INTRODUCTION

Banana trees and plantains belong to the genus Musa and are the fourth most important crop in the world, only below rice, wheat, and maize (^{Jones, 2000}), and they are the most important tropical fruit due to its international consumption. These species are planted in over 120 countries, which produce over 100 million tons per year (^{Heslop-Harrison and Schwarzacher, 2007}), which constitute the food staple for more than 400 million people, since they contain large amounts of carbohydrates, vitamins, and minerals. Latin American countries are the main exporters of fresh fruit to the United States and Europe. However, these countries only export 15 % of the production, and the rest is for local consumption and industrialization. Mexico in 2014 recorded a surface planted with 74 584.52 ha, which produced 2,150,519.90 tons worth $6,305,790.11 (^{SIAP, 2015}), which are exploited mainly in three producing regions: Gulf of Mexico (Tabasco, Veracruz, and Oaxaca), South Pacific (Chiapas), and Central Pacific (Colima, Michoacán, Jalisco, and Nayarit) and more than 70 thousand families depend on this, directly or indirectly. The cultivar Gran Enano (Subgrupo Cavendish, Musa AAA) is the most important, in commercial terms. Pests and diseases have disseminated mainly during the distribution of germplasms of musaceae from the Southeast to new agricultural zones (Latin America and the Caribbean), since its dissemination occurs naturally at a smaller scale, and hardly at long distances (^{Jones, 2000}); these can also spread by shoots, corms, etc. (Table 1.) The diseases of the most quarantine and economic importance faced by bananas in Mexico and other countries, mainly in Latin America, are the leaf spot disease (Mycosphaerella eumusae), Panama disease tropical race 4 (Fusarium oxysporum f. sp. cúbense (E.F. Smith) Snyder and Hansen), banana moko disease (Rolstonia solanacearum), banana bacterial wilting (Xanthomonas campestri pv. musareum), bunchy top virus (BTV), bract mosaic virus (BBrMV), banana streak virus (BSV), and the banana mosaic virus (BMV) (Jones, 2002; ^{Blomme et al., 2013}; ^{Ploetz et al, 2015}), all of which could cause the Mexican banana industry to collapse, if phytosanitary measures and integrated management are not carried out.

Table 1 Possibilities of banana plant pathogens of infecting several vegetative parts of the plant and disseminating by these to other producing regions. 

Key: + pathogen can infect material.

-: pathogen cannot infect material.

?: pathogen could infect but is not confirmed.

LEAF SPOT DISEASE

Introduction. At least 20 species of Mycosphaerella have been described as affecting banana tree leaves and fruit, although only three cause significant losses (^{Arzanlou et al., 2008}; ^{Churchill, 2011}) Mycosphaerella fijiensis, causal agent of the Black Sigatoka (BS) is the most important on a global scale, and it has replaced Mycosphaerella musicola (Yellow Sigatoka) in the humid tropics, and Mycosphaerella eumusae causes the leaf spot disease. Despite the fact that none of the three kills its host, these pathogens may have an impact on the yield, premature maturation, and the quality of the fruit. BS has caused a loss in the production of diverse banana cultivars, and has made it increasingly difficult to produce Cavendish for export. The presence of leaf spot disease has been confirmed in Southern India, Sri Lanka, Thailand, Malaysia, Vietnam, Mauritius, and Nigeria (^{Carlier et al., 2000}). Latin America and Caribbean countries are free of this disease (Jones, 2002; ^{Ploetz et al., 2015}).

Symptoms. The initial symptoms of the disease appear as small, light brown streaks on the leaves. When the density of infection is low, the streaks develop oval or ellipsoidal-shaped lesions with a gray center. However, when the density of infection is high, the lesions coalesce and necrotize, forming large areas of foliar tissue. These symptoms are similar to those produced by diseases such as black and yellow Sigatoka. This disease affects diverse genomic groups of banana, including Cavendish (^{Carlier et al., 2000}).

