Main causative agents of bean diseases
The main bacterial pathogens of bean (Phaseolus) diseases:
- Corynebacterium flaccumfaciens pv. flaccumfaciens (Bacteria wilt);
- Pseudomonas syringae pv. syringae (Bacterial brown spot);
- Xanthomonas phaseoli, X. campestris pv. phaseoli (Common blight);
- Xanthomonas phaseoli pv. fuscans (Fuscous blight);
- Pseudomonas phaseolicola, P. syringae pv. phaseolicola (Halo blight).
Major fungal pathogens of bean diseases:
- Alternaria spp. (Alternaria leaf and pod spot);
- Isriopsis griseola (Angular leaf spot);
- Colletotrichum lindemuthianum, other species of Colletotrichum spp. (Anthracnose);
- Ascochyta spp. (Ascochyta leaf spot);
- Cercospora cruenta (Cercospora leaf spot);
- Phytophthora phaseoli (Downy mildew);
- Peronspora viciae (Fava downy mildew);
- Fusarium solani f. sp. phaseoli (Fusarium root rot);
- Fusarium oxysporum f. sp. phaseoli (Fusarium yellows);
- Botrytis cinerea (Gray mold);
- Phyllosticta phaseolina (Phyllosticta leaf spot);
- Diaporthe phaseolorum (Pod blight);
- Erysiphe polygoni (Powdery mildew);
- Pythium spp., also Aphanomyces, and Thielaviopsis spp. (Pythium root rots and damping-off);
- Rhizoctonia solani (Rhizoctonia);
- Uromyces phaseoli, other species of Uromyces spp. (Rust);
- Elsinoe phaseoli (Scab, lima bean);
- Colletotrichum truncatum (Stem anthracnose);
- Verticillium albo-atrum (Verticillium wilt);
- Sclerotinia sclerotiorum (White mold).
Viruses:
- Bean Common Mosaic Virus, BCMV;
- Bean Golden Mosaic Virus, BGMV;
- Bean Pod Mottle Virus, BPMV;
- Bean Southern Mosaic Virus, BSMV;
- Bean Yellow Mosaic Virus, BYMV, strain of bean;
- Cucumber Mosaic Virus, CMV, strain of bean;
- Sugar Beet Curly Top Virus, SBCTV;
- Peanut Stunt Virus.
Root rot
Aphanomyces root rot
Aphanomyces root rot (Aphanomyces root rot) was not a problem in beans (Phaseolus) until 1979, when a strain of Aphanomyces euteiches was identified in Wisconsin causing severe root rot (Pfender and Hagedorn, 1982). It has since been found on crops in several states in the United States and Australia.
The infection may begin early in the life of the plant or develop later, after the soil of the field has become waterlogged for a short time. The fungus affects the bark of the root system, resulting in stunting and yellowing, after which the plant wilts and dies. A. euteiches can often be found together with Pythium spp. since the latter pathogen is more common, but it is favored by similar conditions. The host range of A. euteiches f. sp. phaseoli includes alfalfa (alfalfa) as well as string beans and dry beans.
Some resistance has been found in breeding lines, but their agronomic characteristics are too far removed from commercial traits in the near future. Heavy irrigation is associated with infection, so soil moisture content and temperature are key factors in disease control in large-scale irrigated bean production. The use of green fertilizer, especially mustard, has proven useful in suppressing A. euteiches and other pathogens (Parke and Rand, 1989). The application of brown mustard, grown as a green fertilizer and then crushed and rolled into the soil, has a fumigant effect on the pathogen. Other work has shown the value of other green fertilizers along with an integrated approach to control, using resistant varieties and proper soil management, to reduce inoculum levels in the soil (Tu, 1991).
Fusarium root rot
Fusarium root rot, Fusarium foot rot, is found in most bean growing areas worldwide. The disease usually causes the most damage when the roots first become stressed by soil compaction, drought, waterlogging, or oxygen stress. Early symptoms are stunting, chlorosis of the upper leaves, and redness or browning at the base of the stem. Roots usually turn black and the base of the stem may collapse. Sometimes symptoms may appear on short sections of plants along the row when soil conditions cause stress.
Loss of plants due to infection results in lower yields. Depending on the pockets of infection in the field, yield loss can be severe. Where plants are low in height, the efficiency of mechanized harvesting is reduced.
The pathogen Fusarium solani f. sp. phaseoli remains in the soil as chlamydospores for many years. Chlamydospores germinate in the presence of host root exudates and infect root tissues. Legume fusariasis isolates from the United Kingdom have also been shown to be pathogenic on pea roots, but the symptoms are not as severe as on beans (Phaseolus).
