Management of fusarium wilt in beans

An aggressive disease in beans, favored by temperatures above 20ºC and sandy, acidic, compacted soils with poor drainage, fusarium wilt has a powerful dissemination channel in infected seeds. To prevent damage, treating seeds with fungicides or biological agents is an important ally for the producer.

The intense cultivation of beans at various times of the year, together with the use of irrigation via central pivot, exposes crops to a series of factors limiting factors, including, for example, diseases caused by soil-borne pathogens. Stands out the wilting or yellowing of fusarium, caused by the fungus Fusarium oxysporum Schlecht. f. sp. phaseoli Kendrick & Snyder (Fop). In Brazil, the disease occurs throughout the country and can lead to losses of around 30% in Brazilian grain production. 

the complex Fusarium oxysporum is the causal agent of vascular diseases in many plant species, and the isolates that are pathogenic to the same host, or that have the same range of hosts, are designated formae speciales; it is known that more than 70 have already were described.

When there is a high level of specialization, formae specialis can be associated with pathogenicity in only one species hostess. Lots of formae speciales can be subdivided into races, based on its virulence in different cultivars of a host. The races that cause wilting or yellowing specifically in the bean, receive the taxonomy of Fusarium oxysporum f.sp. phaseoli (Fop).

Fusarium oxysporum f.sp. phaseoli belongs to the phylum Ascomycota, order Hypocreales, family Nectriaceae. The fungus produces three types of spores asexual, called macroconidia, microconidia and chlamydospores.

The survival of The fungus occurs mainly in the soil, where it lives saprophytically in the remains of culture and organic matter. In the long term, the fungus survives in the form of chlamydospores in the absence of their host. Furthermore, other species may behave as alternative hosts of Fusarium oxysporum f.sp. phaseoli, including Dolichos Lablab L. (lablab), phaseolus lunatus L. (lima beans), Mucuna lands (mucuna black), Canavalia ensiforms (L.) (pork beans), Crotalaris spectabilis (sunn hemp), Cajanus cajan (pigeon pea).

The spread of the fungus from one field to another occurs mainly by infected seeds, or as spore adhered to its surface. This process is further facilitated by the wind and irrigation water, which carry conidia and infested soil from plants killed by the pathogen and by contaminated agricultural implements.

The pathogen is capable of penetrate the host's root tissues, as well as the hypocotyl and older root tissues, usually from wounds or openings natural. The infection can occur at any stage of the crop cycle, at an optimum temperature of 28°C. The "drought" harvest is considered the critical period of pathogen infection, especially when the plant is at stages between V4 (third trifoliate leaf) and R6 (flourishing). Nematodes Meloidogyne unknown, Meloidogyne javanica e Pratylenchus spp. are responsible through wounds and openings that may favor the entry of the pathogen through the bean root system.

After penetration, the fungal hyphae are able to colonize the plant, reaching your vascular system. The fungus remains confined to the xylem vessels until disease reaches more advanced stages of development, causing symptoms of wilting in plants. Eventually, the pathogen moves to other tissues and begins the process of sporulation on the surface of the dead plant.

 The severity of the disease is greater at 20°C and, in field conditions, pathogen development is favored at temperatures between 26ºC to 28°C. Sandy, acidic soils, under water stress conditions, compacted and poor drainage can also worsen the severity of the disease.

Fusarium wilt management

For disease control It is important to prevent the introduction of the pathogen into unharmed areas, as once presence in the field is detected, eradication of the inoculum becomes very difficult through cultural practices and chemical control.

Currently, the disease has been a problem, as its occurrence is related to the introduction in new areas of bean cultivation, due to the use of carrier grains in F. oxysporum and by intense cultivation of beans in the same areas infected with the pathogen. These circumstances force, in a certain way, producers to live with the disease in the field.

Although the use of resistant cultivars is the most efficient option for control the disease, many producers still choose to use cultivars susceptible, due to the desirable agronomic characteristics and the preference of the consumer market. Among the most used cultivars in the country, around 70% correspond to those of the Carioca group.

In this way, the analysis of the sanitary quality of the seeds, together with the treatment, consists of fundamental preventive measures for managing the disease.

Considering integrated disease management, the Seed treatment is an effective method for preventing seed disease, either by through fungicides or the use of biological agents. This practice has the objectives are not only to eradicate or reduce the inoculum carried by seeds, as well as providing partial protection to the seeds against pathogens, or otherwise present in the soil.

Among the benefits offered by the treatment of seeds, the low cost and the guarantee of crop establishment, as it offers protection against fungi of soil during the pre- and post-emergence periods of plants.

