Fungal Diseases Of Beans: Tips For Treating Root Rot In Bean Plants
As if the gardener doesn’t have enough to contend with above the ground, root rots can be serious and often undiagnosed diseases of plants. While you combat the usual visible insect damage and diseases, this insidious soil dwelling fungus is quietly destroying your bean roots. Common fungus on bean plants can be discerned with the naked eye, but to see the damage associated with root rot, you need to dig up the plant. Fortunately, such fungal diseases of beans can be combated successfully with a little preparation and know how.
What Causes Fungus on Bean Plants?
Root rot in bean plants is produced by several different soil dwelling fungi. It might stem from a Fusarium, Rhizoctonia, or Pythium species, but that doesn’t really matter. What matters is the effect it has on your crop. Harvest yields are diminished, plant vigor is compromised and, in some cases, the entire plant may die. Bean root rot control starts before planting with careful cultural considerations.
As mentioned, most bean root diseases are caused by any one of three different fungi. These fungi persist in soil, often for several years. They live on decomposing vegetation left from previous season’s plants. The fungi is most dangerous in mid to late season production of susceptible crops.
When plants are unstressed, the disease does very little damage beyond some vigor loss. However, in areas that have experienced extreme heat, drought, poor soil, diminished nutrition, or oxygen deprivation due to compaction, the disease takes hold on those traumatized plants.
Other plants that are susceptible and actually support the formation of colonies of the fungus that causes bean root diseases are potatoes, sugar beets, soybeans, and sunflowers.
Symptoms of Bean Root Diseases
The most common signs of root rot are subtle and hard to discern at first. Bean plants may be stunted and turn yellow, exhibiting signs of malnourishment. Symptoms of root rot in bean plants can start at emergence or even in mature plants. The dry bean varieties are more commonly affected than snap beans.
If you pull up a plant, most of the fungi will cause water soaked lesions on roots. The color of the roots will be brick red. Scraping a root will reveal a dark interior. In many cases, the side roots rot away and the tap roots become hollow and dry. If there is sufficient moisture, lateral roots may form off of the taproot but these will be spindly and mostly ineffectual.
Bean Root Rot Control Methods
Fungal diseases of beans are actually very simple to prevent. The most important control is crop rotation. Because the fungi persist in soil for years, they will attack the crop annually if it is planted in the same area. Without food, over time the fungus will die. Avoid planting any of the other host plants listed above as well.
Clean up infected plant matter and destroy it rather than chopping it into the soil to compost. Do not feed the spent plants to animals, as the fungus will be borne in their manure and can spread if used in the crop area.
Plant items like corn and small grains for the next three years. Recovery of diseased plants by formation of lateral root shoots can be accomplished by providing adequate water, nutrition, and ventilation.
Seedling And Root Diseases Of Beans
» Diseases that attack seedlings and root systems of bean plants can result in significant yield losses.
» Several soilborne fungal and water mold pathogens infect bean seedlings and root system.
» Disease management strategies include crop rotation, seed treatments, selection of appropriate planting times and locations, soil management practices, and the use of disease resistant varieties when available.
SEEDLING AND ROOT DISEASES
Seedling blights, damping-off, and root rots are caused by a number of pathogens including Aphanomyces euteiches and several species of Pythium (fungal-like water molds), the fungal pathogens Rhizoctonia solani and Fusarium solani f. sp. phaseoli, and several species of root knot nematode. These pathogens reside in the soil and are spread by anything that moves soil, including equipment, animals, and water flowing through infested fields.
Several species of Pythium cause seed rots, pre- and post-emergence damping-off, stem and root rots, and even some pod rots. Losses can be as high as 100% when the disease is severe. 2 Infections of root systems cause slightly sunken, tan lesions that coalesce. Both primary and secondary roots are affected, resulting in a wet, soft rot of large portions of the root system. Seedlings become stunted, wilt, and may eventually “damp-off” (Figure 1). 2 Under certain conditions, Pythium can splash up into the canopy and infect pods, resulting in lesions that start at the tips of the pods and progress towards the stem end. 3,4
Figure 1. Post-emergence damping-off caused by Pythium sp. University of Illinois Extension.
Pythium belongs to the water mold group of fungal-like organisms that thrive in wet conditions. So Pythium diseases typically occur following periods of high soil moisture. Some species of Pythium are favored by cool temperatures, while others do best in warm conditions.
Resistance is available in beans, but it is currently used in only a few commercial cultivars. To control this disease, plant beans in fields with well drained soils, use wide spacings between plants, and avoid over-irrigating during the seedling stage. Seed treatments and/or pre-plant (in furrow) soil treatments with fungicides containing mefenoxam can be effective. 5 Avoid planting in fields with a history of the disease. Crop rotations with cereal crops can help prevent a buildup of the pathogens. 2
APHANOMYCES ROOT ROT
Like Pythium, Aphanomyces belongs to the water mold group of organisms that thrive in wet conditions. Infection can occur at any time during the growing season, often just after seedling emergence. The disease can develop over a wide range of temperatures but does best between 68 and 82 °F. Saturated soils predispose roots to infection and are needed for the swimming spores of the pathogen to move to plant roots. The disease is most severe in locations with poor soil drainage, compacted soils, high clay content, or that receive excessive irrigation. Once in a soil, spores of the pathogen can remain viable for up to ten years. 5
The initial symptoms of Aphanomyces root rot are gray, water-soaked roots that become soft and brown to back. Root systems become small and non-functional, resulting in wilting, chlorosis, and necrosis of above ground plant parts. Infected root systems have reduced levels of Rhizobium root nodulation. The root lesions can extend upward onto the hypocotyls (lower stems). Seedlings become stunted and less competitive, but rarely die (damp-off). 5 The most severe symptoms are seen when dry conditions occur after a wet period that is conducive for infection.