Bean varieties vary in their sensitivity to Fusarium root rot, but none are completely resistant. Some progress in developing resistance or tolerance has recently been made through research on root growth traits (Roman-Aviles et al., 2004), and this along with soil management will be the most practical form of control.
Rhizoctonia root rot
Rhizoctonia root rot, Rhizoctonia root rot, is common throughout the world and is one of the most economically important root and hypocotyl diseases in beans. Losses occur in all types of cultivation, whether plowing and tillage, minimum tillage or direct seeding systems. The disease often occurs in conjunction with Fusarium foot rot, but the characteristic depressed reddish-edged lesions at the base of the stem are usually a lesion of Rhizoctonia solani. This disease usually occurs in warm weather and is not as much of a problem in Northern Europe as Fusarium solani.
Resistance to this disease is available in breeding lines, and some moderate resistance is present in commercial varieties; however, soil management is paramount to minimize risk and alleviate root stress early in the growing season.
Southern blight (Sclerotinia rolfsii)
Southern blight is serious in many tropical and subtropical regions of the world, where high temperatures and moist soil occur throughout the growing season. The causative agent is the fungus Sclerotinia rolfsii, which has a very wide range of hosts among both monocotyledonous and dicotyledonous plants, including many vegetables. Early symptoms are similar to those of its relative S. sclerotiorum, except that most of the infection continues just above and just below the ground, where the base of the stem is girdled and the plant dies. The sclerotia produced in the mycelium then serve as a source of inoculum for other hosts.
Deep cultivation with soil turning over buries the sclerotia and limits contact between the sclerotia and the bean stems. Some resistant varieties are available, and some progress has been made in using biocontrol agents such as Trichoderma and Gliocladium for use as fungus antagonists.
White mold (Sclerotinia sclerotiorum)
White mold is found in most regions of the world except the tropics, and affects a wide range of host crops. It is also a disease of peas, and much of what is used for peas also applies to beans.
The causative agent is Sclerotinia sclerotiorum.
Infection occurs during periods of leaf moisture and optimum temperature; ascospores are released from apothecia, which in turn are formed from germinating sclerocytes left in the soil from a previous infected crop (Boland and Hall, 1987; Phillips, 1994).
Infection leads to destruction of stem tissue and the formation of dense white mycelium that covers the stems and leaves and infects the pods. Large sclerotia form in the infected tissue. The rate at which the infection spreads is rapid, and widespread loss of crop and bean quality occurs. Cross-infection from harvested fresh bean pods to healthy bean pods in storage or packaging can also occur, further reducing marketability.
Some resistant varieties are known in breeding lines, but there are few immune varieties, and selecting varieties for resistance is difficult because plant architecture can also influence infection (Schwartz and Singh, 2013). Biocontrol agents such as Coniothyrium minitans have been used and have been successful in reducing sclerocytes. The inoculant requires multiple applications to establish populations, and often the time between crops of beans in the field is too long for the agent to survive.
Botrytis pod rot (Botrytis cinerea)
Botrytis pod rot, or grey mold, is ubiquitous and present worldwide, infecting many crops and weeds during periods of high humidity.
The pathogen is Botrytis cinerea.
Infestation occurs on the pod, usually after first infecting the wilted petals, which either adhere to the pod after it wilts or reach the leaf petiole, where the fungus can then infect the stem, causing it to girdle.
Botrytis is also a very important postharvest disease of fresh beans. Damage to the pods leads to rejection of the bean crop for the fresh market or for processing. In dry beans, the pods disintegrate and the seeds may be exposed, resulting in blight, and if the seeds are also infected, the beans will shatter.
Wet conditions are favorable for B. cinerea, and the likelihood of a problem is greater if the weather is wet or if watering is excessive during flowering and pod setting, which allows the petals to remain attached to the ends of the pods. A fungicide program can be used, but in many cases the fungus has developed resistance to the commonly used active ingredients.
Anthracnose (Colletotrichum lindemuthianum)
Anthracnose is distributed throughout the world, but is more common in temperate and subtropical areas than in the tropics. It is found in North and South America, as well as in Europe, Africa, Australia and Asia. It is one of the most important diseases because it causes significant yield loss and damage from pod spoilage. The disease is seed-borne, and once spores appear in warm, humid conditions, the spores spread very quickly by rain splashes to surrounding plants. The disease is especially harmful during the phase of sprouting and fruit formation.