Causal agent. The name of the fungus proposed by ^{Crous and Mourichon (2002}) for thi disease is Mycosphaerella eumusae (anamorfo Pseudocercospora eumusae). This fungus is more phylogenetically related with the sequence data of the ITS to M. fijiensis than to M. musicola, M. Musae, and Phaeoseptoria musae, fungi which affect the foliar area of the plant. It is the first report of this new disease, and the creation of information is required on different aspects of the biology of the fungus, host-pathogen interaction, and the genetic analysis of the genetic structure of the populations (^{Carlier et al., 2000}).

Control. Applying the same fungicides used for the control of Black Sigatoka negra and Yellow Sigatoka can control the leaf spot disease, although studies must be carried out on the tolerance to fungicides in order to prevent the development of tolerant strains. To date there are no studies on the reaction of different banana genotypes to this disease.

PANAMA DISEASE TROPICAL RACE 4

Introduction. The Panama disease (Fusarium oxysporum f. sp. cubense: FOC) is and has been the most destructive disease for some banana cultivations [Roatán or Gros Michel (Musa AAA) and Manzano (Musa AAB)] and for plantains [Pera or Cuadrado (Musa ABB)] in Mexico and the world. Once it is introduced in a field, it is difficult to control (^{Ploetz, 2015}). In the 1960s and early 1970s, nearly 40 thousand ha of the cultivar Gros Michel were devastated in Mexico, forcing to change it for another resistant cultivar of the subgroup Cavendish. In 1994, there were approximately 4,000 ha of Manzano banana trees in Mexico, but this surface was reduced by the Panama disease, leaving only 1,260 ha in 2004. The disease has been spread by means of dissemination material (corms and/or shoots) and there are no efficient control measures. FOC's Tropical race 4 is a potentially dangerous threat for Latin America and the Caribbean, since it severely affects Cavendish banana trees (^{Ploetz, 2015}). Nowadays, the disease is present in some countries in Asia and Africa (Fig. 1). The main means of dissemination of the fungus is via shoots and rhizomes (corms), which are used as a way to spread the crop. Te pathogen can also be spread in the soil and water, as well as in agricultural tools, machinery, work boots, and the black banana weevil (^{Ploetz et al., 2015}).

Figure 1 Regions under the effect of damage by tropical race 4 of Fusarium oxysporum f. sp. cubense. 

Causal agent. The Panama disease is caused by the fungus Fusarium oxysporum Schlect. f. sp. cubense (E.F. Smith) Snyder & Hansen. The sexual phase (teleomorfo) of the fungus is unknown. The development in vitro of the pathogen in a potato dextrose agar (PDA) culture medium is of 4 to 7 mm per day at 24 °C, forming cultures with abundant aerial mycelia and white to purple pigment. In general, F. oxysporum strains cannot be distinguished morphologically. Micro and macroconidia are produced in branched and non-branched monophialide, whereas clamidospores are terminal and intercalated with an average of 7 to 11 microns in diameter, generally globular, and they form on their own or in pairs in the hyphae or conidia (^{Ploetz, 2015}).

Control. It is important to raise awareness in producers and the public in general regarding the nature and severity of the problem, as well as to double efforts in the phytosanitary epidemiological surveillance. In this sense, Mexico must implement the available tools to face it with the necessary knowledge and studies, such as the use of vitro plants to begin a cultivation, crop rotation, having an adequate drainage system, the use of biological control agents, incorporating organic matter, induced mutagenesis, knowledge of the genetics of populations and their pathogenicity are some of the studies that could be key to strengthen the knowledge on the pathogen. In Mexico, SAGARPA, through SENASICA, is taking actions of a phytosanitary epidemiological surveillance for the timely detection of RT4, by training technicians and producers, exploring and monitoring banana producing areas, raising awareness in producers regarding the risks of moving plant material. They are an important measure to prevent the entry and spreading of the pathogen.

