and is spread by infected seed or bean plant debris. Incidence of the disease can be reduced by good farming practices.
Sclerotinia. White fungal growth occurs on the stems and pods, which rot and die. The disease is soil-borne and occurs under wet conditions, especially with a dense leaf canopy. Incidence can be reduced by a good crop rotation system and by avoiding prolonged wet conditions.
Root rot complex (Pythium, fusarium root rot and rhizoctonia). These three diseases often occur together, especially under stress conditions such as water-logging. These diseases affect roots and the lower stems of plants, which die off, followed in severe cases by the death of the plant. There is no treatment for this disease complex. It is important to choose the correct planting time so that stress conditions (for example, cold soil during planting) are avoided.
GOOD AGRONOMIC PRINCIPLES FOR REDUCING THE EFFECT OF BEAN DISEASES
- Plant only disease-free, certified seed (obtainable in small or large quantities from commercial seed companies).
- Do not retain beans for use as seed the following season, and do not buy seed from other farmers, casual traders or prepacked beans in shops. Disease infection of seed is not always visible.
- Practise crop rotation with a narrow-leaf crop such as maize, wheat or barley, and only plant beans every third or fourth year.
- Do NOT plant beans after broad-leaved crops such as potatoes.
- Choose a suitable planting date in order to avoid cold, wet conditions at planting or hot, dry conditions during the growing season.
- Do not over-water plants and make sure that fields are well-drained (avoid water-logging).
- Plough bean debris in thoroughly after harvesting. Do not leave piles of bean plants after threshing. Bean plants removed from the field could be burnt or fed to animals.
- Remove volunteer beans from between other crops and keep beans weed-free.
- Do not hoe fields or move irrigation pipes while plants are wet.
- Hoe uninfected or young plantings before doing older or infected plantings.
- Clean tools and machinery before working the next field.
- Wash clothing and clean shoes after having worked in an infected field.
- Be careful not to spread disease from small plots of green beans to a field of dry beans.
- Avoid unnecessary movement of people and animals in bean fields.
- Apply fungicides before a disease infection becomes serious, usually during flowering and before pods mature.
Also read: Guidelines to grow green beans
- This article was written by Merion Liebenberg and first appeared in Farming SA.