Pathogen: Colletotrichum lindemuthianum fungus (Sace, et Magn.) Br. et Cav.
Symptoms of infestation are very characteristic: black or brown fringed leaf lesions appear first, running along the leaf veins, as well as a blight effect. The diseased part of the tissue often falls out, forming so-called “shoots”. Later, the pods become infected, on which deep round brownish-red merging indented lesions, black in the center, form. In the recesses of the spots, especially in wet weather, whitish-orange or pink pads of conidial sporulation of the fungus form. Conidia are colorless, oblong-cylindrical with rounded ends. The fungus infects the seeds through the pods, on which the same spots are formed as on the pods. The disease appears as brown-red spots on seedpods, with light-colored spots in the center. On stems and petioles, dash-shaped, brown spots appear at first, later turning into ulcers. In wet weather, reddish pads appear in the central part of the affected tissue, which are spores of the fungus.
The fungus is preserved by mycelium in diseased plant debris and infected seeds. The disease is transmitted from plant to plant by raindrops. Optimum temperature for anthracnose is about -20°C. Incubation period is 3-7 days.
Some varieties are resistant to most races, but the pathogenicity of the fungus is very diverse. Seed coat infestation can be controlled by fungicide treatment of seeds, but although fungicides can be applied by leafroll, prevention by using healthy seeds produced under drought conditions is the most effective control measure.
Source: commons.wikimedia.org©Howard F. Schwartz, Colorado State University, Bugwood.org (CC BY 3.0)
Bean rust (Uromyces appendiculatus)
Bean rust, English Bean rust, is found all over the world, but is most common in tropical and subtropical areas. In Latin America, it is most severe in Brazil, the Caribbean, Central America and Mexico, as well as in Africa. It rarely occurs in arid climates, only with irrigation. Rust affects yield and pod quality for both the fresh vegetable market and dried bean production and can reduce dried bean yields by up to 30% through defoliation or reduction of leaf area.
Pathogen: obligate fungus Uromyces appendiculatus (probable syn. Uromices phaseoli (Schrot.)).
When pods become infected, the disease can cause deep, dark sores on the surface, making them unfit for sale in fresh or processed foods. Symptoms first appear as small, round rust-colored spots surrounded by a yellow halo. The pustules develop and produce orange-brown spores that are transferred to surrounding leaves and plants by rain splashes or overhead sprinklers. Later, the spots become larger and the spores turn black.
The pathogen persists as teliospores on plant debris. Mycelium on rhizomes of thrush. In spring, teliospores germinate and form basidia with basidiospores that infect beans and beans. In mixed-farm fungi, the basidiospores infect the thistle, on which the eccidial stage develops, and then the eccidiospores infect the main host.
There are a number of fungicides that are more effective when applied early in disease development. The most effective control of bean rust is varietal resistance. Stable resistance is difficult because the pathogenic variability of the fungus is very large. Although most varieties are resistant to several races of rust, only a few are resistant to most local races. Many rust-resistant genes are present in common bean, and some have been used to develop rust-resistant germplasm and varieties (Park et al., 2004). Significant progress has been made in the United States in breeding beans with multiple resistance genes.
Source: commons.wikimedia.org©Jerzy Opioła (CC BY-SA 4.0)
Bacterial diseases
In all cases of bacterial bean diseases, the primary source of infection is seed, so strict phytosanitary procedures should be implemented in bean seed production areas. In arid areas, seed crops should be inspected during production and checked for the presence of bacterial pathogens. In some situations, seed treatment with streptomycin is used to provide surface sterilization, but it is not available in all countries.
Halo blight (Pseudomonas syringae)
Pseudomonas syringae is the most common bacterial seedborne bean disease of temperate climates.
The causative agent is the bacterium Pseudomonas syringae pv. phaseolicola.
Symptoms of the disease on the leaves consist of small, angular, necrotic fatty spots that are surrounded by a chlorotic patch of tissue. Later, depressed, watery oily spots appear on the pods, which deepen into the pod wall until the developing seeds become infected. Because the disease spreads easily during rains or by watering overhead, the effect on the crop usually appears as small, discrete spots that spread quickly in the direction of the prevailing wind. Yield loss is caused by defoliation and plant death, and pod damage can result in culling of the crop when fresh beans are harvested.
Infected seeds or crop residues are the source of infection, but under normal crop rotation, the bacterium does not survive in the soil for more than 1 year.
No effective means of chemical control exists, so the only means of preventing the disease is seed health improvement.
Common blight (Xanthomonas campestris)
Common blight is a bacterial blight that affects leaves and pods and is considered a major problem in most tropical and semi-tropical bean-growing areas.
