Under good growing conditions, beans are among the easiest and most rewarding summertime garden vegetables to grow. However, beans can be stunted during their early growth and development by disease, attacks by pests or poor cultural practices. To ensure a timely and abundant crop, gardeners should be able to identify and remedy these issues at the earliest sign of a problem.
Effects of Temperature
Beans germinate poorly in soils that are too cold. Unlike peas, which can be planted in cool, moist soils and cannot grow in warmer temperatures, beans sprout best in soils of at least 60 degrees Farenheit. Bean seed planted in cool soils tend to rot before germination occurs, or seedlings that do successfully sprout may be slow-growing, look sickly and stunted, or die within days of germination. Overnight temperatures should be well into the 60s for optimum seedling development.
Effects of Soil Organisms
As members of the legume family, beans are able to fix atmospheric nitrogen through nodules the plants form on their roots with the help of soil-borne bacteria. However, in gardens where beans or other legumes have never been grown, beans should be inoculated, or treated, with bacterial rhizobium cultures, available through many farm suppliers. Untreated seed planted in freshly turned soils may germinate well, but may fail to thrive due to lack of adequate bacterial cultures in the soil.
Effects of Water and Sunlight
Though most gardeners know that plants require water and sunlight, it can be a puzzle to figure out exactly how much of these two essential elements a plant needs. As a vegetable, beans need six or more hours daily of direct sunlight; bean plants receiving less will appear to be tall and spindly or stunted and sickly. Similarly, though beans can tolerate moderate dry spells once they are established, germinating seed and new bean seedlings need consistently moist soils to grow properly. Beans growing in soils which receive less than 1 inch of rain or water per week may fail to germinate or come up as stunted, malformed plants.
Diseases of Beans
Several serious diseases of beans can contribute to stunted plant growth. Bean mosaic virus causes mottled foliage and malformed fruit production and can be transmitted by aphids; control generally consists of destroying infected plants and planting successions of crops several weeks in a row. Stem and root rot are caused by several types of fungus that can remain present in soils for years after the initial infection; symptoms include rotted seed, seedling death, wilted seedlings and stunting or death of established plants. As these fungi are most prevalent during cool, wet weather, planting during the appropriate time of year is the most effective method of prevention. Finally, bean rust is another fungal infection that occurs later in the season, showing up first as white pustules on the undersides of leaves, which develop into circular, rust-colored patches. As the disease progresses, affected leaves wither and die. Along with crop rotation, spraying with a fungicide developed specifically for rust is one of the most effective remedies.
Root knot nematodes are the main non-pathogen pest of bean plants. Nematodes, which look like tiny worms but are generally unseen during visual inspection of the plant, cause stunting by burrowing into the root system of bean plants, preventing adequate nutrient and water uptake. Galls formed by the nematodes look similar to beneficial nitrogen-fixing nodules but are incorporated into the root itself rather than as a separate structure attached to the root. Rotating bean crops each year and planting marigolds with beans are two of the most effective ways of combating attacks by root knot nematodes.