BANANA MOKO DISEASE

Introduction. The banana moko disease is characterized by wilting in plants. This disease caused severe losses in 1840 in Guyana and towards the end of the 20th Century in Trinidad, it caused the almost complete destruction of the Moko cultivar. The spreading of the disease in Central America was related, initially, with the development of commercial plantations in Honduras, Panamá, and Costa Rica. In Latin America, it is found in Belize, Colombia, Costa Rica, Ecuador, El Salvador, Granada, Nicaragua, Panama, Surinam, Venezuela, Brazil, Peru, Guatemala, Honduras, Jamaica, and southeastern México (^{Alvarez et al., 2013}). In Mexico nowadays, it is found in the states of Chiapas, Tabasco, and Nayarit.

Symptoms. Moko affects most cultivars of commercial musaceae (table fruit and for cooking), especially Cuadrado (ABB), affecting it in all stages of development and the external symptoms vary according to the path and the state of infection and the strain of the pathogen. Symptoms caused by the moko are similar to those produced by the Panama disease. Both cause vascular wilting and decoloring of the vascular vessels. Symptoms caused by the moko after infection in the roots of young plants are characteristical: the first, second, and third youngest leaves turn yellow-green, and eventually collapse in or on top of the union of the petiole and the leaf. The oldest leaves (the lowest) also wilt and collapse rapidly. Where the infection enters via lesions caused by infected trimming tools, shoots turn black and become stunted in 2 to 4 weeks and can grow a leaf that expands and later wilts. The most characteristic symtpom of moko after the invasion of races transmitted by insects via the peduncule is the male flower turning black, stopping its development, and the bacteria can trickle in drops from the flower. The fruit may become yellow and the skin becomes cracked. When cutting, the fruit's pulp displays a firm brown rotting, which later turns gray.

Agente causal. The banana moko disease is caused by the race 2 of the bacteria Ralstonia solanacearum, which has shown a large genetic diversity, which is why it has been named the species complex R. solanacearum (RSSC) (Fegan and Prior, 2005). This pathogen belongs to phylotype II, equivalent to division 2 and the isolates included between biovars 1, 2, and 2T; and are isolated mainly in the Americas.

Control. In cultivations damaged bymoko, it is crucial to continually have plant inspection and eradication programs, as well as to disinfect trimming and desuckering tools. It is important to detect the disease initially, and in a timely manner, along with a strict tool disinfection, the destruction of diseased and adjacent plants with herbicides. The plants treated must be left in their place in the field. Likewise, the male flower must be removed after the last female hand has emerged. In cultivations where producers are reluctant to carry out this type of practice, moko may become epidemic in Mexico (^{Alvarez et al., 2013}).

BANANA BACTERIAL WILT

Introduction. Bacterial wilt is caused by Xanthomonas campestri pv. musacearum, and is a potential threat to banana tree cultivations. First of all, this disease was found affecting ensete (Ensete ventricosum) plants in Ethiopia, approximately 30 years ago, and later, natural infections were diagnosed in banana trees in the same country. This disease is now one of the most important threats for banana tree cultivations in East Africa and nearby regions. Its spreading is quick, as is the expansive form of propagating in banana trees, seemingly due to the ease with which the pathogen is transmitted by insects that visit the inflorescences. So far no resistance to the disease has been observed in any of the banana tree varieties commonly cultivated in Uganda, and in some areas affected, communities have lost entire productions. As with other bacterial diseases in banana trees, it is likely that some types, especially Bluggoe, are more vulnerable to transmission by insects than others. Preventing the transmission by insects in this and other varieties may provide a way to reduce the spreading of the disease (^{Tripathi et al., 2009}).

Causal agent. The bacterial wilt is caused by Xanthomonas campestris pv. Musacearum. The dissemination of these bacteria is rapid and efficiently transmitted by contact (field tools) causing considerable losses. It has been found attacking banana trees of all genotypes. The sources of inoculant are residues of diseased plants, contaminated soil, products used for sales. The disease may be triggered by insect-transmitted diseases (at altitudes below 1700 m.a.s.l.) from diseased plants to the male flowers of healthy plants, through the splash of raindrops, from rhizomes obtained from infected plants or via the transmission by contact in unflowered plants, from which wilt, the rotting of fruits, and lethal wilt develop.