The causative agent is Xanthomonas campestris pv. phaseoli.
Symptoms of leaf infestation appear as water-soaked spots that expand and the surrounding tissue becomes necrotic and is bordered by an area of lemon-colored tissue. Eventually, the leaves take on a scalded appearance. Pod lesions are sunken and brown in color.
The disease can be very destructive during periods of warm, wet weather when yield and bean quality losses occur.
Bacterial wilt (Curtobacterium flaccumfasciens)
Bacterial wilt causes bean plants to wilt during periods of moisture deficiency. It has been reported in the USA, Tunisia, Australia, Greece, Canada and Colombia as well as other production regions.
The causative agent is Curtobacterium flaccumfasciens.
Viruses
Schwartz et al. (2005) described a number of viral diseases associated with Phaseolus bean, but the most common serious viruses are bean mosaic virus (BCMV), bean curly-top virus (BCTV), yellow bean mosaic virus (BYMV), and cucumber mosaic virus (CMV).
Bean common mosaic virus (BCMV)
Bean common mosaic virus (BCMV) is found worldwide, and in areas where susceptible varieties are grown, the disease can cause severe yield losses, poor pod set and pod development. The virus can survive in host weeds and, to a small extent, in infected seeds. The virus is then transmitted by aphid vectors, including Acyrthosiphon pisum, Macrosiphon euphorbiaea, Myzus persicae, and Aphis fabae. Irregularly shaped pale and dark areas appear on the surface of the three-leaf bean, most often the leaves curl downward and may grow longer than the rest. Some beans may show brown discoloration on leaf veins and stems. Most commercial varieties are resistant to BCMV, but dried bean varieties may be more susceptible. Effective aphid control and planting before aphid activity begins will also reduce the risk of infection.
Bean curly top virus (BCTV)
Bean curly top virus (BCTV) is found in the western United States and British Columbia. The seedlings develop as dwarf plants, with severe wrinkling and downward curling of the leaves. Plants are severely dwarfed and wispy. Leaves become brittle and flowers may wilt. The virus has several hosts, which are perennial or winter annual plants, such as Russian thistle or bramble (Salsola tragu), mustard, and sugar beet. The virus is carried by the beet leaf beetle (Circulifer tenullus), and warm, dry weather early in the season promotes migration of the virus from wintering leaf beetle hosts to newly emerged beans. There are a number of resistant varieties because treatment against leafminer is usually too late to be effective.
Bean yellow mosaic virus (BYMV)
Bean yellow mosaic virus (BYMV) develops in beans any time before flowering. Leaves may be shriveled and pointed with some vein brightening. Leaves may droop, yellow mottling may appear on older leaves, and plants may be stunted. BYMV virus is widespread throughout the world and has several legume and nonlegume hosts. The virus is transmitted by aphids, and infection can occur on individual plants as well as groups of plants. Depending on the degree of infestation, yields may be reduced and pods may be damaged or incomplete. BYMV is also known as Phaseolus virus 2 and can be transmitted by seeds of some legumes, including V. faba. Several aphid vectors can transmit the virus between beans from wintering hosts. The main vector is Acyrthosiphon pisum; the virus is resistant and has a wide range of hosts, including peas, clover, alfalfa, gladiolus, freesia, lupine, soybeans, and wild legumes. There are some resistant dry bean varieties, but early aphid control on flowering crops is the most effective way to prevent widespread infestation.
Cucumber mosaic virus (CMV)
Cucumber mosaic virus (CMV) is found in many countries, including Europe and the Far East. Whole bean fields are known to be affected in the United States, but economic losses depend on the timing of infection. Late infestation decreases pod quality.
Symptoms include narrowing and sharpening of the leaves with mosaic developing later. Other symptoms include curling leaves, green and chlorotic mottling and dark green streaking of the veins. Some strains of CMV can be transmitted by bean seeds, but the disease is easily transmitted by several aphid vectors. The virus has a wide range of weed hosts, including several perennial species.
Healthy seeds are a useful preventative, but aphid control is more important. Some commercial varieties are resistant to CMV, and work is underway to develop new varieties that are resistant to this virus.
Sources
Modern technologies in vegetable production / Dr. A.A. Autko [etc.]; edited by A.A. Autko. – National Academy of Sciences of Belarus, Institute of Vegetable Growing. – Minsk : Belarus. nauvuka, 2012. – 490 p., [16] l. ill.
Peas and beans. Crop production science in horticulture / Antony J. Biddle. 2017. UK.