For bean cultivation, chemical treatment of seeds is a measure widely used by farmers to manage various fungi. In addition to acting as protectors, fungicides applied to seeds, mainly prevent the penetration of fungi infecting roots and those attached to the seed on the surface, thus avoiding the colonization of aerial or root organs.

Currently, for the control of Fop, the Agrofirt System, from the Ministry of Agriculture, Livestock and Supply, presents the fungicide capitan and the fungicide-insecticide metalaxyl-M, fludioxonil, thiabendazole and thiamethoxam. Regarding organic products, there are only two organic-based products Trichoderma spp., registered for the culture of bean, recommended for the control of Fusarium solani e Rhizoctonia solani, represented by species of T. asperellum and T. harzianum (Agrofit, 2015).

Importance of the seed

Especially for bean cultivation, it is very common for producers to use as seed the material from areas destined for grains. In the 2013/2014 harvest, the use of certified seeds in Brazil was approximately 20%, representing a much lower value when compared to other crops (ABRASEM YEARBOOK, 2014).

The quality of these "saved" seeds are characterized, in general, by presenting low purity, high degree of humidity, low germination and vigor, infestation by insects and, mainly, due to the presence of associated pathogens. 

Among the diseases of economic importance for bean cultivation, around 50% are disseminated by seeds and are responsible for significant losses in production. The fungi that represent this picture are Alternaria spp., Colletotrichum lindemuthianum, Pseudocercospora griseola, Fusarium oxysporum f.sp. phaseoli, Fusarium solani f.sp. phaseoli, Macrophomina phaseolina, Rhizoctonia solani, Sclerotinia sclerotiorum, Sclerotium rolfsii and others.

In the case of wilt of fusarium, bean seed is the main source of inoculum and can still transmit the pathogen externally, in the form of spores adhered to the surface of the integument, or internally, as dormant mycelium. Already it was found that the structures of the pathogen present in the seeds remained viable during a storage period of more than eight months and they can constitute the primary inoculum for the development of epidemics in bean crops. 

In this way, work that involve the effect of transmission of this pathogen via bean seeds are relevant, as they lead to understanding how the disease can affect factors essential for good productivity, such as seed germination and vigor.

O study

A study carried out in the Seed Pathology Laboratory, located in the Department of Phytopathology and Nematology at the Luiz de Queiroz College of Agriculture (Piracicaba/SP) aimed to verify the effect of inoculating the fungus Fusarium oxysporum f.sp. phaseoli (Fop) in bean seeds in laboratory and greenhouse conditions. Three were used isolated from Fop, grown on plates Petri dish with PDA culture medium. Bean seeds cultivar 'IAC Alvorada' were placed on the pathogen colonies, remaining in contact for 48, 96 and 144 hours under photoperiod alternating 12 hours of light and temperature of 20ºC ±2°C. The treatments controls consisted of seeds without the pathogen. The effect of different periods of seed contact with Fop was evaluated through health, germination and speed index tests seedling emergence. 

In According to the health analysis, it was found that the inoculation method was very efficient, as 100% of incidence of the pathogen in seeds, regardless of contact times.

In Table 1 it is observed that there was a significant reduction in seed germination after 96 hours of contact with the pathogen, regardless of the isolate. Seed contact for 144 hours led to a 40% to 50% reduction in germination.

You emergency speed indices (Table 2) and emergency percentages (Table 3) corroborate the results obtained in the germination test, confirming the reduction in vigor as the exposure time of the seeds with the pathogen, regardless of isolate.

For the shortest contact period, it was observed bean plants with typical symptoms of fusarium wilt, characterized by necrosis of the plant neck and yellowing of the leaves. Already in treatments where the inoculation time was 96 hours and 144 hours, it was not identified such symptoms. Based on this fact and the emergency percentage of the seeds (Table 3), it can be inferred that the 48-hour period allowed the development of plants and subsequently, the reproduction of symptoms, and, in On the other hand, the periods of 96 hours and 144 hours were decisive in death of the seeds.

Beans

O common bean, Phaseolus L., is a plant species whose grains make up an important protein source in nutrition of Brazilians. Currently, Brazil is positioned as the third largest producer of beans in the world, with an annual production of approximately three million tons, being surpassed by Myanmar and India.

In harvest from 2015/2016, national production reached 3.182,7 million tons, concentrating mainly in the states of Paraná, Minas Gerais, Mato Grosso, Goiás and São Paulo (CONAB, 2016).

Article published in issue 215 of Cultivar Grandes Culturas, April 2017.