Control. It is ideal for the movement of all parts of the banana tree, enset, and other musaceae (except as cultures of pathogen-free axenic tissues) to be restricted from areas affected by the disease, both inside and between countries in rick of the disease. All fruits in the affected areas must be consumed or processed locally to reduce the risk of spreading to a minimum; bunches of fruits that display premature maturation or other external symptoms are unlikely to be commercialized, although bunches that appear to be externally normal, yet are decolored when cut are a particular risk: they can be objects of commercialization, discarded, and serve as possible sources of infection. In practice, these controls and greater surveillance can rarely be complied with, backed by immediate phytosanitary and eradication measures, they must be instigated in areas in which the introduction of the disease could cause economic losses.

BANANA BUNCHY TOP VIRUS (BBTV)

Introduction. The bunchy top virus is the most important virosis that affects banana tree cultivations and can cause their devastation. The first report of the disease was recorded in the island of Fiji in 1879. In the 1920s, it virtually finished the banana industry in Australia (Staiton et al., 2015). The disease has spread around the South Pacific, Asia, and Africa. The BBTV has not been found in Mexico, or other banana-producing areas of Latin America or the Caribbean.

Symptoms. The first symptoms appear as streaks and dark green dots of variable lengths in leaves and petioles. Later, leaves develop a marginal chlorosis, their development is reduced, they become stunted and erect, appearing to present offsetting in the higher parts of the plant. This symptom gives it the name of "bunchy top". When plants are infected at an early stage, the bunch does not emerge, and if it does, the quality of the fruit is poor. The disorganization in the development of the plants is due to the virus affecting the phloem cells, which reproduce in an excessive and disorganized manner.

Causal agent. The virus that causes the bunchy top belongs to the family of Nanoviridae, genus Babuvirus. It contains six components of the genome, each of which is approximately 1,000 nt long and are named DNA-R, DNA-U3, DNA-S, DNA-M, DNA-C, and DNA-N (formally DNA-1 to DNA-6, respectively, according to ^{King et al., 2012}). BBTV is transmitted persistently by the black banana aphid Pentalonia nigronervosa Coq. (^{Thomas and Iskra-Caruana, 2000}). The BBTV is not transmitted by sap. However, it may be transmitted via corms, shoots, and in plants obtained by tissue cultures (Thomas et al., 1995). All species of musaceae are vulnerable to the BBTV.

Control. Quarantines are the main way to avoid the introduction of the BBTV in areas or regions free of the disease. In regions with BBTV problems, the use of virus-free plant material is recommended in plantations. The eradication of infected plants that serve as a source of inoculant is an important sanitary measure. Restricting the movement and use of cultivation material, regular inspections of all the plants affected by the BBVt, and the continuous education and extension programs for producers, are measures that can be used in regions where the disease is present (^{Thomas and Iskra-Caruana, 2000}).

BANANA BRACT MOSAIC VIRUS (BBRMV)

Introduction. This disease is relatively new and it occurs only in the Philippines and India. It was first recorded in 1979 (Magnaye and Espino, 1990). The bract mosaic is more common in cooking ABB cultivars, although it also causes serious problems in Cavendish bananas (AAA). In general, it causes losses in production of over 40 %. This virus is included in the list of viruses of quarantine importance (^{Frison and Putter, 1989}). The banana-producing regions in Mexico and the rest of Latin America and the Caribbean are free of the BBrMV (Jones, 2002).

Symptoms. The infection caused by BBrMV can take place in any stage of development and part of the plant, being very distinctive of the disease. The symptoms in the petioles consist of yellow dots or streaks. On the leaves, symptoms may or may not be visible. In the pseudostem brown stripes appear when removing dead leaves. The typical syndrome that gives the disease its name is a characteristic mosaic in the floral bracts, and that can, in some cases, be found in the fruits (Magnaye and Espino, 1990).