Good resistance is available for Aphanomyces root rot in commercial bean cultivars, and resistance is one of the best methods for managing this disease. Management strategies should also include avoiding heavily infested fields, and practices that promote good soil drainage. Crop rotation and the use of fungicides have not been very effective. However, the use of cover crops, particularly cruciferous species (e.g. mustard and radish) have been shown to help control Aphanomyces root rot. 5
RHIZOCTONIA ROOT ROT
The fungus Rhizoctonia solani can also attack bean plants at any time during the growing season. However, it is most important as a seed decay and seedling damping-off pathogen. Infection by R. solani results in sunken, reddish colored lesions on the lower stems and tap roots of older plants (Figure 2). These lesions can girdle roots and stems, killing roots and weakening plants. 3,4
Figure 2. Rhizoctonia lesions on bean hypocotyls. University of Illinois Extension.
Rhizoctonia root rot is most severe at temperatures between 60 and 65 °F, with the size and number of lesions reduced above 70 °F. Soil moisture levels have little effect on disease development. However, the disease is more severe in areas with compacted soils. Plants are most susceptible in the seedling stages, and infections on plants more than four-weeks old usually have little impact on yield. 2
Seed treatment and soil applied fungicides containing azoxystrobin, PCNB, or fludioxonil can be used to manage Rhizoctonia root rot. 4 Cultural practices that encourage rapid emergence, such as planting in warm soils and shallow planting, can be used to reduce disease severity.
FUSARIUM ROOT ROT
Symptoms of Fusarium root rot include reddish lesions on the tap root and lower stem that can enlarge and coalesce to rot the entire root system (Figure 3). 1 Plants become stunted, and plant death can occur. However, adventitious roots can form above the lesions, keeping plants alive.
Figure 3. Reddish root discoloration from Fusarium root rot. Howard F. Schwartz, Colorado State University, Bugwood.org.
Fusarium root rot is favored by warm (72 to 90 °F), wet conditions and acidic soils. Compacted soils and infection by Pythium or nematodes can increase disease severity. 1
Resistance to Fusarium is not readily available in commercial bean varieties. Cultural practices, such as proper fertilization, adjusting soil pH levels, and alleviating soil compaction can help reduce disease levels. Seed treatment fungicides containing fludioxonil can protect seedlings from infection. 1,2,4
ROOT KNOT NEMATODE
Root knot of beans is caused by several species of the root knot nematode. Meloidogyne hapla is found in colder regions of the U. S. and other Meloidogyne species predominate in warmer areas. The disease is more common in the southern parts of the U. S. than it is in the north. Severe infections can result in yield losses of up to 90%. The disease is most severe in sandy, well-drained soils at temperatures between 77 and 86 °F. 1
Infected roots become swollen as root galls up to ½ inch in diameter develop on both primary and secondary roots (Figure 4). Roots are shortened and thickened, and the over all root system size is reduced. Above ground symptoms include yellowing, stunting, and wilting of stems and leaves. 2,6
Figure 4. Root gall symptoms of root knot nematode infection. Howard F. Schwartz, Colorado State University, Bugwood.org.
Rotation with cereal crops, deep plowing, and soil flooding can help lower inoculum levels of the nematode. 2 High levels of resistance to root knot are not available in commercial snap bean varieties however some varieties are reported to have low levels of resistance. Fumigant and non-fumigant nematicides can be used to lower nematode levels, but the use of these products may not be economically justified, and environmental and worker safety factors need to be considered. 6
1 Hagedorn, D.J. and Inglis, D.A. 1986. Handbook of bean diseases. A3374. University of Wisconsin-Extension.
2 Schwartz, H.F., Steadman, J.R., Hall, R., and Forster, R.L. 2005. Compendium of bean diseases second edition. American Phytopathological Society, St. Paul, MN.
3 Seebold, K.W. 2014. Bean diseases. Plant Pathology Fact Sheet, PPFS-VG-16. University of Kentucky.
4 Reiners, S. et al. 2016. Cornell integrated crop and pest management guidelines for commercial vegetable production.
5 Huges, T.J. and Grau, C.R. 2013. Aphanomyces root rot or common root rot of legumes. The Plant Health Instructor. DOI: 10.1094/PHI-1-2007-0418-01.
6 Noling, J.W. 2015. Nematode management in beans and peas (bush, beans, pole beans, lima beans, southern peas, English peas, Chinese peas, or snow peas). University of Florida-IFAS. INY-020.
For additional agronomic information, please contact your local seed representative. Developed in partnership with Technology Development & Agronomy by Monsanto.
Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about bean diseases. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professional dealing with this specific crop.
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Several species of root-knot nematodes (Meloidogyne spp.) attack green beans and are important pests in temperate areas. Aboveground symptoms include yellowing, stunting, leaf edge necrosis and wilting. The nematodes penetrate the root tissue and cause swollen areas called galls. Roots are short and thick, and fewer lateral roots are produced, resulting in a reduced root system. To help control nematode populations, rotate beans with grain crops or corn. Plant resistant cultivars such as the snap bean variety "Harvester."