Causal agent. The BBrMV is transmitted non-persistently by the aphid Pentalonia nigronevosa, Aphis gossypii, and Rhopalosiphum maydis (Magnaye and Espino, 1990). The genus Musa and Abacá are the only hosts for this virus (Thomas et al., 1997), sin there have been studies using the inoculation of sap infected with the virus in indicative herb plants and they have not presented the symptoms of the disease (Magnaye and Espino, 1990). The BBrMV belongs to the group of the potyviruses, and it presents three strips of protein with an estimated sixe of 31, 37, and 39 kDa. The particles are 750 nm x 11 nm and it internally produces inclusions typical of the potyvirus (Thomas et al., 1997). Purified virions contain a protein layer of 38-39 kDa (Thomas et al., 1997). The analysis of nucleotide sequences indicates that the BBrMV is unique; it is a new potyvirus (^{Thomas et al., 2000}). The symptoms of bracts can aid in the diagnosis of the virus. However, it is ideal to confirm the presence of the BBrMV using serology tests with the ELISA and molecular techniques such as PCR (Thomas et al., 1997). The concentration of virions in infected plants is relatively low, and virions cannot usually be found quickly by placing sap under the electron microscope (^{Thomas et al., 2000}).

Control. Control measures include early detection and the immediate eradication of infected plants. For new plantations, it is suggested that material free of the virus is used, produced by tissue culture.

BANANA STREAK VIRUS (BSV)

Introduction. The BSV was described for the first time in Ivory Coast in 1958. Later, it was found in Asia and Australia. However, in some Latin American and Caribbean countries (Brazil, Colombia, Costa Rica, Cuba, Ecuador, Haiti, Honduras, Jamaica, Nicaragua, Puerto Rico, Venezuela, and in Florida U.S.A.) its presence has been confirmed. Its damage is not well-defined, since in some areas it seems to be a light problem, whereas in other areas, such as Ivory Coast, it can cause losses of up to 90% (^{Frison and Sharrock, 1998}). BSV is different to other plant viruses, since its DNA is integrated in the genome of Musa and Ensete plants. BSV is transmitted by the citrus woodlouse, Planococcus citri and Pseudococcus sp., which colonize the lower part of leaves, bunches, and flowers, although the main medium of dissemination has been planting material, since BSV is not transmitted mechanically. There is strong evidence that indicates that it is transmitted by seeds and infected plant material. There is a serious obstacle in the movement of Musa germplasm, as well as the resistant hybrids generated in improvement programs, since no new ones have been incorporated into commercial production, because they are frequently infected with BSV. This virus is not transmitted through the soil, and it is unlikely that it may be transmitted by work tools. In Mexico there are no official reports of its presence. However, a study carried out in cultivations in Veracruz based on the reproduction of the symptomatology and molecular analyses, concludes on the presence of BSV (^{Nabor-Romero, 2014}).

Symptoms. The initial symptoms of BSV are presented as a mosaic-type interveinal chlorosis, separate lines and diamond-shaped patterns along the veins. With age the affected foliar tissue becomes progressively darker, and eventually turns black or dark brown. The death of the tips and the internal necrosis of the pseudostem in banana trees have been related with the BVS infection. The disease also reduces plant vigor and growth, reduces the weight of bunches, and causes distortion in fruits. Symptoms may disappear or appear in certain seasons, and not all leaves may display symptoms of streaking (^{Frison y Sharrock, 1998}).

Causal agent. BVS belongs to the genus Bandavirus, family Caulimoviridae (Comstock and Lockhart, 1990). It contains a 7.5 kb double chain DNA genome, covered in bacilliform particles sized 30 x 130 mm (Lockhart and Olszewski, 1996). A high degree of heterogeneity of the genome has been observed between different BSV isolates, serologically, genomically, and biologically, which makes the indexation of virus-free plant material troublesome (Lockhart and Olszewski, 1996). All the progeny that arises from the mother plant develops symptoms. The BSV hosts are restricted to the genus Musa (^{Frison and Sharrock, 1998}).

Control. Eradicating infected plants can control the Banana Streak Virus, destroying them with insecticides to kill the mealybug. The virus can be transmitted in plants from tissue cultures, therefore they must be certified plants (^{Frison and Sharrock, 1998}). Due to its heterogenous nature, BSV is the greatest obstacle for a reliable serological detection.

BANANA MOSAIC VIRUS (CMV OR BMV)

Introduction. The banana mosaic virus is a disease with global distribution, and is known by several names: infectious chlorosis, heart rot, cucumber mosaic virus, or leaf rot. The virus was observed for the first time in Australia in 1930, and since then, its presence has been confirmed in most areas in which banana is grown (^{Lockhart and Jones, 2000}). This virosis is of little economic importance. Symptoms tend to be mild, appear and disappear periodically, causing no apparent economic damage. There are no reports on the presence of the banana mosaic virus in Mexico. However, the cucumber mosaic is a virus that has been found in several vegetable species (cucurbits and Solanaceae) and weeds. Studies are required to confirm its presence and evaluate its economic importance.

Symptoms. The leaves of affected plants display interveinal chlorosis, and in severe cases, the internal part of the leaf has displayed rotting, along with the central cylinder. Plants become stunted and the quality of the fruit is affected. This occurs in very particular parts. In general, plants with mild symptoms recover, being able to transform them into an endemic diseas of no economic importance.

Causal agent. The banana mosaic virus (BMV) is caused by a cucumovirus and it has spherical particles with 28-30 nm in diameter; it only contains polyhedral-shaped RNA. Some CMV isolates have three genomic RNA and one subgenomic. There are many races differentiated biologically and molecularly. CMV is transmitted in a non-persistent manner by diverse species of aphids: Aphis gossyphii, Rhopalosphum maydis, Myzus persicae, Macrosiphum pisi, and Rhopalosiphum prunifoliae. Among the hosts of CMV are plants of the family curcubitaceae, solanaceae, and other vegetables.

Control. The infection by the banana mosaic virus has no significant impact on worldwide production. The establishment of a management program is required to prevent or reduce the incidence of the disease. Using virus-free plants, avoiding the cultivation of curcubitaceae or other crops that could host the virus near banana cultivation, destroying weeds that are vulnerable to CMV, remove infected plants and disinfecting tools with sodium hypochlorite or heat (^{Frison and Putter, 1989}). It is also important to maintain an adequate control over the CMV vector aphids. Rather, BBTV is considered the most harmful viral disease that attacks musaceae, and is distributed in Africa, India, Southeast Asia, the Pacific Islands, and in Australia. This pathogen is transmitted by the banana aphid (Pentalonia nigronervosa Coq.), by the rhizomes of infected plants and also through vitroplants from infected plants. Its management requires strict programs for the detection of diseased plants, sanitation, and vector control, as well as a certified plantation material program. It can be diagnosed using the PCR multiplex or PCR in real time. On the other hand, BBrMV is found in India and confirmed in some countries in Southeast Asia. Like BBTV, it is transmitted by infected plant material and by the corn aphid (Ropalosiphum maidis) in a non-persistent manner; there are precedents of its transmission in vitroplants multiplied from infected donors. There is a method for its diagnosis using ELISA, IC-PCR, and RT-PCR.

CONCLUSIONS

As shown earlier, most pathogens mentioned above exist in Southeast Asia, and therefore, if they are introduced and disseminated in Latin America and the Caribbean, they could seriously affect the production of the crop. For this reason, one of the recommendations made to travelers that visit this region is: do NOT visit farms in which any of these diseases have been reported, do NOT pick up or transport soil or plants nor parts of plants of any genus or species, do NOT buy souvenirs made with banana plant material. The acknowledgement of the importance of combining efforts by federal and state plant health organisms, by research institutions, universities and producers is crucial to prevent the entry of these and other pathogens. Training producers and establishing measures that tend to reduce the risks of establishing and disseminating these pathogens in Mexico is vital.